Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Quantification of Smoothness Index

Differences Related to Long-Term

Pavement Performance Equipment Type

PUBLICATION NO. FHWA-HRT-05-054 SEPTEMBER 2005
FOREWORD 

The main objective of this project is to quantify and resolve the differences in the longitudinal 

profile and roughness indices that are attributable to the different profiling equipment that have 

been used in the LTPP program. The Long-Term Pavement Performance (LTPP) program was 

designed as a 20-year study of pavement performance. A major data collection effort at LTPP 

test sections is the collection of longitudinal profile data using inertial profilers. Three types of 

inertial profilers have been used since the inception of the LTPP program: (1) K.J. Law 

Engineers DNC 690 incandescent profilers, (2) K.J. Law Engineers T-6600 infrared-system 

profilers, and (3) ICC laser profilers. The following analyses were performed for this research 

project: (1) investigate data collection characteristics and compare profile data collected by the 

different inertial profilers, (2) compare International Roughness Index (IRI) values obtained by 

the different inertial profilers, (3) investigate factors that contribute to differences in IRI for data 

obtained from profilers and Dipstick®, and (4) identify problems with equipment functionality 

and current data collection and processing procedures. The analysis indicated good agreement of 

IRI values among the different inertial profilers that have been used in the LTPP program. 

Steve Chase, Acting Director 

Office of Infrastructure Research 

 and Development 

NOTICE 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

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contents or use thereof. This report does not constitute a standard, specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trade and manufacturers’ 

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Technical Report Documentation Page

1. Report No. 

FHWA–HRT–05–054 

2. Government Accession No. 3. Recipient's Catalog No. 

4. Title and Subtitle 

QUANTIFICATION OF SMOOTHNESS INDEX 

DIFFERENCES RELATED TO LTPP EQUIPMENT TYPE 

5. Report Date 

September 2005 

6. Performing Organization Code 

7. Author(s) 

R.W. Perera and S.D. Kohn

8. Performing Organization Report No. 

9. Performing Organization Name and Address 

Soil and Materials Engineers, Inc. 

43980 Plymouth Oaks Blvd. 

Plymouth, MI 48170 

10. Work Unit No. (TRAIS) 

11. Contract or Grant No. 

DTFH61-02-D-00137 

12. Sponsoring Agency Name and Address 

Office of Research, Development, and Technology 

Office of Infrastructure R&D

Federal Highway Administration 

6300 Georgetown Pike

13. Type of Report and Period Covered 

Final Report 

January–December 2004

McLean, VA 22101 14. Sponsoring Agency Code 

15. Supplementary Notes 

Contracting Officer’s Technical Representative (COTR): Larry Wiser, HRDI-13. 

This research was conducted in collaboration with the University of Michigan Transportation Research 

Institute (UMTRI) through participation and contributions from Steven Karamihas. Work was 

performed as a subcontract to Construction Technology Laboratories, Inc. (CTL), Columbia, MD. 

Dr. Shiraz Tayabji served as the project manager for the CTL contract. 

16. Abstract 

The Long-Term Pavement Performance (LTPP) program was designed as a 20-year study of pavement 

performance. A major data collection effort at LTPP test sections is the collection of longitudinal 

profile data using inertial profilers. Three types of inertial profilers have been used since the inception 

of the LTPP program: (1) K.J. Law Engineers DNC 690 incandescent profilers, (2) K.J. Law Engineers 

T-6600 infrared-system profilers, and (3) International Cybernetics Corporation (ICC) laser profilers. 

The following analyses were performed for this research project: (1) investigate data collection 

characteristics and compare profile data collected by the different inertial profilers, (2) compare 

International Roughness Index (IRI) values obtained by the different inertial profilers, (3) investigate 

factors that contribute to differences in IRI for data obtained from profilers and Dipstick®, and 

(4) identify problems with equipment functionality and current data collection and processing 

procedures. The analyses indicated good agreement of IRI values among the different inertial profilers 

that have been used in the LTPP program. 

17. Key Words 

IRI, inertial profilers, Dipstick, pavement data collection, 

pavement profile, profile measurement, profiler, LTPP. 

18. Distribution Statement 

No restrictions. This document is 

available to the public through the 

National Technical Information 

Service, Springfield, VA 22161.

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

157 

22. Price 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2

 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Approp e riate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)

ii 
TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION...........................................................................1

LONG-TERM PAVEMENT PERFORMANCE PROGRAM................................................... 1 

DATA COLLECTION AT GPS AND SPS SECTIONS ........................................................... 2 

DEVICES FOR PROFILE DATA COLLECTION ................................................................... 4 

RESEARCH OBJECTIVES ....................................................................................................... 4 

ORGANIZATION OF THE REPORT....................................................................................... 5 

CHAPTER 2: PROFILING DEVICES USED IN THE LTPP PROGRAM.....7

INERTIAL PROFILERS............................................................................................................ 7

K.J. Law Engineers DNC 690 Profiler ................................................................................... 7 

K.J. Law Engineers T-6600 Profiler....................................................................................... 8 

International Cybernetics Corporation Profiler ...................................................................... 9 

DIFFERENCES AMONG THE INERTIAL PROFILERS...................................................... 10 

Height-Sensor Type and Footprint........................................................................................ 10 

Sensor Spacing...................................................................................................................... 10 

Number of Sensors................................................................................................................ 10 

Location of Height Sensors................................................................................................... 11

Measurement Range of Height Sensors................................................................................ 11 

Data Recording Interval........................................................................................................ 12 

Data Filtering Methods ......................................................................................................... 12 

DIPSTICK................................................................................................................................. 12 

MANUALS FOR PROFILER OPERATIONS ........................................................................ 12 

COMPUTATION OF ROUGHNESS INDICES ..................................................................... 13 

CHAPTER 3: PROFILER COMPARISON STUDIES.....................................15

INTRODUCTION .................................................................................................................... 15 

LTPP PROFILER COMPARISON STUDIES......................................................................... 15 

Overview............................................................................................................................... 15 

Purpose of Comparison Test................................................................................................. 16 

Selection of Test Sections..................................................................................................... 17 

Collection of Reference Elevation Measurements................................................................ 18 

Profiler Data Collection ........................................................................................................ 18 

Computation of IRI Values................................................................................................... 19 

Analysis of Data from LTPP Comparison Studies ............................................................... 19 

LTPP PROFILER VERIFICATION STUDIES....................................................................... 19 

OTHER PROFILER COMPARISON/ANALYTICAL STUDIES.......................................... 19 

PIARC Comparison .............................................................................................................. 19 

Road Profiler User Group Comparisons: 1993 and 1994 ..................................................... 20 

LTPP Profile Variability Analysis........................................................................................ 20 

iii 
CHAPTER 4: ANALYTICAL PROCEDURES .................................................21

INTRODUCTION .................................................................................................................... 21 

ANALYTICAL TECHNIQUES AND SOFTWARE .............................................................. 21 

Roughness Profiles................................................................................................................ 21 

Power Spectral Density Plots................................................................................................ 24 

Data Filtering ........................................................................................................................ 24 

Cross Correlation .................................................................................................................. 26 

RoadRuf Software................................................................................................................. 28 

ANALYTICAL APPROACH .................................................................................................. 29 

CHAPTER 5: DATA COLLECTION CHARACTERISTICS AND 

COMPARISON OF DATA COLLECTED BY LTPP’S PROFILERS ...........31

CHARACTERISTICS OF DATA COLLECTED BY LTPP’S PROFILERS......................... 31 

K.J. Law Engineers DNC 690 Profiler ................................................................................. 31 

K.J. Law Engineers T-6600 Profiler..................................................................................... 31 

International Cybernetics Corporation Profiler .................................................................... 32 

COMPARISON OF K.J. LAW ENGINEERS DNC 690 AND T-6600 PROFILERS ............ 33 

Comparison of Profile Data .................................................................................................. 33 

Comparison of IRI Values .................................................................................................... 36 

Cross Correlation of IRI........................................................................................................ 44 

Analysis of Variance and Regression Analysis of IRI.......................................................... 45 

COMPARISON OF K.J. LAW ENGINEERS T-6600 AND ICC PROFILERS ..................... 46 

Comparison of Profile Data .................................................................................................. 46 

Comparison of IRI Values .................................................................................................... 53 

Cross Correlation of IRI........................................................................................................ 59 

ANOVA and Regression Analysis of IRI............................................................................. 60 

EFFECTS OF APPLYING A MOVING AVERAGE ONTO PROFILE DATA .................... 60 

Faulted Pavement.................................................................................................................. 61 

Effects of Downward Features.............................................................................................. 61 

Effects of Sharp Upward Features ........................................................................................ 64 

Smooth Asphalt Pavement.................................................................................................... 64 

SUMMARY OF THE FINDINGS ........................................................................................... 66 

CHAPTER 6: DIFFERENCES BETWEEN DIPSTICK AND 

PROFILER IRI ......................................................................................................69

INTRODUCTION .................................................................................................................... 69 

FACTORS CONTRIBUTING TO DIFFERENCES BETWEEN DIPSTICK AND 

PROFILER IRI ....................................................................................................................... 69 

Sampling Qualities of Dipstick............................................................................................. 69 

Variations in the Path Followed by the Profiler.................................................................... 73 

Features Recorded by the Profiler That Are Missed or Underestimated by Dipstick .......... 75 

Averaging Effects of Profiler Data ....................................................................................... 77 

Dipstick Data Errors ............................................................................................................. 78 

IRI Computation Procedure for Dipstick Data ..................................................................... 81 

iv 
DISCUSSION OF DIFFERENCES IN IRI BETWEEN DIPSTICK AND THE 

PROFILERS ........................................................................................................................... 82 

EXPECTED DIFFERENCES BETWEEN DIPSTICK AND PROFILER IRI........................ 88 

SUMMARY OF THE FINDINGS ........................................................................................... 90 

CHAPTER 7: OTHER FINDINGS FROM ANALYSIS OF THE DATA ......93

INTRODUCTION .................................................................................................................... 93 

IRI VALUES COMPUTED USING PROQUAL .................................................................... 93 

ACCELEROMETER EFFECTS ON THE K.J. LAW ENGINEERS T-6600 PROFILER 

DATA ..................................................................................................................................... 94 

OBSERVATIONS ON SHORT-WAVELENGTH DATA COLLECTED BY THE 

K.J. LAW ENGINEERS T-6600 PROFILERS...................................................................... 97 

Data from the 2000 Profiler Comparison.............................................................................. 97 

Evaluation of Data from the 1998 LTPP Profiler Comparison........................................... 100 

IRI DIFFERENCES FOR THE SOUTHERN PROFILER DURING THE 1991 PROFILER 

COMPARISON .................................................................................................................... 103 

SUMMARY OF THE FINDINGS ......................................................................................... 103 

CHAPTER 8: CONCLUSIONS AND RECOMMENDATIONS .................. 105

DATA COLLECTED BY INERTIAL PROFILERS............................................................. 105 

EFFECT OF APPLYING A MOVING AVERAGE ONTO PROFILE DATA .................... 105 

COMPARISON OF THE K.J. LAW ENGINEERS DNC 690 AND T-6600 PROFILERS.. 106 

COMPARISON OF THE K.J. LAW ENGINEERS T-6600 AND ICC PROFILERS .......... 106 

IRI VALUES .......................................................................................................................... 106 

DIFFERENCES IN IRI BETWEEN PROFILERS AND DIPSTICK.................................... 107 

REPEATABILITY OF THE K.J. LAW ENGINEERS T-6600 PROFILER ......................... 108 

SHORT-WAVELENGTH ERRORS IN DATA COLLECTED BY THE K.J. LAW 

ENGINEERS T-6600 PROFILER........................................................................................ 108 

Spikes in PSD Plots ............................................................................................................ 108 

Western K.J. Law Engineers T-6600 Profiler Data ............................................................ 109 

RECOMMENDATIONS FOR IMPROVING CURRENT LTPP DATA COLLECTION 

AND DATA PROCESSING PROCEDURES ..................................................................... 109 

RECOMMENDATIONS ON LTPP PROFILER COMPARISONS ..................................... 110 

APPENDIX A: LTPP PROFILER COMPARISON STUDIES..................... 111

APPENDIX B: PROFILER VERIFICATION STUDIES.............................. 131

REFERENCES.................................................................................................... 143

v 
LIST OF FIGURES 

Figure 1. LTPP regions................................................................................................................... 3 

Figure 2. K.J. Law Engineers DNC 690 profiler with a motor-home body. .................................. 7 

Figure 3. K.J. Law Engineers DNC 690 profiler housed in a van. ................................................. 7 

Figure 4. K.J. Law Engineers T-6600 profiler................................................................................ 9 

Figure 5. ICC MDR 4086L3 profiler.............................................................................................. 9 

Figure 6. Height-sensor footprints. ............................................................................................... 11 

Figure 7. Schematic view of Dipstick........................................................................................... 13 

Figure 8. Roughness of a roadway expressed in 10-m (33-ft) segments...................................... 21 

Figure 9. Example of a roughness profile..................................................................................... 22 

Figure 10. IRI obtained from two repeat runs............................................................................... 23 

Figure 11. Roughness profiles at 10-m (33-ft) base length for two runs...................................... 23 

Figure 12. Example of a PSD plot. ............................................................................................... 24 

Figure 13. Profile recorded by a profiler. ..................................................................................... 25 

Figure 14. Profile after being subjected to a 5-m (16-ft) high-pass filter..................................... 25 

Figure 15. Profile after being subjected to a 10-m (33-ft) low-pass filter. ................................... 26 

Figure 16. Profile after being subjected to a band-pass filter. ...................................................... 26 

Figure 17. Three IRI filtered profiles with an average correlation greater than 0.995................. 27 

Figure 18. Three IRI filtered profiles with an average correlation of 0.84.(21) ............................. 28 

Figure 19. PSD plot of data collected by the K.J. Law Engineers DNC 690 profiler. ................. 31 

Figure 20. PSD plot of data collected by the K.J. Law Engineers T-6600 profiler...................... 32 

Figure 21. PSD plot of data collected by the ICC profiler............................................................ 33 

Figure 22. Data collected by the North Central K.J. Law Engineers DNC 690 and T-6600 

profilers at the smooth AC site during the 1996 verification test.............................................. 34 

Figure 23. Data collected by the North Central K.J. Law Engineers DNC 690 and T-6600 

profilers at the rough AC site during the 1996 verification test. ............................................... 34 

Figure 24. PSD plot of data collected by the K.J. Law Engineers DNC 690 and T-6600 

profilers...................................................................................................................................... 35 

Figure 25. PSD plots of K.J. Law Engineers DNC 690 profiler data and ProQual-processed 

T-6600 profiler data ................................................................................................................... 36 

Figure 26. Relationship between IRI from the K.J. Law Engineers DNC 690 and T-6600 

profilers...................................................................................................................................... 39

Figure 27. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

All regions.................................................................................................................................. 40 

Figure 28. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

North Atlantic region ................................................................................................................. 41 

Figure 29. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

North Central region .................................................................................................................. 42 

Figure 30. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

Southern region.......................................................................................................................... 43 

Figure 31. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

Western region........................................................................................................................... 44 

Figure 32. Comparison of ICC and K.J. Law Engineers profiles: Western site 320209.............. 47 

Figure 33. Comparison of ICC and K.J. Law Engineers profiles: Western site 069107.............. 48 

Figure 34. Response of the IRI filter. ........................................................................................... 48 

vi 
Figure 35. PSD plot of 25-mm (1-inch) data collected by the North Central ICC and 

K.J. Law Engineers profilers at the chip-seal section during the 2002 verification test............ 49

Figure 36. Closeup view of 25-mm (1-inch) profile data collected by North Central ICC and 

K.J. Law Engineers profilers on a chip-seal pavement.............................................................. 50

Figure 37. Readings taken over a joint by the two profilers......................................................... 52 

Figure 38. Profile data obtained by the ICC and K.J. Law Engineers profilers at a concrete site.52 

Figure 39. Relationship between IRI from the K.J. Law Engineers and ICC profilers................ 54 

Figure 40. Differences in IRI between the K.J. Law Engineers and ICC profilers. ..................... 54 

Figure 41. Differences in IRI between the K.J. Law Engineers and ICC profilers: 

North Atlantic region. ................................................................................................................ 55 

Figure 42. Differences in IRI between the K.J. Law Engineers and ICC profilers: 

North Central region. ................................................................................................................. 56

Figure 43. Differences in IRI between the K.J. Law Engineers and ICC profilers: 

Southern region.......................................................................................................................... 58

Figure 44. Differences in IRI between the K.J. Law Engineers and ICC profilers: 

Western region........................................................................................................................... 59 

Figure 45. Profile distortion caused by the application of a moving average onto data collected 

over a fault. ................................................................................................................................ 61 

Figure 46. Profile distortion caused by the application of a moving average onto data collected 

over a patched crack................................................................................................................... 62 

Figure 47. Profile distortion caused by the application of a moving average onto data collected 

over a crack................................................................................................................................ 63 

Figure 48. Application of a moving average onto data collected for a concrete pavement.......... 64 

Figure 49. Application of a moving average onto a profile containing a sharp upward feature. . 65 

Figure 50. The 25-mm (1-inch) data and 150-mm (5.9-inch) averaged data from a smooth AC 

section. ....................................................................................................................................... 65 

Figure 51. Offset profile plot of 25-mm (1-inch) data and averaged 150-mm (5.9-inch) data 

collected from a smooth AC pavement...................................................................................... 66 

Figure 52. Dipstick response to a sinusoid with a wavelength equal to the footpad spacing of 

Dipstick...................................................................................................................................... 70 

Figure 53. Gain plot of Dipstick. .................................................................................................. 71 

Figure 54. An example of aliasing................................................................................................ 72 

Figure 55. Roughness profiles for nine runs that show good agreement...................................... 74 

Figure 56. Roughness profiles for two profile runs that show variations..................................... 75 

Figure 57. Roughness profiles along the left wheelpath for three profilers.................................. 75 

Figure 58. Measurement of cracks by a profiler and Dipstick...................................................... 76 

Figure 59. Measurement of a downward feature by a profiler and Dipstick. ............................... 77 

Figure 60. Illustration of artificial profile used by ProQual for computing IRI. .......................... 78 

Figure 61. Left-wheelpath 10-m (33-ft) base-length roughness profiles for profiler and 

Dipstick at site 5......................................................................................................................... 79 

Figure 62. Left-wheelpath profiles for profiler and Dipstick at site 5.......................................... 80 

Figure 63. Right-wheelpath profiles for profiler and Dipstick at site 1........................................ 80 

Figure 64. Roughness profiles for a profiler and Dipstick showing good agreement in roughness 

distribution. ................................................................................................................................ 83 

Figure 65. Roughness profiles for a profiler and Dipstick showing moderate agreement in 

roughness distribution................................................................................................................ 84 

vii 
Figure 66. Roughness profiles for the case with the lowest cross correlation.............................. 86 

Figure 67. Roughness profiles for the case with the highest cross correlation............................. 86 

Figure 68. Overlaid right-sensor profiles of the K.J. Law Engineers DNC 690 profiler.............. 95 

Figure 69. Overlaid right-sensor profiles of the K.J. Law Engineers T-6600 profiler. ................ 96 

Figure 70. Overlaid right-sensor profiles from the K.J. Law Engineers DNC 690 and 

T-6600 profilers. ........................................................................................................................ 96 

Figure 71. Right-sensor profile data collected by the Western and North Central profilers. ....... 98 

Figure 72. PSD plots of the right-sensor data collected by the North Central and Western 

profilers at site 2......................................................................................................................... 98 

Figure 73. PSD plot of the left-sensor profile data from the North Atlantic profiler. ................ 100 

Figure 74. PSD plots of 25-mm (1-inch) right-sensor data from the North Central and Western 

profilers.................................................................................................................................... 101 

Figure 75. PSD plots of ProQual-processed right-sensor data from the North Central 

and Western profilers............................................................................................................... 102 

viii 
LIST OF TABLES 

Table 1. GPS experiments............................................................................................................... 2 

Table 2. SPS experiments. .............................................................................................................. 2 

Table 3. Changes in IRI caused by lateral variations in the longitudinal path. ............................ 37 

Table 4. IRI values from the 1996 verification test. ..................................................................... 39 

Table 5. Results of cross correlation between the K.J. Law Engineers DNC 690 and 

T-6600 profilers. ........................................................................................................................ 45 

Table 6. Standard deviations of filtered elevation values............................................................. 51 

Table 7. Results of cross correlation between the K.J. Law Engineers T-6600 and 

ICC profilers. ............................................................................................................................. 60 

Table 8. IRI values for Dipstick data computed using ProQual and RoadRuf. ............................ 82 

Table 9. Results of cross-correlation analysis: Left wheelpath. ................................................... 84 

Table 10. Results of cross-correlation analysis: Right wheelpath. ............................................... 85 

Table 11. Cross correlation of IRI for North Atlantic and Western profilers............................... 87 

Table 12. Differences between the K.J. Law Engineers DNC 690 profiler IRI and 

Dipstick IRI................................................................................................................................ 89 

Table 13. Differences between the K.J. Law Engineers T-6600 profiler IRI and Dipstick IRI. .. 89 

Table 14. Differences between the ICC profiler IRI and Dipstick IRI......................................... 90 

Table 15. IRI values computed using ProQual and RoadRuf....................................................... 94 

Table 16. Comparison of the IRI from the 25-mm (1-inch) data with the IRI from ProQual. ..... 95 

Table 17. IRI values along the left wheelpath (1991)................................................................. 112 

Table 18. IRI values along the right wheelpath (1991). ............................................................. 113 

Table 19. Differences between profiler IRI and Dipstick IRI: Left wheelpath (1991)............... 113 

Table 20. Differences between profiler IRI and Dipstick IRI: Right wheelpath (1991). ........... 113 

Table 21. Standard deviations of IRI for 80-km/h (50-mi/h) runs: Left wheelpath (1991)........ 114 

Table 22. Standard deviations of IRI for 80-km/h (50-mi/h) runs: Right wheelpath (1991)...... 114 

Table 23. IRI values along the left wheelpath (1992)................................................................. 116 

Table 24. IRI values along the right wheelpath (1992). ............................................................. 116 

Table 25. Differences between profiler IRI and Dipstick IRI: Left wheelpath (1992)............... 117 

Table 26. Differences between profiler IRI and Dipstick IRI: Right wheelpath (1992). ........... 117 

Table 27. Standard deviations of IRI for 80-km/h (50-mi/h) runs: Left wheelpath (1992)........ 117 

Table 28. Standard deviations of IRI for 80-km/h (50-mi/h) runs: Right wheelpath (1992)...... 118 

Table 29. IRI values along the left wheelpath (1998)................................................................. 120 

Table 30. IRI values along the right wheelpath (1998). ............................................................. 120 

Table 31. Differences between the profiler IRI and Dipstick IRI: Left wheelpath (1998)......... 120

Table 32. Differences between the profiler IRI and Dipstick IRI: Right wheelpath (1998). ..... 121

Table 33. Standard deviations of IRI: Left wheelpath (1998). ................................................... 121 

Table 34. Standard deviations of IRI: Right wheelpath (1998).................................................. 121 

Table 35. IRI values along the left wheelpath (2000)................................................................. 123 

Table 36. IRI values along the right wheelpath (2000). ............................................................. 123 

Table 37. Differences between the profiler IRI and the walking profiler IRI (2000)................. 123 

Table 38. Standard deviations of IRI: Left wheelpath (2000). ................................................... 124 

Table 39. Standard deviations of IRI: Right wheelpath (2000).................................................. 124 

Table 40. IRI values along the left wheelpath (2003)................................................................. 126 

Table 41. IRI values along the right wheelpath (2003). ............................................................. 126 

ix 
Table 42. Differences between the profiler IRI and Dipstick IRI: Left wheelpath (2003)......... 127 

Table 43. Differences between the profiler IRI and Dipstick IRI: Right wheelpath (2003). ..... 127 

Table 44. Standard deviations of IRI: Left wheelpath (2003). ................................................... 127 

Table 45. Standard deviations of IRI: Right wheelpath (2003).................................................. 128 

Table 46. Profiler IRI for 80-km/h (50-mi/h) runs and Dipstick IRI: North Atlantic region. .... 132 

Table 47. Differences between the profiler IRI and the Dipstick IRI: North Atlantic region. ... 132 

Table 48. Profiler IRI for 80-km/h (50-mi/h) runs and Dipstick IRI: North Central region. ..... 134 

Table 49. Differences between the profiler IRI and the Dipstick IRI: North Central region. .... 134 

Table 50. Profiler IRI for 80-km/h (50-mi/h) runs and Dipstick IRI: Southern region.............. 135 

Table 51. Differences between the profiler IRI and the Dipstick IRI: Southern region............. 136 

Table 52. Profiler IRI for 80-km/h (50-mi/h) runs and Dipstick IRI: Western region. .............. 137 

Table 53. Differences between the profiler IRI and the Dipstick IRI: Western region.............. 138 

Table 54. IRI values obtained from the 2002 verification study. ............................................... 141 

x 
ACRONYMS AND ABBREVIATIONS 

AC Asphalt Concrete 

ANOVA Analysis of Variance 

ARRB Australian Road Research Board 

DMI Distance Measuring Instrument 

DOT Department of Transportation 

FHWA Federal Highway Administration 

GPS General Pavement Studies 

ICC International Cybernetics Corporation 

IRI International Roughness Index 

LTPP Long-Term Pavement Performance 

Mn/DOT Minnesota Department of Transportation 

Mn/ROAD Minnesota Road Research Project 

NCHRP National Cooperative Highway Research Program 

PCC Portland Cement Concrete 

PSD Power Spectral Density 

RMSVA Root Mean Square Vertical Acceleration 

RN Ride Number 

RSC Regional Support Contractor 

SHRP Strategic Highway Research Program 

SPS Specific Pavement Studies 

TxDOT Texas Department of Transportation 

UMTRI University of Michigan Transportation Research Institute 

 

xi 

CHAPTER 1: INTRODUCTION 

LONG-TERM PAVEMENT PERFORMANCE PROGRAM 

The Strategic Highway Research Program (SHRP) was a 5-year, $150-million research program 

that began in 1987. The research areas targeted under SHRP were asphalt, pavement 

performance, concrete and structures, and highway operations. One aspect of SHRP was the 

Long-Term Pavement Performance (LTPP) program. The LTPP program was designed as a 20-

year study. The first 5 years of the program were administrated by SHRP and, afterwards, 

administration of the program was transferred to the Federal Highway Administration (FHWA). 

The objectives of the LTPP program are to: 

• Evaluate existing design methods. 

• Develop improved design methods and strategies for rehabilitating existing pavements. 

• Develop improved design equations for new and reconstructed pavements. 

• Determine the effects of loading, environment, material properties and variability, 

construction quality, and maintenance levels on pavement distress and performance. 

• Determine the effects of specific design features on pavement performance. 

• Establish a national long-term pavement database to support SHRP objectives and future 

needs. 

To accomplish these objectives, the LTPP program was divided into two complementary 

programs. The first program, General Pavement Studies (GPS), uses inservice pavement test 

sections in either their original design phase or in their first overlay phase. The second program, 

Specific Pavement Studies (SPS), investigates the effects of specific design features on pavement 

performance.

Under the GPS program, more than 800 test sections were established on inservice pavements in 

all 50 States and in Canada. Each GPS section is 152.4 meters (m) (500 feet (ft)) long, and is 

located in the outside traffic lane. The GPS sections are categorized into different experiments 

based on the pavement type as shown in table 1. The GPS sections generally represent 

pavements that incorporate materials and structural designs used in standard engineering 

practices in the United States and Canada. The objective of the GPS program is to use the data 

collected at the GPS sections to develop improved pavement design procedures. The SPS 

experiments are designed to study the effects of specific design features on pavement 

performance. Each SPS experiment consists of multiple test sections. The SPS experiments that 

were designed for the LTPP program are shown in table 2. 

1 
Table 1. GPS experiments. 

GPS 

Experiment Description 

Number 

GPS-1 Asphalt Concrete on Granular Base 

GPS-2 Asphalt Concrete on Stabilized Base 

GPS-3 Jointed Plain Concrete 

GPS-4 Jointed Reinforced Concrete 

GPS-5 Continuously Reinforced Concrete 

GPS-6 Asphalt Concrete Overlay of Asphalt Pavements 

GPS-7 Asphalt Overlay of Concrete Pavements 

GPS-9 Unbonded Concrete Overlay of Concrete Pavements 

Table 2. SPS experiments. 

SPS 

Experiment Description 

Number 

SPS-1 Strategic Study of Structural Factors for Flexible Pavements 

SPS-2 Strategic Study of Structural Factors for Rigid Pavements 

SPS-3 Preventive Maintenance Effectiveness for Flexible Pavements 

SPS-4 Preventive Maintenance Effectiveness for Rigid Pavements 

SPS-5 Rehabilitation of Asphalt Concrete Pavements 

SPS-6 Rehabilitation of Jointed Concrete Pavements 

SPS-7 Bonded Concrete Overlay of Concrete Pavements 

SPS-8 Study of Environmental Factors in the Absence of Heavy Loads 

SPS-9 Validation of SHRP Asphalt Specifications and Mix Design 

DATA COLLECTION AT GPS AND SPS SECTIONS 

The GPS and SPS test sections are monitored at regular intervals to collect deflection, profile, 

and distress data. For purposes of data collection, the United States and the Canadian Provinces 

have been subdivided into four regions: (1) North Atlantic, (2) North Central, (3) Southern, and 

(4) Western. Each region is served by a Regional Support Contractor (RSC) who performs data 

collection at the test sections located within its region. The regional boundaries defining the 

jurisdiction of each RSC are shown in figure 1. 

One of the major data collection efforts in the LTPP program is the collection of longitudinal 

profile data at LTPP test sections. Longitudinal profile data are collected using an inertial 

profiler (except for test sections located in Alaska, Hawaii, and Puerto Rico, where data are 

collected using Dipstick®, a hand-operated device manufactured by the Face Company®). 

2 
 

Figure 1. LTPP regions. 

3 
Profile data at LTPP test sections are collected along the two wheelpaths. The collected profile 

data are processed to compute roughness indices such as the International Roughness Index 

(IRI), Root Mean Square Vertical Acceleration (RMSVA), Slope Variance, and the Mays Index. 

The computed roughness parameters and the profile data are stored in the LTPP database after 

undergoing quality control checks. The data in the LTPP database are available to the research 

community. 

DEVICES FOR PROFILE DATA COLLECTION 

Each RSC operates an inertial profiler to collect longitudinal profile data. From the inception of 

the LTPP program until the end of 1996, profile data at test sections were collected using a 

model DNC 690 incandescent profiler manufactured by K.J. Law Engineers. In late 1996, each 

RSC replaced their model DNC 690 profiler with a model T-6600 infrared profiler manufactured 

by K.J. Law Engineers. In September 2002, each RSC replaced their T-6600 profiler with an 

International Cybernetics Corporation (ICC) MDR 4086L3 laser profiler. At test sections located 

in Alaska, Hawaii, and Puerto Rico, longitudinal profile data collection is performed using 

Dipstick. 

RESEARCH OBJECTIVES 

As described previously, data collection at LTPP test sections has been performed using three 

types of inertial profilers. Differences in the sampling interval, filtering procedures, and sensing 

devices for these profilers could lead to differences in profiles and smoothness index values 

among the devices. To ensure that high-quality and repeatable data are collected with each 

device, it is important to confirm the compatibility of the indices obtained using these devices. 

An analysis of LTPP profile data and equipment is necessary for quantifying the differences in 

IRI values among these profiling devices. The end result of this analysis will be an improvement 

in the quality of future LTPP data collection and an understanding of how to use the current 

LTPP profile data for analysis. Another useful result is quantification of the tolerances with 

which these profilers agree so that studies of roughness progression may be done with the 

knowledge of the differences among the devices. 

The main objective of this project is to quantify and resolve the differences in the longitudinal 

profile and roughness indices that are attributable to the different profiling equipment that have 

been used in the LTPP program. Under this research project, the following activities were carried 

out to meet the project objective: 

• Review of reports on LTPP profiler comparison studies that have been performed in the 

past and review of other literature on Dipstick testing and profiler comparisons. 

• Quantification of differences in IRI related to different profiling equipment and 

investigation of factors causing differences in IRI among the different inertial profiler 

types. 

4 
• Use of data collected for LTPP profiler comparison studies to investigate factors causing 

differences in IRI obtained from Dipstick and different types of profilers. 

• Identification of problems with equipment functionality, and current data collection and 

data processing procedures. Provision of recommendations for modifying current data 

collection and data processing procedures. 

• Development of a table listing the expected range of differences among the IRI values 

collected using LTPP’s profilers and Dipstick, and provision of recommendations for 

recalculation of IRI based on the findings. 

• Preparation of a final report that describes the analyses performed, findings from the 

analyses, and recommendations for improvements in LTPP data collection and 

processing procedures. 

All analyses were performed using the data that were collected during LTPP profiler comparison 

studies. 

ORGANIZATION OF THE REPORT 

Chapter 2 presents a description of profiling devices that have been used in the LTPP program to 

collect longitudinal profile data. Chapter 3 presents an overview of LTPP profiler comparison 

studies that have been performed since the inception of the LTPP program. Chapter 4 presents a 

description of analytical procedures that were used in this research project to analyze profile 

data. Chapter 5 presents a description of the data collection characteristics of LTPP’s profilers 

and a comparison of the devices. Chapter 6 presents the factors that can cause differences in IRI 

obtained from profilers and Dipstick. Chapter 7 presents several other findings from analysis of 

the profile data obtained from LTPP profiler comparison studies. Chapter 8 presents conclusions 

and recommendations for improvements to current procedures used in the collection and 

processing of profile data in the LTPP program. 

5 

CHAPTER 2: PROFILING DEVICES USED IN THE LTPP PROGRAM 

INERTIAL PROFILERS 

A brief description of each of the inertial profilers that have been used in the LTPP program is 

presented in the following sections. 

K.J. Law Engineers DNC 690 Profiler 

Three of LTPP’s DNC 690 profilers were identical, and the host vehicle used for these profilers 

was a Ford E 350 chassis that had a motor home body built onto it (see figure 2). The fourth 

DNC 690 profiler had the same profiling equipment as the other three profilers; however, the 

host vehicle was a passenger van (see figure 3). This profiler was used to collect profile data in 

the North Central region. 

Figure 2. K.J. Law Engineers DNC 690 profiler with a motor home body. 

Figure 3. K.J. Law Engineers DNC 690 profiler housed in a van. 

7 
All of these profilers were equipped with two incandescent sensors manufactured by K.J. Law 

Engineers that collected data along the two wheelpaths. The sensors were fixed to the vehicle 

body and were located between the axles of the vehicle. The spacing between the two sensors in 

the profilers was 1,676 millimeters (mm) (66 inches), except for the passenger van-based profiler 

that had a sensor spacing of 1,422 mm (56 inches). The incandescent sensors emitted a beam of 

light onto the pavement surface, and the reflected light signal was detected by a rotating mirror 

that was located inside the sensor. 

The data collected by these profilers could become contaminated if the receiver in the sensor 

picked up any sunlight. If the sensor detected sunlight, the result would be spikes in the profile 

data. A shroud was installed around the sensors in these profilers to prevent contamination of the 

profile data by sunlight. However, there were instances when sunlight did get under the shroud 

(particularly on rough roads or when the sun angle was low) and cause spikes to appear in the 

profile data. 

Another problem that occurred with the incandescent sensors was caused by the insufficient 

reflectivity of some of the pavement surfaces—the light signal was not being reflected back to 

the sensor. This condition usually happened on pavements having a dark-colored surface, such as 

a newly placed asphalt surface, or when there was a change in reflectivity of the pavement 

surface. This condition was referred to as “lost lock.” When this condition occurred, it appears 

that only the accelerometer signal was used to compute the profile, and this resulted in an 

incorrect profile being recorded. 

The height-sensor footprint of an incandescent sensor (which is the area covered by the beam of 

light emitted by the sensor) was 150 mm by 6 mm (5.9 inches by 0.24 inches), with the 150-mm 

(5.9-inch) side being perpendicular to the direction of travel. It is believed that the incandescent 

sensors had a measurement range of 125 mm (4.9 inches). 

This profiler recorded data at 152.4-mm (6-inch) intervals. However, the profiler collected data 

at 25.4-mm (1-inch) intervals and then applied a 304.8-mm (12-inch) moving average onto the 

data before recording the data. 

K.J. Law Engineers T-6600 Profiler 

In 1996, FHWA purchased four K.J. Law Engineers T-6600 profilers (see figure 4) to replace the 

K.J. Law Engineers DNC 690 profilers. The T-6600 profilers collected data for the LTPP 

program from late 1996 until July 2002. 

These profilers were equipped with three infrared height sensors manufactured by K.J. Law 

Engineers, which were mounted on a profiler bar located on the front of the vehicle. Two of the 

sensors collected data along the wheelpaths, while the third sensor collected data along the center 

of the lane. The spacing between the two outer sensors in the profiler was 1676 mm (66 inches). 

The infrared sensors had an elliptical footprint that was 38 mm by 6 mm (1.5 inches by 

0.24 inches), with the 38-mm (1.5-inch) side being perpendicular to the direction of travel. These 

sensors had a measurement range of 125 mm (4.9 inches). The data collected by the infrared 

8 
height sensors were not affected by ambient light. These profilers recorded profile data at 25-mm 

(l-inch) intervals. 

Figure 4. K.J. Law Engineers T-6600 profiler. 

International Cybernetics Corporation Profiler 

In July 2002, FHWA purchased four new ICC MDR 4086L3 profilers (see figure 5) to replace 

the K.J. Law Engineers T-6600 profilers. The ICC profilers began collecting profile data for the 

LTPP program in August 2002, and currently are used to collect profile data. 

Figure 5. ICC MDR 4086L3 profiler. 

These profilers were equipped with three Selcom® Systems laser sensors mounted on a profiler 

bar located on the front of the vehicle. Two sensors collect data along the wheelpaths, while the 

third sensor collects data along the center of the lane. The spacing between the two outer sensors 

is 1676 mm (66 inches). The footprint of a laser sensor is circular, and has a diameter of about 

1.5 mm (0.06 inches). The laser sensors have a measurement range of 200 mm (7.9 inches). The 

readings obtained by the laser sensors are not affected by ambient light. The ICC profilers do not 

record profile data, but rather they record in a file the signals measured by the height sensors and 

the accelerometers, and the distance data from the distance measuring instrument (DMI). After 

9 
data collection has been completed, a computer program is used to generate profile data at 

25-mm (l-inch) intervals. 

DIFFERENCES AMONG THE INERTIAL PROFILERS 

Several differences among the three types of inertial profilers that have been used to collect 

profile data for the LTPP program are: 

• Height-sensor type and footprint. 

• Sensor spacing. 

• Number of sensors. 

• Location of height sensors. 

• Measurement range of height sensors. 

• Data recording interval. 

• Data filtering methods. 

Height-Sensor Type and Footprint 

The DNC 690, T-6600, and ICC profilers were equipped with incandescent sensors, infrared 

sensors, and laser sensors, respectively. The height-sensor data collected by the DNC 690 

profiler could get contaminated by sunlight getting into the sensor through the shroud covering 

the sensors. The data collection capabilities of the infrared sensors on the T-6600 profiler and the 

laser sensors on the ICC profilers were not affected by ambient light. Another problem with the 

DNC 690 profilers was the occurrence of lost lock. This problem did not occur in either the 

T-6600 profilers or the ICC profilers. 

There were differences in the height-sensor footprint size among the three profilers. The 

DNC 690 profilers had a footprint size of 150 mm by 6 mm (5.9 inches by 0.24 inches); the 

150-mm (5.9-inch) side was perpendicular to the direction of travel. The T-6600 profilers had an 

elliptical footprint that was 38 mm by 6 mm (1.5 inches by 0.24 inches); the 38-mm (1.5-inch) 

side was perpendicular to the direction of travel. The ICC profilers were equipped with laser 

height sensors that had a circular footprint of about 1.5 mm (0.06 inches) in diameter. Figure 6 

shows the relative size of the sensor footprints for the three height sensors. 

Sensor Spacing 

The spacing between the two outer sensors for all three profilers was 1,676 mm (66 inches), 

except for the DNC 690 profiler operated by the North Central region. This profiler had a sensor 

spacing of 1,422 mm (56 inches). 

Number of Sensors 

The DNC 690 profilers were equipped with two height sensors for collecting profile data along 

the wheelpaths. The T-6600 profilers and the ICC profilers had three sensors for collecting 

profile data (two sensors collected data along the wheelpaths, and the third sensor was located at 

the midpoint between the two outer sensors).

10 
INCANDESCENT INFRARED LASER

1 inch = 25.4 mm 

Figure 6. Height-sensor footprints. 

Location of Height Sensors 

In the DNC 690 profilers, the height sensors were located midway between the two axles of the 

vehicle. The sensors in the T-6600 profilers and the ICC profilers were housed inside a sensor 

bar that was mounted on the front of the vehicle. 

Measurement Range of Height Sensors 

The ICC profilers were equipped with Selcom laser sensors that had a measurement range of 

200 mm (7.9 inches). The T-6600 profilers that were equipped with infrared sensors had a 

measurement range of 125 mm (4.9 inches). It is believed that the incandescent sensors that were 

used on the DNC 690 profilers had a similar measurement range. A National Cooperative 

Highway Research Program (NCHRP) study that analyzed data from roads having a roughness 

of up to 4.5 meters per kilometer (m/km) (285 inches per mile (inches/mi)) found that the range 

of vertical movement that was expected in a vehicle between the axles (where the sensors on the 

DNC 690 profiler were located) was well within the measurement range of the sensors on the 

DNC 690 profiler.(1) Therefore, it is unlikely that the height sensors on the DNC 690 profiler 

exceeded the measurement range while collecting data. On a road with a given roughness value, 

the range of movement that is experienced by the profiler bar that is located on the front of the 

vehicle is much more than the movement that occurs in the vehicle body between the axles. 

Therefore, on any given road, the height sensors of the T-6600 profiler that were mounted on the 

front profiler bar measured much more movement than that measured by the height sensors on 

the DNC 690 profiler. There is a possibility that the measurement range of the height sensors on 

the T-6600 profiler may have been exceeded at extremely rough locations. If this occurred, it is 

believed that the reading obtained at the cutoff limit of the height sensor was used to compute the 

profile at that location. The 200-mm (7.9-inch) height-sensor range for the ICC profilers is 

expected to be sufficient for collecting data on rough LTPP sections without the height sensors 

exceeding the measurement range. 

11 
Data Recording Interval 

The DNC 690 profilers collected profile data at 25.4-mm (1-inch) intervals, and then applied a 

304.8-mm (12-inch) moving average onto the data and recorded the data at 152.4-mm (6-inch) 

intervals. The T-6600 profilers recorded profile data at 25-mm (1-inch) intervals. The ICC 

profilers do not record profile data; however, they record data obtained from the height sensors, 

accelerometers, and DMI. It is possible to obtain profile data at 25-mm (1-inch) intervals from 

these data. 

Data Filtering Methods 

Details about the filters used in the computation of the profile data for all three profiler types are 

not available. The manufacturers of the profilers consider this information to be proprietary. It is 

possible that the filtering methods used with the DNC 690 and T-6600 profilers may be similar, 

since the same manufacturer built both of these profilers. Differences in the filtering techniques 

used in the K.J. Law Engineers profilers and the ICC profilers are expected. A 100-m (328-ft) 

upper-wavelength cutoff filter is applied to the data obtained from the T-6600 and ICC profilers. 

The data collected by the DNC 690 profiler were subjected to a 91-m (300-ft) upper-wavelength 

cutoff filter. 

DIPSTICK 

In the LTPP program, longitudinal profile data collection at the test sections located in Alaska, 

Hawaii, and Puerto Rico is performed using Dipstick, a hand-operated device manufactured by 

the Face Company. Dipstick has a digital inclinometer that measures the elevation difference 

between the two footpads (see figure 7).(2) The diameter of the footpads of the Dipsticks used in 

the LTPP program is approximately 32 mm (1.25 inches). The spacing between the centers of the 

two footpads is 304.8 mm (12 inches). Dipstick is walked along a test section, and at each 

position it displays the elevation difference between the two footpads, which is recorded in a data 

collection form. The individual readings are then added to get the elevation profile. Dipstick is 

used during LTPP profiler comparisons to obtain reference elevations along the two wheelpaths 

at the test sections. 

In 1989, the Center for Transportation Research at the University of Texas at Austin investigated 

the ability of Dipstick to measure road profiles.(3) This investigation showed that when properly 

calibrated and operated, Dipstick could give profiles as good as those from rod-and-level 

surveys, but at a fraction of the time and cost. 

MANUALS FOR PROFILER OPERATIONS 

Manuals have been developed that document the operational procedures to be followed when 

measuring pavement profiles for the LTPP program using an inertial profiler or Dipstick. These 

manuals cover field testing procedures, data collection procedures, calibration of equipment, 

record keeping, and maintenance of equipment. The operational procedures for the DNC 690 

profiler are documented in a SHRP report (report no. SHRP-P-378).(4) The operational 

procedures for the T-6600 profiler are contained in a legacy document written by the LTPP 

12 
technical support contractor.(5) Operational procedures for the ICC profiler are described in the 

LTPP Manual for Profile Measurements and Processing, Version 4.1.

(6) 

Figure 7. Schematic view of Dipstick.(2)

OMPUTATION OF ROUGHNESS INDICES 

he longitudinal profile data collected by the inertial profilers and Dipsticks are used to compute 

he DNC 690 profilers recorded profile data at 152.4-mm (6-inch) intervals in a binary format. 

rofile data obtained at 25-mm (1-inch) intervals are available for both the T-6600 and ICC 

nto 

the 

1 inch = 25.4 mm 

C

T

roughness indices such as IRI, RMSVA, and Slope Variance. In the LTPP program, roughness 

indices from profile data are computed using FHWA’s ProQual software.(7,8) This program uses

the IRI computation algorithm that is presented in a World Bank document.(9)

T

ProQual converted this data to an ASCII format, and then used the data to compute roughness 

indices. The computed roughness indices and the profile data are stored in the LTPP database. 

P

profilers. ProQual imports this data and then applies a 300-mm (11.8-inch) moving average o

the 25-mm (1-inch) data, and extracts data points at 150-mm (5.9-inch) intervals. Roughness 

indices are computed using the data that are at 150-mm (5.9-inch) intervals. The computed 

roughness indices and the averaged profile data that are at 150-mm (5.9-inch) intervals are 

uploaded to the LTPP database. The raw profile data files that contain the data recorded by 

profilers are stored at the regional offices. 

13 
When computing IRI values from the Dipstick data, the following procedure is used in ProQual:

1. Sum the individual Dipstick readings to obtain the elevation profile of a wheelpath. 

2. Rotate the profile for a wheelpath by 180 degrees so that an additional length of 152.4 m 

(500 ft) is available before the start of the section. 

3. Apply the long-wavelength cutoff filter that is used in the profiler to filter the data so that 

wavelengths greater than 100 m (328 ft) are removed. 

4. Extract the filtered profile from the start of the test section to the end of the test section. 

5. Compute roughness indices using this filtered profile. 

14 
CHAPTER 3: PROFILER COMPARISON STUDIES 

INTRODUCTION 

Studies have been conducted at regular intervals to compare the output from the four LTPP 

profilers. For each study, several test sections were laid out, and reference profile measurements 

along each wheelpath were obtained using Dipstick. Thereafter, profile measurements were 

performed on the test sections by the profilers. The primary method for checking if the profilers 

were functioning accurately was to compare the IRI values computed from Dipstick data with the 

values computed from the data obtained from the profilers. The repeatability of the profilers was 

evaluated by analyzing the standard deviations of the IRI, which were computed using the IRI 

values obtained from repeat measurements on a test section. In the profiler comparison studies 

that have been performed since 1998, in addition to comparing the IRI values, profiles obtained 

by the profilers were compared to evaluate profiler repeatability and reproducibility. The details 

of these comparison tests are presented later in this chapter. 

Whenever FHWA purchased new profilers, the profilers underwent rigorous testing to ensure 

that they met the requirements that were specified in the contract documents. After each regional 

contractor took delivery of a new profiler, a comparison of the new profiler and the old profiler 

was performed in each region before collecting data with the new profiler. The purpose of these 

verification tests was to compare the output from the old and the new profilers. Details about 

these verification tests are presented later in this chapter. 

 

In this research project, a literature review was also performed to gather information on other 

profiler comparison studies that have been performed in the past. The results of this literature 

review are presented later in this chapter. 

LTPP PROFILER COMPARISON STUDIES 

Overview 

The following six LTPP profiler comparison studies have been held since the start of the LTPP 

program: 

• 1990: Austin, TX. 

• 1991: Ann Arbor, MI. 

• 1992: Ames, IA. 

• 1998: Urbana, IL. 

• 2000: College Station, TX. 

• 2003: Minnesota Road Research Project (Mn/ROAD) of the Minnesota Department of 

Transportation (Mn/DOT). 

This section presents a brief description of the following activities related to an LTPP profiler 

comparison: (1) purpose of comparison test, (2) selection of test sections, (3) collection of 

15 
reference elevation measurements, (4) profiler data collection, (5) computation of IRI values, and 

(6) analysis of data from LTPP comparison studies. 

Purpose of Comparison Test 

The purpose of performing a comparison test of the four LTPP profilers is to ensure that the 

profilers are collecting accurate, repeatable, and reproducible data. Currently, the following 

analyses are performed on the data collected during a comparison test:(6)

• Evaluation of the static accuracy of the profiler height sensors: Performed before data 

collection, this test checks the static accuracy of the height sensors using a set of blocks 

to determine whether the readings are within a specified tolerance. 

• Evaluation of the results from the bounce test: The bounce test determines whether the 

height-sensor readings and accelerometer readings are canceling each other. The results 

of this test for each of the four profilers are compared to determine whether all four 

profilers are providing similar results. 

• Evaluation of the accuracy of the DMI: Performed before data collection, this test 

determines whether the DMI meets specified bias and precision criteria. A 300-m- 

(984-ft-) long section is laid out to perform this test. 

• Evaluation of the repeatability of the IRI values obtained by the profilers and a 

comparison of the IRI values obtained by profilers with those obtained using Dipstick: 

The IRI values obtained from the repeat runs of a profiler on a test section are used to 

evaluate the precision (repeatability) of a profiler. The IRI values are also used to 

evaluate the bias of a profiler with respect to Dipstick. For comparison studies that have 

been performed since 1998, the following precision and bias criteria have been evaluated: 

(1) determination of whether the precision of the IRI along a wheelpath (obtained by 

computing the standard deviations of the IRI for the repeat profiler runs) is less than 

0.04 m/km (2.5 inches/mi), and (2) determination of whether the difference in IRI for a 

wheelpath between the average profiler IRI (which is calculated by averaging IRI 

obtained from the five profiler runs) and the Dipstick IRI is within ±0.16 m/km 

(±10 inches/mi). 

• Evaluation of the repeatability of the profile data: The point-to-point repeatability for 

each profiler along each wheelpath is computed to evaluate the repeatability of the profile 

data. 

• Comparison of the profile data obtained by the four profilers: One representative run for 

each profiler is selected for a test section and overlaid profile plots for each wheelpath at 

each test section are prepared to compare the data collected by the four profilers. 

For comparison tests performed before 1998, an evaluation of the profile data was not 

performed; the analysis of the data was confined to IRI values. 

16 
Selection of Test Sections 

The current LTPP procedures for performing a profiler comparison indicate that five test 

sections, which satisfy the following criteria, should be selected:(6)

• Section 1: Smooth Asphalt: Asphalt concrete (AC) pavement with an average IRI for the 

two wheelpaths of less than 1.6 m/km (101 inches/mi). 

• Section 2: Rough Asphalt: AC pavement with an average IRI for the two wheelpaths of 

greater than 2.2 m/km (139 inches/mi). 

• Section 3: Smooth Concrete: Jointed portland cement concrete (PCC) pavement with an 

average IRI for the two wheelpaths of less than 1.6 m/km (101 inches/mi). 

• Section 4: Rough Concrete: Jointed PCC pavement with an average IRI for the two 

wheelpaths of greater than 2.2 m/km (139 inches/mi). 

• Section 5: Chip-sealed section. 

The comparison study performed in 1990 used six test sections, the studies in 1991 and 1992 

used eight test sections, the study in 1998 used four test sections, and the studies in 2000 and 

2003 used five test sections. 

The following guidelines are specified for selecting test sections:(6)

• Each test section should be 152.4 m (500 ft) in length, with the beginning and end 

marked. 

• The test sections should be located on flat, tangential sections that have sufficient length 

at each end to allow for acceleration to a constant speed before the section and safe 

deceleration past its end. 

• The speed limit of the roadway containing the test sections should be at least 

80 kilometers per hour (km/h) (50 miles per hour (mi/h)). 

• The test sections should have a marked outside lane-edge stripe that can be used as a 

lateral reference when profiling the test section. 

• The AC sections should not be concrete sections that have been overlaid with asphalt. 

• Where possible, test sections should be located within a centralized locale, with short 

travel distances between each test section to reduce travel time. 

17 
Collection of Reference Elevation Measurements 

Dipstick has been used to collect reference elevation data for all profiler comparison studies, 

except for the study in 2000. In the 2000 study, the reference profile measurements were 

obtained using an Australian Road Research Board (ARRB) walking profiler. In the 1992 

comparison, rod-and-level measurements were obtained in addition to Dipstick measurements. 

Dipstick measurements are performed along both wheelpaths at all test sections. The current 

procedures for performing Dipstick measurements are outlined in the LTPP Manual for Profile 

Measurements and Processing.

(6) As described in this document, Dipstick data collection from a 

test section is performed according to the following sequence: 

1. Start data collection from the beginning of the section along the right wheelpath. 

2. When the end of the section is reached, go across the lane toward the left wheelpath. 

3. Perform measurements along the left wheelpath toward the beginning of the section. 

4. After reaching the start of the section, go across the lane and terminate data collection at 

the right wheelpath. 

It is not known whether this procedure for Dipstick data collection was followed when data were 

collected for the 1990 study. However, this procedure for Dipstick data collection was used for 

all other profiler comparison studies, except for the study performed in 1991 in Michigan. In the 

Michigan study, Dipstick measurements were first made along a wheelpath from the beginning 

of the section to the end of the section and, thereafter, measurements were made along the same 

path from the end of the section to the beginning of the section. This resulted in two profiles 

being available for each wheelpath. (Note: This was the Dipstick measurement procedure used in 

the LTPP program at that time.) 

Dipstick measurements on PCC test sections were performed in the afternoon, at the same 

approximate time of day as expected for the collection of profiler data for all comparison studies. 

This procedure was followed to avoid the slab curling effects that may be present in the PCC 

pavements in the morning. 

Profiler Data Collection 

Current test procedures indicate that data collection at the test sections should be performed at a 

test speed of 80 km/h (50 mi/h).(6) Data collection at PCC sections was performed in the 

afternoon, at the same approximate time of day as when Dipstick data were collected at the 

sections. At each test section, each profiler collects a set of measurements following the normal 

operating procedures that are used during data collection at LTPP sections. 

During the 1990 profiler comparison, profile testing was performed at test speeds of 56 and 

80 km/h (35 and 50 mi/h). In the profiler comparison studies that were performed in 1991 and 

1992, profile testing was performed at test speeds of 64 and 80 km/h (40 and 50 mi/h). For all 

other comparison studies, profile testing was performed only at 80 km/h (50 mi/h). 

18 
The left wheelpath for all test sections was marked using paint dots for the profiler comparisons 

that were conducted in 1990 and 1991. In these studies, the profiler driver was asked to align the 

vehicle along the left wheelpath when profiling the test sections. In the 2000 comparison, the 

wheelpaths of two sections were marked with paint dots. In the 2003 comparison, both 

wheelpaths on all test sections were marked with paint dots. The location of the wheelpaths was 

not marked for the comparisons that were held in 1992 and 1998. In these studies, the profiler 

driver judged the location of the wheelpaths and aligned the profiler along the wheelpaths when 

profiling the test sections. 

Computation of IRI Values 

The computation of IRI values from the profiler data was performed by each region using the 

ProQual software.(7,8) The number of runs that were used in the analysis for the different 

comparison studies was either five or six. Each RSC selected the profile runs that were to be 

used in the analysis from all available repeat runs. Each RSC prepared a table that included the 

left- and right-wheelpath IRI values for all selected runs and submitted the table and a brief 

report to FHWA and its technical support contractor. 

Analysis of Data from LTPP Comparison Studies 

The analysis of data obtained from the LTPP profiler comparison studies has been performed by 

the LTPP technical support contractor. Reports documenting the analyses and findings for all 

profiler comparison studies are available. (See references 10, 11, 12, 13, 14, and 15.) Summaries 

of the findings from each profiler comparison study are presented in appendix A. 

LTPP PROFILER VERIFICATION STUDIES 

Profiler equipment changes occurred in the LTPP program in 1996 and 2002. In 1996, each RSC 

replaced their K.J. Law Engineers DNC 690 profiler with a K.J. Law Engineers T-6600 profiler. 

In 2002, each RSC replaced their T-6600 profiler with an ICC MDR 4086L3 profiler. On each of 

these occasions, after accepting delivery of the new profiler, each RSC performed a comparison 

of the old and new profilers before collecting data with the new profiler. Details and findings 

from these two verification studies are presented in appendix B. 

OTHER PROFILER COMPARISON/ANALYTICAL STUDIES 

A literature review was performed to gather information on other studies that have been 

performed to compare IRI from profilers and reference devices. The purpose of obtaining 

information about other studies was to determine if they contained any explanations regarding 

the differences in IRI between inertial profilers and reference devices that could be useful for this 

research project. In addition, reports on other studies that have analyzed LTPP profile data were 

reviewed. 

PIARC Comparison 

19 
In 1998, World Road Association (PIARC) Committee C1 on Surface Characteristics conducted 

a global experiment to investigate the performance of various high-speed profiling equipment.(16)

Test sections were established in the United States, Japan, and Europe (The Netherlands and 

Germany). Reference profile measurements at the test sections were conducted using Dipstick 

and the ARRB walking profiler. The number of high-speed profilers that performed 

measurements in the United States, Japan, and Europe were 4, 7, and 25, respectively. The 

profilers that took measurements at the test sections in the United States generally showed good 

agreement in IRI. However, there was wide variability in the IRI values that were obtained by 

the different profilers at the test sections located in Japan and Europe. In addition, there were 

some large discrepancies in the IRI values obtained from the reference devices and the highspeed profilers at many sections. The causes of the differences in IRI were not investigated in 

this project. 

Road Profiler User Group Comparisons: 1993 and 1994 

Four regional test centers were used for these studies performed in 1993 and 1994—

Pennsylvania, South Dakota, Nevada, and Mississippi.(17,18) Eight test sections were used at each 

regional center, except for Nevada, where six test sections were used. Dipstick measurements 

were obtained at all test sections. LTPP’s DNC 690 profilers were used in these two 

comparisons. An evaluation of the results indicated that at each of the regional test centers, 

LTPP’s profilers were among the profilers that best matched the Dipstick IRI. 

LTPP Profile Variability Analysis 

This study visually reviewed all LTPP profile data that were collected between June 1989 and 

October 1997 for saturation spikes, lost lock, shifted start, wrong location, out of study, and other 

equipment- and operator-related problems.(19) The data for the review were obtained from the 

LTPP database. The profile data for the period under review were collected by the DNC 690 

profilers, except for a few data sets that were collected by the T-6600 profilers. The profiles that 

exhibited problems were divided into two groups—reparable and irreparable profiles. Reparable 

profiles included profiles with saturation spikes that were not marked. These data were 

reprocessed, and the IRI values in the LTPP database were updated. Data that exhibited problems 

that could not be repaired were deleted from the LTPP database. 

20 
CHAPTER 4: ANALYTICAL PROCEDURES 

INTRODUCTION 

This section describes the analytical techniques and software that were used in this research 

project, and the overall approach that was used to analyze data collected for the various LTPP 

profiler comparison and verification studies. The following analytical techniques/software were 

used for data analysis: 

• Roughness profiles. 

• Power spectral density (PSD) plots. 

• Data filtering. 

• Cross correlation. 

• Road Profile Analysis Software (RoadRufTM).1

ANALYTICAL TECHNIQUES AND SOFTWARE 

Roughness Profiles 

The roughness of a section of roadway can be expressed by the IRI, which indicates the average 

roughness for that road section. However, the roughness within this section of roadway can vary. 

For example, consider a 100-m- (328-ft-) long section of roadway that has a roughness of 

1.23 m/km (6.5 ft/mi). This road section can be divided into 10 equidistant segments, where the 

length of each segment is 10 m (33 ft). Figure 8 shows the roughness of each of these 10-m (33-

ft) segments. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 2 3 4 5 6 7 8 9 10

Segment Number

IRI (m/km)

 

1 m/km = 5.28 ft/mi 

Figure 8. Roughness of a roadway expressed in 10-m (33-ft) segments. 

 

1 Initially funded by FHWA, this software was developed by the University of Michigan’s Transportation 

Research Institute. 

21 
As shown in this figure, the roughness values for the 10-m (33-ft) segments are variable, with 

segment 1, which has an IRI of 1.42 m/km (7.5 ft/mi), having the highest roughness, and 

segment 9, which has an IRI of 1.01 m/km (5.3 ft/mi), having the lowest roughness. 

Instead of using a single value to characterize the roughness of a roadway, a roughness profile 

can be used to show how roughness varies with distance along the roadway. Figure 9 shows a 

roughness profile based on a 10-m (33-ft) base length for the same section of roadway whose 

roughness distribution was shown in figure 8. In figure 9, the roughness value for a specific 

location is the average roughness over a 10-m (33-ft) length (i.e., the base length of the 

roughness profile) that is centered at that location. For example, the roughness shown at a 

distance of 25 m (82 ft) is the average roughness from 20 to 30 m (66 to 98 ft). The highest 

roughness value in the roughness profile occurs at a distance of 50 m (164 ft), and, therefore, the 

10-m (33-ft) stretch of the road that has the highest roughness is between 45 and 55 m (148 and 

181 ft). A roughness profile can be constructed for any base length. A detailed description of 

roughness profiles is presented by Sayers.(20)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 20 40 60 80 100

Distance (m)

IRI (m/km)

1 m = 3.28 ft. 

1 m/km = 5.28 ft/mi 

Figure 9. Example of a roughness profile. 

When evaluating the IRI repeatability of a profiler, or when comparing IRI values obtained by 

different profilers, an evaluation of roughness profiles provides much more information than just 

evaluating the IRI value that is obtained for the entire road section. For example, consider the 

following case where two repeat runs were conducted by a profiler (not LTPP’s profiler) on a 

test section that was 180 m (590 ft) long. The left-wheelpath IRI values for the two runs were 

1.66 and 1.63 m/km (105 and 103 inches/mi). The IRI values obtained for the two repeat runs 

were very similar, which seems to indicate that the profiler is capable of collecting repeatable 

data. Figure 10 shows the IRI values for the two runs reported at 10-m (33-ft) intervals. 

 

This figure shows that at some segments, there was a considerable difference between the IRI 

values obtained for the two runs. However, averaging IRI over the 152.4-m- (500-ft-) long 

section caused these variations to smooth out and gave an overall IRI value for the two repeat 

runs that was very similar to the individual value for each run. 

22 
Figure 11 shows the roughness profiles for a 10-m (33-ft) base length for the two runs whose IRI 

values are shown in figure 10. As described previously, for the 180-m- (590-ft-) long section, the 

two runs have IRI values of 1.66 and 1.63 m/km (105 and 103 inches/mi). Although the IRI over 

the entire section is very similar for the two runs, figure 11 shows there are differences in the 

spatial distribution of the roughness for the two runs. The roughness profiles present much more 

information than what is presented in the bar charts shown in figure 10 because the roughness 

profiles show how the roughness captured by the two runs varies throughout the roadway. 

0.0

0.5

1.0

1.5

2.0

2.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Segment Number

IRI (m/km)

Run 1

Run 2

Figure 10. IRI obtained from two repeat runs. 

1 m = 3.28 ft 

1 m/km = 5.28 ft/mi 

1 m/km = 5.28 ft/mi 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 15 30 45 60 75 90 105 120 135 150 165 180

Distance (m)

IRI (m/km)

Run 1

Run 2

Run 1

Run 2

Figure 11. Roughness profiles at 10-m (33-ft) base length for two runs. 

As illustrated in this example, it is important to recognize that, just because the IRI from repeat 

runs agree well, or the IRI from two devices agree well at a pavement section, it does not 

23 
necessarily mean that the two devices are collecting similar profile data. Roughness profiles can 

be used to determine whether repeat runs from a profiler or profile runs from different devices 

are giving a similar spatial distribution of IRI along the section. 

Power Spectral Density Plots 

A road profile encompasses a spectrum of sinusoidal wavelengths. A PSD function is a statistical 

representation of the importance of the various wavelengths contained in the profile.(2) The PSD 

function of the profile slope best shows the differences in the roughness properties because the 

basic spectrum of roughness over the wave numbers is more uniform.(2) In this research project, 

PSD plots that use the profile slope were used in the analyses. Figure 12 shows an example of a 

PSD plot of a road profile. This plot presents a view of the distribution of the wavelengths that 

are contained within the road profile. The x-axis of the PSD plot represents the wave number. 

The wave number is the inverse of the wavelength. If prominent wavelengths are present in a 

profile, such wavelengths will show up as dominant spikes in the PSD plot. 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 12. Example of a PSD plot. 

Data Filtering 

The profiles obtained by the T-6600 and ICC profilers have been filtered with a 100-m (328-ft) 

upper-wavelength cutoff filter, while the data obtained by the DNC 690 profilers have been 

filtered with a 91-m (300-ft) upper-wavelength cutoff filter. The profile data can be further 

filtered during data analysis to look at details within the profile. The types of filters that are 

commonly used in analyses are high-pass filters, low-pass filters, and band-pass filters. A highpass filter removes wavelengths greater than a specified value. A low-pass filter removes 

wavelengths less than a specified value. A band-pass filter keeps the wavelengths within a 

specified waveband and removes all other wavelengths. 

The following example illustrates the application of filtering techniques to profile data. Figure 13 

shows a plot of a typical profile that was obtained from LTPP’s T-6600 profiler. Figures 14, 15, 

24 
and 16, respectively, show this profile after it has been subjected to a 5-m (16-ft) high-pass filter, 

a 10-m (33-ft) low-pass filter, and a band-pass filter that has a lower wavelength of 5 m (16 ft) 

and an upper wavelength of 10 m (33 ft). 

The profile plot shown in figure 14 has all wavelengths that are greater than 5 m (16 ft) removed. 

The profile plot shown in figure 15 has all wavelengths less than 10 m (33 ft) removed. The plot 

shown in figure 16 contains only the wavelengths between 5 and 10 m (16 and 33 ft). 

-15

-10

-5

0

5

10

0 20 40 60 80 100 120 140 160

Distance (m)

Elevation (mm)

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 13. Profile recorded by a profiler. 

-10

-8

-6

-4

-2

0

2

4

0 20 40 60 80 100 120 140 160

Distance (m)

Elevation (mm)

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 14. Profile after being subjected to a 5-m (16-ft) high-pass filter. 

25 
-2

-1

0

1

2

3

4

5

0 20 40 60 80 100 120 140 160

Distance (m)

Elevation (mm)

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 15. Profile after being subjected to a 10-m (33-ft) low-pass filter. 

-4

-3

-2

-1

0

1

2

3

0 20 40 60 80 100 120 140 160

Distance (m)

Elevation (mm)

25.4 mm = 1 inch 

Figure 16. Profile after being subjected to a band-pass filter. 

1 m = 3.28 ft 

Cross Correlation 

The cross-correlation method for analyzing road profiles, which is an objective procedure for 

rating the agreement between profile measurements, was developed by Karamihas.(21) This 

procedure is based on the cross-correlation function described by Bendant and Piersol.(22) The 

description of the cross-correlation procedure presented in this section was obtained from 

University of Michigan Transportation Research Institute (UMTRI) report 2002-36.(21)

The cross-correlation method can be used to rate agreement between profiles in a given 

waveband. It can also be applied to rate agreement between the devices for any given roughness 

index, including IRI. This procedure provides a single, unitless rating agreement (ranging from 

0 to 1) that describes how well two profiles correlate with each other. 

26 
Consider the example presented previously during the discussion on roughness profiles, where 

two repeat runs from a profiler provided very good agreement in overall IRI; however, there 

were significant differences in the distribution of roughness within the section. The crosscorrelation method can be applied to this situation to obtain a value between 0 and 1 that will 

indicate how well the profiles agreed with each other in their ability to measure IRI over the 

section. This method compares the magnitude as well as the spatial distribution of the roughness 

within the section when computing the value of the correlation. When the cross-correlation 

method is used to compare the IRI for two profiles, both profiles are first filtered with the IRI 

filter that is contained in the IRI algorithm. Afterwards, the cross-correlation method is used on 

these filtered profiles to obtain a cross-correlation rating. To have a high rating between the two 

filtered profiles, the same level of roughness should be present in the two filtered profiles and, in 

addition, the rough features in both profiles must appear at the same location and have the same 

shape. 

Cross correlation is superior to direct comparison of IRI values because it compares the overall 

roughness, as well as the spatial distribution of the roughness. Figure 17 shows an example of 

three repeat measurements made by a profiler after they have passed through the filters in the IRI 

algorithm. The filtered signals compare well with each other and have an average cross 

correlation that is higher than 0.995. The average cross correlation was computed by comparing 

two profiles at a time for all possible combinations and then computing the average of the crosscorrelation values. The traces shown in figure 17 overlay so well that they are barely 

distinguishable from each other. 

Figure 17. Three IRI filtered profiles with an average correlation greater than 0.995.(21) 

Figure 18 provides an example of a moderate correlation. It shows the repeat measurements from 

1 m/km = 5.28 ft/mi 

1 m = 3.28 ft 

60 65 70 75 80 85 90

Distance (m) 

-10 

-5 

0 

5 

10 

Filtered Profile (m/km)

the same device on a different pavement section after they have passed through the filters of the 

IRI algorithm. The profiles compare fairly well, with an average correlation of 0.84. The traces 

do not overlay nearly as well as the traces shown in figure 17, and significant differences in the 

IRI filtered profiles are noted at some locations. 

27 
60 65 70 75 80 85 90 

Distance (m) 

-5 

0 

5 

10 

 IRI filtered profiles with an average correlation of 0.84.(21)

The cross- avebands. 

or example, the agreement between two profiles can be evaluated for short wavelengths, 

re represents repeatability when it is applied to 

o measurements of the same profile by the same device, reproducibility when it is applied to 

lation method is to ensure that the two profiles 

re properly synchronized so that the start location for both profilers is the same. Any error in the 

y or reproducibility is that the 

rofiles were obtained along the same path. Lateral variability during profiling can cause 

erence 

ools for interpreting longitudinal road profiles.(23)

oadRuf was developed at UMTRI and was funded by FHWA. It is free software and can be 

s. 

nd 

Filtered Profile (m/km)

1 m = 3.28 ft 

1 m/km = 5.28 ft/mi 

Figure 18. Three

correlation method can also be used to compare two profiles over different w

F

medium wavelengths, and long wavelengths. 

The rating agreement provided by this procedu

tw

two measurements of the same profile by different devices, and accuracy when a measurement

from one of the devices is deemed to be correct. 

An important step before applying the cross-corre

a

DMI of a profiler could have an impact on the results obtained from this method when it is 

applied to evaluate the reproducibility or accuracy of a profiler. 

Another important assumption made when evaluating repeatabilit

p

differences in the profile features that are measured, and these will be interpreted as an 

equipment factor during cross-correlation analysis. When this method is used to compare a

profiler with a reference device, it is assumed that the measurements obtained by the ref

device are error free. 

RoadRuf Software 

RoadRuf is an integrated set of computer t

R

downloaded from the Internet. RoadRuf contains a variety of tools for analyzing road profile

The tools available in RoadRuf that were used in this research project are: (1) computing IRI a

Ride Number (RN) values, (2) plotting profile data, (3) evaluating roughness profiles, and (4) 

plotting PSD. 

28 
ANALYTICAL APPROACH 

cal approach that was used in this research project to analyze 

ata obtained from LTPP profiler comparison and verification studies. When analyzing profile 

n a test 

ction. Initially, the following analyses were performed for each profiler, at each test section, 

 a visual evaluation of the profiles. 

Distinct features observed in the profile were noted. 

2. lter, a 10-m (33-ft) high-pass filter, 

and a band-pass filter having a wavelength cutoff between 1 and 30 m (3 and 100 ft), and 

3. roughness profiles computed for a 10-m (33-ft) base 

length by overlaying the roughness profiles obtained for the repeat runs. If differences in 

s 

4. ces in the PSD plots are noted, evaluate 

the profile data to identify the profile features that cause the PSD plots to be different. 

After p

ach test section. Thereafter, for each LTPP comparison, the following analyses were performed 

 four of LTPP’s profilers and perform a visual 

evaluation of the profile data. Note any distinct features that are different among the 

2. profile data using a 1-m (0.3-ft) high-pass filter, a 10-m (33-ft) high-pass filter, 

and a band-pass filter having a wavelength cutoff between 1 and 30 m (3 and 100 ft), and 

3. gth roughness profiles for the four profilers and 

Dipstick, and evaluate the repeatability of the roughness profiles. If major differences 

 that 

This section describes the analyti

d

data obtained from the T-6600 and ICC profilers, the 25-mm (1-inch) data were used. For the 

DNC 690 profiler, only the data recorded at 152.4-mm (6-inch) intervals are available. 

During LTPP profiler comparison studies, each profiler performs replicate profile runs o

se

separately for the left- and right-wheelpath profile data: 

1. Overlay the repeat runs of the profiler and perform

Filter the profile data using a 1-m (0.3-ft) high-pass fi

evaluate the distinct features in the profile. Also look at the differences in the profile 

features among the repeat runs. 

Evaluate the repeatability of the 

the roughness profiles are noted, evaluate the profile data to identify the profile feature

that cause the roughness profiles to be different. 

Compare PSD plots of the repeat runs. If differen

erforming this evaluation, a representative profile run was selected for each profiler at 

e

separately for the left and right wheelpaths: 

1. Overlay the profile runs obtained from

profiles. 

Filter the 

compare the profile features recorded by the four profilers. Note any distinct features that 

are different among the profiles. 

Overlay the 10-m (33-ft) base-len

among the roughness profiles are noted, look at the profile data to identify features

cause the roughness profiles to be different. 

29 
4. Overlay the PSD plots for the four profilers and determine whether there are differences 

. Perform a cross-correlation analysis between the profiler and Dipstick data. 

hanges in LTPP’s profiling equipment occurred in 1996 and 2002. On both of these occasions, 

. Overlay the profile data from the two profilers and perform a visual evaluation of the 

e 

. Filter the profile data using a 1-m (3-ft) high-pass filter, a 10-m (33-ft) high-pass filter, 

d 

. Overlay the 10-m (33-ft) base-length roughness profiles for the two profilers to evaluate 

. Compare the PSD plots for the two profilers to determine whether the wavelength 

. Perform a cross-correlation analysis on a subset of the profiles to evaluate the agreement 

in the PSD plots. If major differences are noted, analyze the profile data for the profiler 

that is different to identify the features in the profile that cause the difference. 

5

C

before using the new profiler, each region performed a verification study between their old 

profiler and the new profiler. Data obtained from these two studies were used to evaluate the 

profile data collected by the two profilers. When comparing the data from the DNC 690 and 

T-6600 profilers, the ProQual-processed data, which is averaged profile data, were used. When 

comparing the profile data between the T-6600 and ICC profilers, profile data at 25-mm (1-inch)

intervals were used. The following analyses were conducted at each test section and were 

performed separately for the left and right wheelpaths: 

1

profiles. Determine whether there are differences in the features contained in the profil

data. 

2

and a band-pass filter having a wavelength cutoff between 1 and 30 m (3 and 100 ft), an

evaluate the distinct features in the profile. Also look at the differences in the profile 

features among the repeat runs. 

3

the agreement of the roughness distribution within a section. If major differences among 

the roughness profiles are noted, look at the profile data to identify the features that cause

the roughness profiles to be different. 

4

distribution for the two profilers is similar. 

5

among the profiles collected by the two profilers. 

30 
CHAPTER 5: DATA COLLECTION CHARACTERISTICS AND 

COMPARISON OF DATA COLLECTED BY LTPP’S PROFILERS 

CHARACTERISTICS OF DATA COLLECTED BY LTPP’S PROFILERS 

K.J. Law Engineers DNC 690 Profiler 

The DNC 690 profiler collected profile data at 25.4-mm (1-inch) intervals, and then applied a 

304.8-mm (12-inch) moving average onto the data and recorded the data at 152.4-mm (6-inch) 

intervals. The data collected by this profiler at LTPP sections and the IRI values computed from 

the profile data are available in the LTPP database. Figure 19 shows a PSD plot of the data 

collected by this profiler. The PSD plot shows a sharp drop after a wave number of 1 cycle/m 

(0.3 cycle/ft), which corresponds to a wavelength of 1 m (3 ft). This sharp drop in the PSD plot is 

an indication that a moving average has been applied to the profile data. The application of the 

moving average onto the profile data attenuates wavelengths less than 1 m (3 ft). 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 19. PSD plot of data collected by the K.J. Law Engineers DNC 690 profiler. 

K.J. Law Engineers T-6600 Profiler 

The T-6600 profiler recorded profile data at 25-mm (1-inch) intervals. In the LTPP program, 

these data are processed using the ProQual software, which applies a 300-mm (11.8-inch) 

moving average onto the 25-mm (1-inch) interval profile data, and then extracts profile data 

points at 150-mm intervals. ProQual computes the IRI using these averaged data. The IRI values 

and the averaged 150-mm (5.9-inch) interval profile data for LTPP sections are available in the 

LTPP database. 

31 
Figure 20 shows a PSD plot of the 25-mm (1-inch) data collected by the T-6600 profiler and the 

PSD plot of the same data after it has been processed using ProQual. Figure 20 shows that there 

is a significant difference in the profile content between the two profilers for wave numbers 

greater than 1 cycle/m (0.3 cycle/ft), which corresponds to wavelengths less than 1 m (3 ft). The 

sharp dropoff seen in the PSD plot for 150-mm (5.9-inch) data for wave numbers greater than 

1 cycle/m (0.3 cycle/ft) occurs because the moving average attenuates wavelengths less than 1 m 

(3 ft) in the profile. 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 20. PSD plot of data collected by the K.J. Law Engineers T-6600 profiler. 

International Cybernetics Corporation Profiler 

The ICC profilers do not record profile data, but rather record the data collected by the height 

sensors, accelerometers, and the DMI. These data can be used to generate profiles with a 25-mm 

(1-inch) sampling interval. In the LTPP program, these 25-mm (1-inch) data are processed using 

the ProQual software, which uses the same procedure as described for the T-6600 profiler. As in 

the case of the T-6600 profiler, IRI is computed using the averaged data that are at 150-mm 

(5.9-inch) intervals. The computed IRI values and the averaged profile data for LTPP sections 

are available in the LTPP database. Figure 21 shows a PSD plot of the 25-mm (1-inch) data 

collected by the ICC profiler and the PSD plot of the same data after it had been processed using 

ProQual. 

The trend between the 25-mm (1-inch) data and the 150-mm (5.9-inch) data seen in this figure is 

similar to the trend that was observed for the T-6600 profiler, where the application of the 

moving average caused profile features that have a wave number greater than 1 cycle/m 

(0.3 cycle/ft) to become attenuated. 

32 
COMPARISON OF K.J. LAW ENGINEERS DNC 690 AND T-6600 PROFILERS 

Comparison of Profile Data 

The DNC 690 profiler recorded profile data at 152.4-mm (6-inch) intervals, while the T-6600 

profiler recorded profile data at 25-mm (1-inch) intervals. The DNC 690 and T-6600 profilers 

applied a 91-m (300-ft) and 100-m (328-ft) upper-wavelength cutoff filter to the profile data, 

respectively. 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 21. PSD plot of data collected by the ICC profiler. 

Figure 22 shows overlaid profile plots of data collected at the same site by the DNC 690 and 

T-6600 profilers. These data are from the smooth AC section that was used by the North Central 

region for the 1996 profiler verification test.(24) Figure 23 shows a similar plot at the rough AC 

section used by the North Central region for the same study. 

33 
-8

-6

-4

-2

0

2

4

6

8

10

0 20 40 60 80 100 120 140 160

Distance (m)

Elevation (mm)

DNC 690

T-6600

2.54 mm = 1 inch 

1 m = 3.28 ft 

Figure 22. Data collected by the North Central K.J. Law Engineers DNC 690 and T-6600 

profilers at the smooth AC site during the 1996 verification test. 

-60

-40

-20

0

20

40

60

0 20 40 60 80 100 120 140 160

Distance (m)

Elevation (mm)

DNC 690

T-6600

T-6600 DNC 690

2.54 mm = 1 inch 

1 m = 3.28 ft 

Figure 23. Data collected by the North Central K.J. Law Engineers DNC 690 and T-6600 

profilers at the rough AC site during the 1996 verification test. 

The data collected by the two profilers overlay extremely well at the smooth AC section, while at 

the rough AC section, the agreement is less when compared to the agreement seen at the smooth 

AC section. At the rough AC section, although there is a slight shift between the two profiles, an 

evaluation of the profile data using filtering techniques shows that similar profile features are 

captured by both profilers. 

The overall shape of a profile plot primarily depends on the long-wavelength content in the 

profile. There is a slight difference in the long-wavelength cutoff limit used by the two profilers, 

which can cause some differences to occur in the profile plots. The long-wavelength content at 

34 
the rough AC site is higher than that at the smooth AC site and, thus, differences among the 

profiles are seen more clearly at the rough AC site. These observations, as well as a comparison 

of other data collected during the 1996 verification test, indicate that the same upper-wavelength 

cutoff filtering technique appears to have been used with the DNC 690 and T-6600 profilers. 

Figure 24 shows PSD plots of left-wheelpath data collected by the DNC 690 profiler, which has 

a sampling interval of 152.4 mm (6 inches), and the T-6600 profiler, which has a sampling 

interval of 25 mm (1 inch), at the smooth AC site in the North Central region during the 1996 

verification test. The PSD plots show good agreement, except for wave numbers greater than 1 

cycle/m (0.3 cycle/ft), which corresponds to wavelengths less than 1 m (3 ft). In this waveband 

range, the profile content in the DNC 690 profiler is attenuated when compared to the T-6600 

profiler. This attenuation is caused by the moving average filter that is applied to the DNC 690 

profiler data before saving the data. 

Figure 25 shows the PSD plot of the two data sets whose PSD plots are shown in figure 24, 

except that the data shown for the T-6600 profiler are the data that were obtained after the 

25-mm (1-inch) data were processed using ProQual. The application of the 300-mm (11.8-inch) 

moving average onto the 25-mm (1-inch) T-6600 profiler data attenuates wavelengths less than 

1 m (3 ft), which corresponds to wave numbers greater than 1 cycle/m (0.3 cycle/ft). The two 

PSD plots shown in figure 25 indicate good agreement. A review of similar plots for other data 

collected during the 1996 verification test showed similar trends. These results confirm that the 

DNC 690 profiler applied a 304.8-mm (12-inch) moving average onto the data before saving the 

data. Since the PSD plots for the two profilers agree well through a range of 0.025 to 1 cycle/m 

(0.008 to 0.3 cycle/ft), which corresponds to wavelengths between 1 and 40 m (3 and 130 ft), the 

IRI values of the two profilers are expected to agree closely. 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 24. PSD plot of data collected by the K.J. Law Engineers DNC 690 and T-6600 profilers. 

35 
Figure 25. PSD plots of K.J. Law Engineers DNC 690 profiler data and ProQual-processed 

num

When comparison testing between the two profilers is performed, the profiler driver should do 

the

Align the profiler along the wheelpaths. 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

T-6600 profiler data. 

The PSD plots also indicate that the spectral content of the data collected at the same site by the 

two profilers was similar. This indicates that the profile features that are recorded by the two 

profilers are similar. (Note: These observations are only valid when the DNC 690 profiler data 

are compared to ProQual-processed T-6600 profiler data. If 25-mm (1-inch) T-6600 profiler data 

are compared to DNC 690 profiler data, differences among the profile data will be seen for wave 

bers greater than 1 cycle/m (0.3 cycle/ft).) The PSD plots also indicated good agreement 

between the two profilers for low wave numbers (long wavelengths), which is an indication that 

the filtering technique used by the two profilers for the upper-wavelength cutoff is similar. The 

slight differences seen in the PSD plots for the low wave numbers (long wavelengths) are 

probably related to the different upper-wavelength cutoff values that were used in the two 

profilers, which are 91 m (300 ft) for the DNC 690 profiler and 100 m (328 ft) for the T-6600 

profiler. 

Comparison of IRI Values 

 following two tasks accurately: 

•

• Maintain a consistent path within the test section. 

If the profiler driver does not correctly align the profiler along the wheelpaths, the longitudinal 

path followed by the sensors of the different profilers will be different. This can cause IRI 

36 
obtained from the different profilers to vary. After aligning the profiler along the wheelpaths, the 

driver should also follow a consistent path within the test section without lateral wander. If the

is variability in the path that is followed within the section, it can result in differences in IRI 

when the two profilers are being compared. 

The ability of a driver to correctly align the profiler along the wheelpaths and to follow a 

consistent path within the section can vary among drivers. Drivers who are more experienced i

profiling c

re 

n 

an probably do these two tasks much better than a driver who is inexperienced in 

rofiling. A driver who is experienced in profiling probably will also be able to follow a more 

g repeat measurements at a site than a person who does not have 

uch experience in operating the profiler. 

ity within the section during profiling, will 

sually have a greater impact on the IRI for sections that have distresses. This is because, in such 

ment features either to be included or missed in 

der is an issue that can sometimes 

omplicate the analysis when the IRI from different profilers are compared. 

 is 

 shown in table 3. As shown in this table, the 

ercent change in IRI will vary for different pavements. Some pavements showed extremely 

hange 

Section IRI (m/km) Percent Change in IRI 

p

consistent path when obtainin

m

Not following the correct wheelpath, or variabil

u

sections, lateral variations can cause certain pave

the profile, thus affecting the IRI. The effect of lateral wan

c

A study performed for an NCHRP project found that variations in the longitudinal path that

followed during profiling could have a significant effect on IRI.(1) In this study, the effect of 

lateral variations in profiling was studied at seven test sections. The percent change in IRI that 

was obtained for each wheelpath at the test sections for a 0.3-m (1-ft) lateral shift in the 

longitudinal path to the left and to the right is

p

large variations in IRI for a 0.3-m (1-ft) shift from the wheelpath. Generally, the percent c

in IRI that was observed along the left wheelpath was less than that obtained for the right 

wheelpath. 

Table 3. Changes in IRI caused by lateral variations in the longitudinal path. 

 Wheelpath Shifted 0.3 m Right Shifted 0.3 m Left 

 Left Right Wheelpath Wheelpath 

 Left Right Left Right 

New AC 0.85 0.98 −4 6 −4 −7 

AC with Transverse Cracks 1.20 1.22 −3 34 8 2 

Old AC 1.85 2.63 −7 0 −17 −22 

1-year-old PCC 0.84 1.41 1 13 −4 −21 

3-year-old PCC 0.59 0.58 −5 14 15 7 

6-year-old PCC 1.41 1.75 4 13 −4 −3

Severely Faulted PCC 3.67 3.83 −1 1 8 −2 

After each r

1 m = 3.28 ft 

1 m/km = 5.28 ft/mi 

egional contractor accepted delivery of the T-6600 profiler in 1996, they performed a 

comparison of this profiler and the DNC 690 profiler. Four test sections were used in each region 

37 
to perform m this 

test are described in appendix B. In the reports e 

IRI value for a wheelpath from the that were pe puted 

u ng different procedures. In this r ct a

obtained from all four regions to compare value DNC 690 and T-6600 

profilers. The IRI values obtained fo sequen 2 tes ere u this a c ite 

1 ern region for the DNC 690 profiler, where sequence 1 values were used because 

sequence 2 data had s pi he ge IRI for each wheelpath at each test section 

wa th run had the least standard deviations in IRI for the 

mean IRI (i.e., the average IRI for ft a ht wheelpaths). The comp verag

va ilers for all reg re in t e 4. Whe l 4 regi ere c red, 

the 16 test sites (3 eelp ons between the 2 profilers 

co

Figure 26 shows the IRI relationship between the two profilers, w e data ob ned for 

h wn. There is very good agreement in the IRI values obtained by the two 

ro one case along the left wheelpath. This data point corresponds to the left 

 this comparison. (See references 24, 25, 26, and 27.) The results obtained fro

prepared by th

 multiple runs

e regional contractors, the averag

rformed at a site was com

si esearch proje , a consistent method w s used on data 

the IRI s between the

r ce ting w sed in nalysis (ex ept at s

 in the West

aturation s

s computed using the IRI for 

kes). T avera

e five s that

 the le nd rig uted a e IRI 

lues for both prof ions a shown abl n al ons w onside

re were a total of 2 wh aths) where IRI comparis

uld be made. 

her tai 32 

w eelpaths are sho

p filers, except for

wheelpath of site 2 in the Western region. An evaluation of the data indicated that the data 

collected by the DNC 690 profiler had a significant number of spikes that were caused by 

sunlight being picked up by the sensor, which resulted in a high IRI value. The correlation 

coefficient for the data shown in figure 26 is 0.98. 

38 
Table 4. IRI values from the 1996 verification test. 

Region Site Description Average IRI (m/km) 

 Left Wheelpath Right Wheelpath

 DNC 690 T-6600 DNC 690 T-6600

North Atlantic 1 Smooth AC 0.86 0.74 0.86 0.88 

North Atlantic 2 Rough AC 2.27 2.11 1.93 1.67 

North Atlantic 3 Smooth PCC 1.22 1.18 1.31 1.34 

North Atlantic 4 Rough PCC 1.87 1.90 2.15 2.24 

 

North Central 1 Smooth AC 1.01 1.05 1.06 1.05 

North Central 2 Rough AC 3.96 4.00 4.92 4.79 

North Central 3 Smooth PCC 1.10 1.10 1.07 1.08 

North Central 4 Rough PCC 2.69 2.67 2.99 2.95 

 

Southern 1 Smooth AC 0.74 0.71 0.74 0.67 

Southern 2 Rough AC 1.63 1.88 1.91 1.80 

Southern 3 Smooth PCC 1.77 1.81 1.75 1.70 

Southern 4 Rough PCC 2.17 2.01 2.50 2.38 

 

Western 1 Smooth AC 0.77 0.78 0.90 0.92 

Western 2 Rough AC 3.74 2.62 2.61 2.57 

Western 3 Smooth PCC 1.04 0.99 0.93 0.89 

Western 4 Rough PCC 2.31 2.35 2.40 2.39 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

DNC 690 IRI (m/km)

T-6600 IRI (m/km)

Left

Right

1 m/km = 5.28 ft/mi 

1 m/km = 5.28 ft/mi 

Figure 26. Relationship between IRI from the K.J. Law Engineers DNC 690 

and T-6600 profilers. 

39 
The difference in IRI between the DNC 690 and T-6600 profilers (DNC 690 IRI – T-6600 IRI) 

was computed along each wheelpath for all of the test sections. These data are shown in figure 

27 as a function of the IRI for the wheelpath, where the IRI for the wheelpath was computed by 

averaging the IRI obtained for that wheelpath by the DNC 690 and T-6600 profilers. Of the 32 

available cases, the difference in IRI was within ±0.10 m/km (±6 inches/mi) for 23 cases. For six 

cases, the difference was between 0.10 and 0.20 m/km (6 and 13 inches/mi). There was one case 

where the difference was between −0.20 and −0.30 m/km (−13 and −19 inches/mi), one case 

where the difference was between 0.20 and 0.30 m/km (13 and 19 inches/mi), and one case 

where the difference was greater than 1.1 m/km (70 inches/mi) (this data point is not shown in 

figure 27). 

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.0 1.0 2.0 3.0 4.0 5.0 6.0

IRI of Wheelpath (m/km)

DNC 690 IRI - T-6600 IRI (m/km)

Left

Right

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.0 1.0 2.0 3.0 4.0 5.0 6.0

IRI of Wheelpath (m/km)

DNC 690 IRI - T-6600 IRI (m/km)

Left

Right

1 m/km = 5.28 ft/mi 

Figure 27. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

All regions. 

An investigation was performed separately for each region to identify the cause of the difference 

in IRI between the two profilers for cases where the difference in IRI was outside ±0.10 m/km 

(±6 inches/mi). In each region, the difference in IRI between the DNC 690 and T-6600 profilers 

(DNC 690 IRI – T-6600 IRI) for each wheelpath was plotted as a function of the IRI for the 

wheelpath, with the IRI for the wheelpath computed by averaging IRI obtained by the two 

profilers for that wheelpath. 

North Atlantic Region 

Figure 28 shows the difference in IRI from the DNC 690 and T-6600 profilers at the test sections 

tested by the North Atlantic profilers as a function of the IRI for the wheelpath. 

A difference in IRI between the DNC 690 and T-6600 profilers (DNC 690 –IRI – T-6600 IRI) 

that was outside ±0.10 m/km (±6 inches/mi) was observed for the following three cases: (1) left 

wheelpath of site 1 (a difference of 0.12 m/km (8 inches/mi)), (2) left wheelpath of site 2 (a 

difference of 0.16 m/km (10 inches/mi)), and (3) right wheelpath of site 2 (a difference of 

0.26 m/km (16 inches/mi)). 

40 
Site 1: The left-wheelpath IRI from the DNC 690 profiler was 0.12 m/km (8 inches/mi) higher 

than IRI obtained by the T-6600 profiler. The DNC 690 profiler conducted six runs on this 

section. IRI along the left wheelpath for these six runs ranged from 0.83 to 0.87 m/km (53 to 

55 inches/mi). The T-6600 profiler conducted nine runs at this site, and the left-wheelpath IRI 

for eight runs ranged from 0.72 to 0.77 m/km (46 to 49 inches/mi); however, there was one run 

that had an IRI of 0.85 m/km (54 inches/mi). The roughness profile of this run overlaid 

extremely well with the roughness profiles obtained by the DNC 690 profiler. This indicates that 

the probable cause of the difference in IRI between the two profilers was a difference in the 

paths that were followed during profiling. 

North Atlantic

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 0.5 1.0 1.5 2.0 2.5

IRI of Wheelpath (m/km)

Left

Right

Site 2

Site 1 Site 2

North Atlantic

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 0.5 1.0 1.5 2.0 2.5

IRI of Wheelpath (m/km)

Left

Right

Site 2

Site 1 Site 2

DNC IRI

–T-6600 (m/km)

1 m/km = 5.28 ft/mi 

Figure 28. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

North Atlantic region. 

Site 2: IRI from the DNC 690 profiler was higher than IRI from the T-6600 profiler by 

0.16 m/km (10 inches/mi) and 0.26 m/km (16 inches/mi) for the left and right wheelpaths, 

respectively. This analysis used IRI values obtained during sequence 2 testing. The average IRI 

from the T-6600 profiler from sequence 1 testing at this site, along the left and right wheelpaths, 

were 2.21 m/km (140 inches/mi) and 1.89 m/km (120 inches/mi), respectively. These values 

compare extremely well with the IRI values obtained for the left and right wheelpaths by the 

DNC 690 profiler (2.27 m/km (144 inches/mi) and 1.93 m/km (122 inches/mi), respectively). 

This section had significant transverse and longitudinal cracking along both wheelpaths 

throughout the test section. Thus, the differences in IRI that were observed between the two 

profilers for sequence 2 testing are attributed to variations in the wheelpaths followed by the two 

profilers. 

North Central Region 

Figure 29 shows the differences in IRI from the DNC 690 and T-6600 profilers at the test 

sections in the North Central region as a function of the IRI for the wheelpath. 

41 
A difference in IRI between the DNC 690 and T-6600 profilers that was outside ±0.10 m/km 

(±6 inches/mi) was observed only along the right wheelpath at site 2, where the IRI from the 

DNC 690 profiler was higher than that from the T-6000 profiler by 0.13 m/km (8 inches/mi). The 

right wheelpath at this site is extremely rough. The right-wheelpath IRI for the five selected runs 

from the DNC 690 profiler ranged from 4.77 to 4.96 m/km (302 to 314 inches/mi), while the IRI 

for the five selected runs from the T-6600 profiler ranged from 4.74 to 4.81 m/km (301 to 

305 inches/mi). These ranges for the two profilers have some overlap. The difference in IRI 

between the two profilers at this site is attributed to variations in the wheelpaths. 

North Central

-0.15

-0.10

Figure 29. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

North Central region. 

Southern Region 

Figure 30 shows the difference in IRI from the DNC 690 and T-6600 profilers at the test sections 

tested by the Southern profilers as a function of the IRI for the wheelpath. 

Differences in IRI between the DNC 690 and T-6600 profilers (DNC 690 –IRI – T-6600 IRI) 

that were outside ±0.10 m/km (±6 inches/mi) were observed for the following four cases: (1) left 

wheelpath of site 2 (a difference of −0.25 m/km (−16 inches/mi)), (2) right wheelpath of site 2 

(a difference of 0.11 m/km (10 inches/mi)), (3) left wheelpath of site 4 (a difference of 

0.16 m/km (10 inches/mi)), and (4) right wheelpath of site 4 (a difference of 0.12 m/km 

(7 inches/mi)). 

1 m/km = 5.28 ft/mi 

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 1.0 2.0 3.0 4.0 5.0 6.0

IRI of Wheelpath (m/km)

Left

Right

Site 2

North Central

-0.15

-0.10

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 1.0 2.0 3.0 4.0 5.0 6.0

IRI of Wheelpath (m/km)

-0.05

Left

Right

DNC IRI

–T-6600 (m/km)

Site 2

42 
Site 2: At this site, IRI from the DNC 690 profiler was lower than that from the T-6600 profiler 

along the left wheelpath by 0.25 m/km (16 inches/mi), but higher than that obtained by the T6600 profiler by 0.11 m/km (7 inches/mi) along the right wheelpath. The T-6600 profiler 

conducted nine runs at this site. Along the left wheelpath, the IRI for the nine runs ranged from 

1.80 to 2.00 m/km (114 to 127 inches/mi), while along the right wheelpath, the IRI ranged from 

1.70 to 1.86 m/km (108 to 118 inches/mi). This indicates there is some transverse variability at 

this site. A comparison of the profile data from the T-6000 and DNC 690 profilers indicated that 

there were localized differences between the profile data and that these caused the difference in 

the IRI from the profilers. The difference in IRI between the two profilers has opposite signs for 

the two wheelpaths. This is usually an indication that the difference in IRI between the two 

profilers is probably related to variations in the profiled paths. No explanation other than 

variability between the profiled paths can be offered to explain the difference in IRI between the 

two profilers at this site. 

Southern

-0.30

Figure 30. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

Southern region. 

Site 4: IRI from the DNC 690 profiler at this site was higher than that obtained by the T-6600 

profiler by 0.16 m/km (10 inches/mi) and 0.12 m/km (8 inches/mi) along the left and right 

wheelpaths, respectively. An evaluation of the roughness profile for the left wheelpath indicated 

that most of the difference in roughness between the two profilers was occurring between 140 m 

(459 ft) and the end of the section. This was caused by differences in the way a feature, which 

was located at approximately 145 m (476 ft), was being measured by the two profilers. An 

evaluation of the roughness profiles for the right wheelpath also showed some localized 

variations in roughness that occurred because of differences in the way features were measured 

by the two profilers. Because this site is fairly rough, with left- and right-wheelpath IRI values of 

approximately 2.10 and 2.14 m/km (133 and 152 inches/mi), respectively, variations in the 

profiled paths are probably the cause of the difference in IRI between the two profilers. 

1 m/km = 5.28 ft/mi 

-0

-0

.20

.10

0.00

0.10

0.20

0.0 0.5 1.0 1.5 2.0 2.5 3.0

IRI of Wheelpath (m/km)

Left

Right

Site 2

Site 2

Site 4

Southern

-0.30

.20

.10

0.00

0.10

0.20

0.0 0.5 1.0 1.5 2.0 2.5 3.0

IRI of Wheelpath (m/km)

-0

-0

Left

Right

Site 2

Site 2

Site 4

DNC IRI

–T-6600 (m/km)

43 
Western Region 

Figure 31 shows the difference in IRI from the DNC 690 and T-6600 profilers at the sections 

tested by the two Western profilers as a function of the IRI for the wheelpath. The difference in 

IRI between the two profilers was within ±0.10 m/km (±6 inches/mi) for all of the cases except 

one. This case was along the left wheelpath at section 2, where the difference in IRI was 1.1 

m/km (70 inches/mi). This occurred because data collected by the DNC 690 profiler was 

contaminated by saturation spikes. This data point is not shown in figure 31. 

Cross Correlation of IRI 

The cross-correlation technique provides a method to compare the magnitude and the spatial 

distribution of IRI between two devices. This technique was used to compare IRI obtained by the 

DNC 690 and T-6600 profilers using data obtained during the 1996 verification testing in the 

North Central and Western regions. When using the cross-correlation technique, the DNC 690 

profiler was considered to be the “correct” device and, thus, the analysis will indicate how well 

the T-6600 profiler reproduced the results from the DNC 690 profiler. 

Western

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 1.0 2.0 3.0

IRI of Wheelpath (m/km)

Left

Right

Western

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 1.0 2.0 3.0

IRI of Wheelpath (m/km)

Left

Right

DNC IRI

–T-6600 (m/km)

1 m/km = 5.28 ft/mi 

Figure 31. Differences in IRI between the K.J. Law Engineers DNC 690 and T-6600 profilers: 

Western region. 

One representative run was selected for each profiler at each site to perform this analysis. In this 

analysis, for the T-6600 profiler, the ProQual-processed averaged data that are at 150-mm 

(5.9-inch) intervals was used, while for the DNC 690 profiler, the 152.4-mm (6-inch) data 

obtained by the profiler was used. Because the data collected by the DNC 690 profiler is 

considered to be the correct data, any deviations in the path followed by the T-6600 profiler from 

the path followed by the DNC 690 profiler will affect the results. The results of the crosscorrelation analysis are presented in table 5. 

44 
Table 5. Results of cross correlation between the K.J. Law Engineers DNC 690 and 

T-6600 profilers. 

Region Site IRI (m/km) Cross Correlation 

 Left Wheelpath Right Wheelpath Left Right 

 DNC 690 T-6600 DNC 690 T-6600 Wheelpath Wheelpath

North Central 1 1.00 1.04 1.06 1.02 0.79 0.91 

North Central 2 3.87 4.10 4.96 4.81 0.81 0.94 

North Central 3 1.06 1.07 1.03 1.07 0.85 0.95 

North Central 4 2.65 2.64 2.90 2.92 0.92 0.96 

Western 1 0.75 0.79 0.88 0.91 0.88 0.94 

Western 2 3.37 2.51 2.62 2.62 0.13 0.85 

Western 3 0.98 0.99 0.94 0.88 0.74 0.74 

Western 4 2.24 2.32 2.40 2.45 0.88 0.93 

1 m/km = 5.28 ft/mi 

The sensor spacing for the two North Central profilers was different. During testing, the two 

profilers aligned the right sensor along a similar path using a camera system. The two North 

Central profilers had very high cross-correlation values along the right wheelpath where the 

camera system was used to judge the wheelpath. This indicates excellent agreement in both the 

IRI magnitude and IRI distribution along that path for the two profilers. The two North Central 

profilers also showed good cross-correlation values along the left wheelpath, too, although the 

values were slightly less than those obtained for the right wheelpath. 

The two profilers in the Western region also had high cross-correlation values at the majority of 

the sections. The data collected along the left wheelpath at site 2 by the DNC 690 profiler were 

contaminated with saturation spikes, thus, a low cross-correlation value was obtained for this 

case. The cross-correlation values at site 3 were somewhat lower than the values obtained for the 

other sites. Site 3 is a concrete site, and evaluation of the profile data indicated that the amount of 

slab curling that was present when the two profilers measured the site was different, and this was 

the cause of the low cross correlation at this site. 

Analysis of Variance and Regression Analysis of IRI 

An analysis of variance (ANOVA) was performed using the IRI values obtained from the 1996 

regional testing to determine whether IRI values obtained by the DNC 690 and T-6600 profilers 

were similar. A two-factor ANOVA was performed using the IRI values obtained for the five 

runs that were selected for computing the average IRI values shown in table 4. During the 1996 

verification test, testing was performed at 16 sections (4 sections per region), and this provided 

32 cases (32 wheelpaths) that could be used in the analysis. Because the data for the left 

wheelpath at site 2 in the Western region for the DNC 690 profiler were erroneous, these data 

were omitted from the analysis. The ANOVA indicated that the profilers were significant at a 

significance level of 0.05. 

Another ANOVA was performed by omitting the data for the left wheelpath in the North Central 

region. This was done because the two profilers in the North Central region have different sensor 

45 
spacing, and they were aligned along the right wheelpath during testing. This analysis also found 

that the profilers were significant at a significance level of 0.05. Thereafter, separate ANOVAs 

were performed for each region. The only case where the profilers were not significant was in 

the Western region. 

Thereafter, for each region, separate ANOVAs were performed for each wheelpath. The only 

cases where the profilers were not significant at a significance level of 0.05 were for the left and 

right wheelpaths in the Western region, and for the left wheelpath in the North Central region. 

A regression analysis was performed for the IRI from the DNC 690 and T-6600 profilers. The 

IRI for the five runs that were selected at each section to compute the average IRI value in the 

previous analysis were used in the regression. This provided 155 pairs of data for the regression 

(i.e., four regions x four sections per region x two wheelpaths per section x five runs per section, 

less the erroneous left-wheelpath runs at section 2 in the Western region). The following 

relationship was obtained from the regression: 

 (1) IRI (T-6600) = 0.982 IRI (DNC 690) + 0.004

where: 

• IRI (T-6600) = IRI from T-6600 profiler for a wheelpath (m/km). 

• IRI (DNC 690) = IRI from DNC 690 profiler for a wheelpath (m/km). 

• R2 = 0.99, standard error = 0.09 m/km. 

The regression analysis indicated that IRI from the two profilers were extremely similar; 

however, IRI obtained from the DNC 690 profiler was predicted to be slightly higher than that 

obtained from the T-6600 profiler. 

COMPARISON OF K.J. LAW ENGINEERS T-6600 AND ICC PROFILERS 

Comparison of Profile Data 

The data obtained from the 2002 verification test were used for this analysis. Both profilers 

applied a 100-m (328-ft) upper-wavelength cutoff filter onto the data. The data collected by both 

profilers along each wheelpath at each site were overlaid to evaluate differences in the data. At 

some sites, the profiles overlaid extremely well; however, at many sites, there were significant 

differences among the profiles. Figure 32 shows an example of a case where close agreement 

was obtained between the profile data from the two profilers. Figure 33 shows an example of a 

case where there were significant differences between the profile plots. The data shown in 

figures 32 and 33 are the 25-mm (1-inch) left-wheelpath data collected by the two Western 

profilers during the 2002 verification test at LTPP sites 320209 and 069107, respectively. 

46 
-10

-8

-6

-4

-2

0

2

4

6

8

0 20 40 60 80 100 120 140

Distance (m)

Elevation (mm)

Law

ICC

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 32. Comparison of ICC and K.J. Law Engineers profiles: Western site 320209. 

An evaluation of all data collected for the 2002 verification test indicated that the profile plots 

from the two profilers usually overlaid well at sites that did not have much long-wavelength 

content. However, differences between the profile plots were noticeable at sites that had more 

long-wavelength content. An evaluation of the profile data using filtering techniques indicated 

that profile features that were present on the pavement were being measured similarly by both 

profilers. These observations indicate that there are differences in the long-wavelength data 

collected by the two profilers. The differences in the long wavelengths appear to be occurring for 

wavelengths greater than approximately 40 m (131 ft). 

47 
-40

-30

-20

-10

0

10

20

30

0 20 40 60 80 100 120 140

Distance (m)

Elevation (mm)

Law

ICC

ICC

Figure 33. Comparison of ICC and K.J. Law Engineers profiles: Western site 069107. 

The response of the quarter car filter that is used in the IRI computation procedure to different 

wavelengths is shown in figure 34.(2) The amplitude of the output sinusoid is the amplitude of the 

input multiplied by the gain shown in the figure, which is dimensionless. IRI is primarily 

influenced by wavelengths ranging from 1.2 to 30.5 m (4 to 100 ft).(2) However, there is still 

some response to wavelengths outside this range. The IRI filter has a maximum sensitivity to 

sinusoids with wavelengths of 2.4 and 15.4 m (7.9 and 50.5 ft). The response is down to 0.5 for 

wavelengths of 1.2 and 30.5 m (4 and 100 ft).(2) 

0

0.5

1.0

1.5

2.0

Gain for Profile Slope

0.1 1 10 100 2 5 2 5 2 5

Wavelength (m)

25.4 mm = 1 inch 

1 m = 3.28 ft 

1 m = 3.28 ft 

Figure 34. Response of the IRI filter.(2)

48 
IRI obtained by the T-6600 and ICC profilers during the 2002 verification test showed good 

agreement.(28) Although there are differences in the long wavelengths between the two profilers, 

good agreement in IRI between the two profilers indicates that the profilers are collecting similar 

data within the wavelength range that is influencing the IRI that was described previously. 

The K.J. Law Engineers profiler uses a Butterworth filter for long-wavelength cutoff, while the 

ICC profiler uses a cotangent filter. Although both profilers are using an upper-wavelength 

cutoff filter of 100 m (328 ft), differences in the filtering techniques used by the two profilers are 

causing some differences in the long-wavelength data between the two profilers. An evaluation 

of the filtering techniques indicated that the Butterworth filter makes a much sharper transition 

from wavelengths that are unmodified to wavelengths that are eliminated than the cotangent 

filter, and this causes differences in the long wavelengths to occur between the two profilers. 

Figure 35 shows a typical PSD plot that was obtained when the 25-mm (1-inch) data from the 

ICC and K.J. Law Engineers profilers were compared. The data shown in figure 35 are those 

obtained by the two North Central profilers at site 5 (which has a chip seal) during the 2002 

verification test. The two PSD plots show good agreement between wave numbers of 0.025 and 

1 cycle/m (0.008 and 0.305 cycle/ft), which correspond to wavelengths between 40 and 1 m 

(131 and 3 ft). However, there are differences between the profilers for wave numbers less than 

0.025 cycle/m (0.008 cycle/ft), which corresponds to wavelengths greater than 40 m (131 ft), and 

wave numbers greater than 1 cycle/m (0.3 cycle/ft), which corresponds to (wavelengths less than 

1 m (3 ft). 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 35. PSD plot of 25-mm (1-inch) data collected by the North Central ICC and 

K.J. Law Engineers profilers at the chip-seal section during the 2002 verification test. 

An examination of PSD plots of data collected by the ICC and K.J. Law Engineers profilers 

during the 2002 regional comparison indicated the following: 

49 
• The ICC and T-6600 profilers generally are collecting similar data between wavelengths 

of 1 and 40 m (3 and 131 ft). 

• There were some differences in the data collected by the two profilers for wavelengths 

greater than 40 m (131 ft). The PSD plots indicated that, usually, the K.J. Law Engineers 

profiler was collecting more spectral content than the ICC profiler for wavelengths 

greater than 40 m (131 ft). 

• For wavelengths below 1 m (3 ft), the ICC profilers usually have slightly more content 

compared to the K.J. Law Engineers profiler. This may be occurring because of the small 

footprint size of the height sensor on the ICC profiler. The laser height sensor on the ICC 

profiler has a 1.5-mm- (0.06-inch-) diameter circular footprint, while the K.J. Law 

Engineers profiler has an elliptical footprint that is 38 by 6 mm (1.5 by 0.24 inches). The 

sensors in the K.J. Law Engineers profiler are reported to be averaging the elevation 

values within its footprint when obtaining height measurements. This can cause elevation 

values obtained by the K.J. Law Engineers profiler to be less than those obtained by the 

ICC profiler in pavements that have coarse texture and in pavements that have narrow 

upward or downward features. 

A closeup view of the 25-mm (1-inch) profile data collected by the two profilers at the site 

(whose PSD plot is shown figure 35) is shown in figure 36. Figure 36 indicates that the profile 

recorded by the ICC profiler shows more profile details compared to those recorded by the K.J. 

Law Engineers profiler. This is why the PSD plots shown in figure 35 indicate a difference 

between the two profilers for wave numbers greater than 1 cycle/m (0.3 cycle/ft). 

-6

-4

-2

0

2

4

6

8

10

12

14

10 11 12 13 14 15

Distance (m)

Elevation (mm)

ICC

Law

1 inch = 25.4 mm 

1 m = 3.28 ft 

Figure 36. Closeup view of 25-mm (1-inch) profile data collected by North Central ICC and 

K.J. Law Engineers profilers on a chip-seal pavement. 

50 
When ProQual processes the 25-mm (1-inch) data by applying a 300-mm (11.8-inch) moving 

average, the short-wavelength features that have wavelengths less than 1 m (3 ft) will become 

attenuated and the PSD plots for the two profilers will show better agreement for wave numbers 

greater than 1 cycle/m (0.3 cycle/ft). 

An analysis was performed to investigate the differences between the two profilers in measuring 

short wavelengths. The data collected by the North Central profilers during the 2002 regional 

verification test were used for this analysis. At each site, one run from each profiler was selected, 

and the analysis was performed on the left-wheelpath data. The analysis was performed on the 

first 76 m (249 ft) of the data from the site. The profile data were filtered to get rid of the long 

wavelengths, so that only the short wavelengths would be present in the profile. Thereafter, the 

standard deviations of the filtered elevation values were computed. The results from this analysis 

are presented in table 6. 

Table 6. Standard deviations of filtered elevation values. 

Site Description Standard Deviations of 

 Filtered Elevation (mm) 

 ICC K.J. Law 

1 Smooth Asphalt 0.0066 0.0064 

2 Rough Asphalt 0.0174 0.0079 

3 Smooth Concrete 0.0092 0.0082 

4 Rough Concrete 0.0115 0.0084 

5 Chip Seal 0.0113 0.0100 

1 inch = 25.4 mm 

The ICC profiler had higher standard deviations for all of the cases. The data shown in table 6 

suggests that the small footprint size of the ICC profiler is measuring more texture-related effects 

and possibly higher depth at narrow downward features, such as cracks, than that measured by 

the T-6600 profiler. 

Comparison of 25-mm (1-inch) data collected by the K.J. Law Engineers and ICC profilers 

indicated that there were differences in the depth of downward features, such as cracks, that were 

measured by the profilers at some sites. However, it is unclear if this difference was caused by 

variations in the paths profiled by the two profilers, or if it was related to differences in the 

footprint sizes of the two height sensors, or differences in the filtering techniques used by the 

two profilers. 

Figure 37 shows a closeup view of the profile data recorded by the two profilers over a joint in a 

concrete pavement. The data shown in this figure were collected by the two North Central 

profilers at the rough PCC section during the 2002 verification test. On this pavement, minor 

variations in the profiled path are not likely to result in a difference in the magnitude of the 

downward feature measured by the profilers when readings are taken on top of the sealant at a 

joint. This is because the joint sealant is expected to be at a constant depth from the slab surface 

over a short lateral distance. However, in cases where there are transverse cracks in the 

pavement, minor variations in the wheelpaths can result in the crack depth being different. Thus, 

comparing the magnitude of the downward feature recorded by the two profilers at a joint 

51 
provides a better choice for comparing the two profilers. The two plots in figure 37 show that the 

depths of the joint as recorded by the two profilers are very similar; however, the depth recorded 

by the ICC profiler is slightly higher. 

-12

-10

-8

-6

-4

-2

0

2

4

6

61.3 61.5 61.7 61.9 62.1 62.3

Distance (m)

Elevation (mm)

ICC

Law

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 37. Readings taken over a joint by the two profilers. 

Figure 38 shows the 25-mm (1-inch) profile data collected by the two profilers at this site after 

the data have been subjected to a 3-m (10-ft) high-pass filter. The slab length in this concrete 

pavement is 13 m (43 ft). Figure 38 shows that both profile plots are showing all joint locations, 

and the depths recorded over the joints by the two profilers are very similar. 

3-m High Pass Filtered (Profiles Offset)

-10

-5

0

5

10

15

0 25 50 75 100 125 150

Distance (m)

Elevation (mm)

Law

ICC

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 38. Profile data obtained by the ICC and K.J. Law Engineers profilers at a concrete site. 

52 
Although the profilers output data at 25-mm (1-inch) intervals, the height sensors in the profilers 

are obtaining data at much closer intervals, and are then averaging the data and using the 

averaged height-sensor value for computing the profile data at 25-mm (1-inch) intervals. If the 

profilers were just getting height-sensor data at 25-mm (1-inch) intervals, there is always the 

possibility that a reading may not be obtained on top of a joint. However, because the height 

sensors are obtaining readings at much closer intervals than 25 mm (1 inch), a reading is always 

obtained on top of the joint sealant. When the height-sensor data are averaged to obtain a reading 

every 25 mm (1 inch), the reading obtained over the joint was of sufficient magnitude for the 

joint to be clearly seen in the filtered profile. 

Another interesting observation seen in figure 37 is that the profile data show the joint to be a 

feature that is spread over a distance of 75 mm (3 inches), when the actual width of the joint is 

on the order of 10 mm (0.4 inches). The joint appears like this in the profile data because of the 

averaging procedure that is used on the height-sensor data and possibly because of the 

application of an anti-aliasing filter onto the profile data. The averaging procedure and the antialiasing filter will also cause some attenuation in the magnitude of the depth of narrow 

downward features such as joints and cracks. 

Comparison of IRI Values 

An analysis of the IRI values obtained from the 2002 verification test was performed to compare 

IRI values obtained by the two profilers.(28) In the North Central region, testing by both profilers 

was performed on the same day. In the North Atlantic region, testing at six of the eight sites was 

performed on the same day by the two profilers, while in the Western region, a similar procedure 

was followed for three of the five sites. In the Southern region, testing at the sites by the ICC 

profiler was performed approximately 1.5 months after testing by the K.J. Law Engineers 

profiler. The IRI values obtained from the testing (average IRI from five runs) and the test dates 

are presented in appendix B. 

There were 23 test sites (46 wheelpaths) where IRI comparisons of the two profilers could be 

made. Figure 39 shows the IRI relationship between the two profilers, where data for 46 

wheelpaths are shown. There is very good agreement in IRI between the two profilers, with the 

correlation coefficient for the two sets of IRI values being 0.99.

53 
0.0

1.0

2.0

3.0

4.0

5.0

0.0 1.0 2.0 3.0 4.0 5.0

K.J. Law IRI (m/km)

ICC IRI (m/km)

Left

Right

Figure 39. Relationship between IRI from the K.J. Law Engineers and ICC profilers. 

The difference in IRI between the K.J. Law Engineers and ICC profilers (K.J. Law IRI – ICC 

IRI) was computed along each wheelpath for all test sections. The differences in IRI are shown 

in figure 40 as a function of the IRI for the wheelpath, where the IRI for the wheelpath was 

computed by averaging IRI obtained for that wheelpath by the ICC and K.J. Law Engineers 

profilers. 

1 m/km = 5.28 ft/mi 

1 m/km = 5.28 ft/mi 

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.0 1.0 2.0 3.0 4.0 5.0

IRI of Wheelpath (m/km)

Law IRI - IRI ICC (m/km)

Left

Right

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.0 1.0 2.0 3.0 4.0 5.0

IRI of Wheelpath (m/km)

Law IRI - IRI ICC (m/km)

Left

Right

Figure 40. Differences in IRI between the K.J. Law Engineers and ICC profilers. 

For the 46 cases, the difference in IRI was within ±0.10 m/km (±6 inches/mi) for 33 cases, 

between 0.10 and 0.20 m/km (6 and 13 inches/mi) for 6 cases, between −0.10 and −0.20 m/km 

(-6 and -13 inches/mi) for 4 cases, between −0.20 and −0.30 m/km (−13 and −19 inches/mi) for 2 

cases, and between 0.30 and 0.40 m/km (19 and 25 inches/mi) for 1 case. An investigation was 

54 
performed separately for each region to identify the cause of the difference in IRI between the 

two profilers for cases where the difference in IRI was outside ±0.10 m/km (±6 inches/mi). 

North Atlantic Region 

Figure 41 shows the difference in IRI between the K.J. Law Engineers and ICC profilers at the 

sections tested by the North Atlantic profilers as a function of the IRI for the wheelpath. 

North Atlantic

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.0 1.0 2.0 3.0 4.0 5.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

364018 251002

251002

245807

North Atlantic

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.0 1.0 2.0 3.0 4.0 5.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

364018 251002

251002

245807

1 m/km = 5.28 ft/mi 

Figure 41. Differences in IRI between the K.J. Law Engineers and ICC profilers: 

North Atlantic region. 

Differences in IRI between the K.J. Law Engineers and ICC profilers (K.J. Law IRI – ICC IRI) 

that were outside ±0.10 m/km (±6 inches/mi) were observed for the following four cases: (1) left 

wheelpath of site 251002 (a difference of −0.23 m/km (−15 inches/mi)), (2) right wheelpath of 

site 251002 (a difference of 0.17 m/km (11 inches/mi)), (3) left wheelpath of site 364018 (a 

difference of -0.23 m/km (−15 inches/mi)), and (4) left wheelpath of site 245807 (a difference of 

0.15 m/km (10 inches/mi)). 

Site 251002: At this site, the K.J. Law Engineers profiler obtained an IRI that was 0.23 m/km 

(15 inches/mi) lower than the IRI from the ICC profiler for the left wheelpath, and an IRI that 

was 0.17 m/km (11 inches/mi) higher than the IRI from the ICC profiler for the right wheelpath. 

Each profiler conducted nine runs on this section. The IRI for the nine runs from the ICC profiler 

ranged from 2.58 to 5.03 m/km (164 to 319 inches/mi) for the left wheelpath, and from 1.24 to 

1.57 m/km (79 to 100 inches/mi) for the right wheelpath. For the K.J. Law Engineers profiler, 

the IRI for the nine runs ranged from 2.56 to 4.81 m/km (162 to 305 inches/mi) for the left 

wheelpath, and from 1.50 to 1.67 m/km (95 to 106 inches/mi) for the right wheelpath. The left 

wheelpath of this section had significant distress. As indicated from the IRI range that was 

obtained for the repeat runs, significant variability in IRI can occur at this site because of 

variability in the profiled path. Investigation of the profile data indicated that the difference in 

IRI between the two profilers at this site was probably caused by differences in the profiled 

paths. 

55 
Site 364018: The K.J. Law Engineers profiler obtained an IRI that was 0.23 m/km 

(15 inches/mi) less than that obtained by the ICC profiler along the left wheelpath at this section. 

Each profiler conducted nine runs on this test section. The IRI for the runs for the ICC profiler 

ranged from 2.64 to 3.18 m/km (167 to 202 inches/mi) for the left wheelpath, and from 2.20 to 

2.39 m/km (139 to 151 inches/mi) for the right wheelpath. For the K.J. Law Engineers profiler, 

IRI ranged from 2.59 to 2.92 m/km (164 to 185 inches/mi) for the left wheelpath, and from 

2.23 to 2.36 m/km (141 to 149 inches/mi) for the right wheelpath. There was a major downward 

feature at a distance of 50 m (164 ft) along the left wheelpath that made a significant contribution 

to the roughness at this site. Variability in the profiled path that caused this feature to be 

measured differently had a significant effect on IRI. Investigation of the profile data and 

roughness profiles at this section indicated that the difference in IRI between the two profilers 

was caused by variability in the paths followed by the two profilers. 

Site 245807: Along the left wheelpath at this site, the IRI from the K.J. Law Engineers profiler 

was 0.15 m/km (10 inches/mi) lower than that obtained by the ICC profiler. An investigation of 

the profile data did not indicate a clear reason for the cause of this difference. 

North Central Region 

Figure 42 shows the difference in IRI between the K.J. Law Engineers and ICC profilers at the 

test sections tested by the North Central profilers as a function of the IRI for the wheelpath. 

Differences in the IRI between the K.J. Law Engineers and ICC profilers (K.J. Law IRI – ICC 

IRI) that were outside ±0.10 m/km (±6 inches/mi) were observed for the following three cases: 

(1) right wheelpath of site 17A002 (a difference of 0.15 m/km (10 inches/mi)), (2) left wheelpath 

of site 17A005 (a difference of 0.35 m/km (22 inches/mi)), and (3) right wheelpath of site 

17A005 (a difference of −0.13 m/km (−8 inches/mi)). 

North Central

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.0 1.0 2.0 3.0 4.0 5.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

17A005

17A005

17A002

North Central

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.0 1.0 2.0 3.0 4.0 5.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

17A005

17A005

17A002

1 m/km = 5.28 ft/mi 

Figure 42. Differences in IRI between the K.J. Law Engineers and ICC profilers: 

North Central region. 

56 
Site 17A002: The IRI from the K.J. Law Engineers profiler was higher than that of the ICC 

profiler by 0.15 m/km (10 inches/mi) along the right wheelpath at this site. However, along the 

left wheelpath, the IRI from the K.J. Law Engineers profiler was 0.10 m/km (6 inches/mi) lower 

than that obtained by the ICC profiler. An investigation of the profile data did not indicate a 

definitive cause for the difference in IRI between the profilers. However, variability in the 

wheelpath is a likely cause for the difference in IRI. In this case, the difference in IRI between 

the K.J. Law Engineers and ICC profilers had opposite signs for the wheelpaths (negative for the 

left wheelpath and positive for the right wheelpath). This is an indication that the two profilers 

followed different wheelpaths. 

Site 17A005: The IRI from the K.J. Law Engineers profiler was higher than that obtained by the 

ICC profiler by 0.35 m/km (22 inches/mi) along the left wheelpath; along the right wheelpath, 

the IRI from the K.J. Law Engineers profiler was lower than that obtained by the ICC profiler by 

0.13 m/km (8 inches/mi). An investigation of the profile data did not indicate a definitive cause 

for the difference in IRI between the profilers. Since there was a reversal in signs for the 

difference in IRI for the two wheelpaths as in the previous case, the differences in IRI between 

the two profilers at this site were probably cased by variability in the paths followed by the two 

profilers. 

Southern Region 

Figure 43 shows the difference in IRI between the K.J. Law Engineers and ICC profilers at the 

sections tested by the Southern profilers as a function of the IRI for the wheelpath. 

Differences in the IRI between the K.J. Law Engineers and ICC profilers (K.J. Law IRI – ICC 

IRI) that were outside ±0.10 m/km (±6 inches/mi) were observed for the following three cases: 

(1) left wheelpath of site 48B350 (a difference of 0.17 m/km (11 inches/mi)), (2) right wheelpath 

of site 48B350 (a difference of −0.13 m/km (−8 inches/mi)), and (3) right wheelpath of site 

485253 (a difference of 0.11 m/km (7 inches/mi)). The data recorded by the ICC profiler could 

not be converted to obtain the 25-mm (1-inch) data, thus, a comparison of the profiles between 

the ICC and K.J. Law Engineers profilers could not be performed. 

57 
Southern Region

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.0 0.5 1.0 1.5 2.0 2.5 3.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

48B350

48B350

485253

Southern Region

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.0 0.5 1.0 1.5 2.0 2.5 3.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

48B350

48B350

485253

1 m/km = 5.28 ft/mi 

Figure 43. Differences in IRI between the K.J. Law Engineers and ICC profilers: 

Southern region. 

At site 48B350, the IRI from the K.J. Law Engineers profiler was higher than that obtained by 

the ICC profiler by 0.17 m/km (11 inches/mi) along the left wheelpath; however, along the right 

wheelpath, the IRI from the K.J. Law Engineers profiler was lower than that obtained by the ICC 

profiler by 0.13 m/km (8 inches/mi). Since the difference in the IRI between the two profilers 

had opposite signs for the left and right wheelpaths, variability in the paths followed by the two 

profilers is a likely cause of the difference in the IRI between the two profilers. 

Western Region 

Figure 44 shows the difference in IRI between the K.J. Law Engineers and ICC profilers at the 

sections tested by the Western profilers as a function of the IRI for the wheelpath. 

Differences in IRI between the K.J. Law Engineers and ICC profilers (K.J. Law IRI – ICC IRI) 

that were outside ±0.10 m/km (±6 inches/mi) were observed for the following three cases: 

(1) left wheelpath of site 320209 (a difference of -0.18 m/km (11 inches/mi)), (2) right wheelpath 

of site 320209 (a difference of -0.15 m/km (−10 inches/mi)), and (3) right wheelpath of site 

067454 (a difference of 0.13 m/km (8 inches/mi)). 

Site 320209: The IRI from the K.J. Law Engineers profiler was lower than that obtained by the 

ICC profiler by 0.18 m/km (11 inches/mi) and 0.15 m/km (10 inches/mi) along the left and right 

wheelpaths, respectively. The two profilers measured this section on different dates. An 

evaluation of the profile data indicated that the amount of slab curling present when the ICC 

profiler profiled the section was slightly higher than the curling that was present when the site 

was profiled by the K.J. Law Engineers profiler. The higher IRI obtained by the ICC profiler is 

attributed to higher slab curling. 

58 
Western Region

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.0 0.5 1.0 1.5 2.0 2.5 3.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

320209

67454

Western Region

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.0 0.5 1.0 1.5 2.0 2.5 3.0

IRI of Wheelpath (m/km)

Law IRI - ICC IRI (m/km)

Left

Right

320209

67454

1 m/km = 5.28 ft/mi 

Figure 44. Differences in IRI between the K.J. Law Engineers and ICC profilers: Western region. 

Site 067454: The IRI from the K.J. Law Engineers profiler was higher than that obtained by the 

ICC profiler by 0.13 m/km (8 inches/mi) along the right wheelpath. Each profiler conducted nine 

runs on this section. The right-wheelpath IRI from the ICC profiler for these runs ranged from 

2.17 to 2.39 m/km (138 to 152 inches/mi), while the IRI from the K.J. Law Engineers profiler for 

the same wheelpath ranged from 2.31 to 2.48 m/km (146 to 157 inches/mi). As seen from these 

values, there is some overlap in the IRI values obtained by the two profilers. Analysis of the 

profile data did not indicate a clear reason why the K.J. Law Engineers profiler IRI would be 

higher. Variability in the profiled paths may be a reason why the IRI values were different 

between the devices. 

Cross Correlation of IRI 

The 25-mm (1-inch) profile data collected by the two North Central profilers during the 2002 

regional verification test were used in this analysis. The first profile run conducted by each 

profiler at the test sites was selected for analysis. The T-6600 profiler was considered to be the 

“correct” device and, thus, the analysis will indicate how well the ICC profiler reproduced the 

results obtained by the T-6600 profiler. No markings were present at the test sections, and the 

driver of each profiler visually judged the location of the wheelpaths when profiling the test 

sections. Any variations in the profiled paths between the two profilers will result in lower crosscorrelation values. The results from the cross-correlation analysis are presented in table 7. 

Good cross-correlation values were obtained for the T-6600 and ICC profilers, which indicate 

good agreement in IRI magnitude and IRI distribution between the two profilers. It should be 

noted that other combinations of runs used for cross-correlation analysis could give different 

results. 

59 
Table 7. Results of cross correlation between the K.J. Law Engineers T-6600 and ICC profilers. 

Site IRI (m/km) Cross Correlation 

 Left Wheelpath Right Wheelpath Left Right 

 K.J. Law ICC K.J. Law ICC Wheelpath Wheelpath 

1: Asphalt 1.05 0.96 1.19 1.20 0.81 0.94 

2: Asphalt 2.71 2.86 2.87 2.72 0.91 0.91 

3: Concrete 1.11 1.10 1.18 1.19 0.82 0.80 

4: Concrete 4.02 4.11 4.14 4.19 0.91 0.93 

5: Chip Seal 3.39 3.01 3.85 4.00 0.78 0.85 

1 m/km = 5.28 ft/mi 

ANOVA and Regression Analysis of IRI 

An ANOVA was performed using the IRI data obtained from the 2002 regional verification test. 

There were 23 sites used for testing, and this provided data for 46 cases (23 sites x 2 wheelpaths 

per site) where an IRI comparison could be performed for the K.J. Law Engineers and ICC 

profilers. For each case, IRI values for five repeat runs were available for both the ICC and K.J. 

Law Engineers profilers. The ANOVA indicated that the profilers were not significant at a 

significance level of 0.05. 

A regression analysis was performed for the IRI from the T-6600 and ICC profilers. The IRI for 

the five runs that were selected at each section to compute the average IRI value for the IRI 

comparison of the two profilers were used in the regression. This provided 230 pairs of data for 

the regression (i.e., 23 sections x 2 wheelpaths x 5 repeat runs). The following relationship was 

obtained from the regression: 

 (2) IRI (ICC) = 1.006 IRI (K.J. Law) – 0.018

where: 

• IRI (ICC) = IRI from ICC profiler for a wheelpath (m/km). 

• IRI (K.J. Law) = IRI from K.J. Law Engineers T-6600 profiler for a wheelpath (m/km). 

• R2 = 0.96, standard error = 0.17. 

The regression equation indicated very good agreement in IRI between the two profilers. 

EFFECTS OF APPLYING A MOVING AVERAGE ONTO PROFILE DATA 

The DNC 690 profilers collected profile data at 25.4-mm (1-inch) intervals and then applied a 

304.8-mm (12-inch) moving average onto the data and recorded profile data at 152.4-mm (6-

inch) intervals. Profile data collected at 25-mm (1-inch) intervals at LTPP test sections are 

available for both the T-6600 and ICC profilers. However, currently, in the LTPP program, these 

25-mm (1-inch) profile data are processed using the ProQual software, which applies a 300-mm 

60 
(11.8-inch) moving average onto the 25-mm (1-inch) profile data, and extracts profile data at 

150-mm (5.9-inch) intervals. The IRI values for the LTPP sections are computed using this 

averaged data, and the averaged data are uploaded to the LTPP database. 

The application of the 300-mm (11.8-inch) moving average onto the 25-mm (1-inch) data 

attenuates very short-wavelength features that are present in the profile, and can also distort 

profile features that are actually present in the pavement. The averaged profile can show features 

that do not actually appear in the pavement, while not showing features that are actually present 

in the pavement. In this section, several examples that show the distortion caused in profile 

features by the application of the moving average are presented. 

Faulted Pavement 

Figure 45 shows how the application of the moving average distorts the profile data collected 

over a fault. This figure shows the 25-mm (1-inch) interval data collected by the North Central 

T-6600 profiler over a faulted crack at the rough PCC section during the 1996 profiler 

verification study. The averaged data also are shown in this figure. The 25-mm (1-inch) data 

clearly define the fault, which is about 13 mm (0.5 inches). However, the application of the 

moving average makes the fault appear as a ramp, where there is a gradual change in elevation of 

about 7 mm (0.3 inches) that occurs over a distance of 0.3 m (1 ft). As seen in this example, the 

application of the moving average distorts the profile and shows a feature that does not actually 

exist on the pavement. 

-20

-15

-10

-5

0

136 136.5 137 137.5

Distance (m)

Elevation (mm)

25 mm 

150 mm

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 45. Profile distortion caused by the application of a moving average onto data collected 

over a fault. 

Effects of Downward Features 

The rough AC section that was used in the 2003 LTPP profiler comparison conducted in 

Minnesota had cracks that had been patched full width across the lane. Figure 46 shows the 

profile data obtained by the North Atlantic ICC profiler over such a patched crack. The figure 

61 
shows the 25-mm (1-inch) data and the averaged data after the 25-mm (1-inch) data were 

processed using ProQual. The 25-mm (1-inch) data indicate that the patched crack is about 9 mm 

(0.4 inches) deeper than the adjacent pavement area. The application of the moving average onto 

the 25-mm (1-inch) data causes the depth of the patched crack to be reduced. 

Distance (m)

-14

-12

-10

-8

-6

-4

-2

0

128 128.5 129 129.5 130

Elevation (mm)

25-mm Data

Averaged 150 mm 

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 46. Profile distortion caused by the application of a moving average onto data collected 

over a patched crack. 

Figure 47 shows the 25-mm (1-inch) data and the averaged 150-mm (5.9-inch) interval data at 

the chip-seal section that was used in the 2003 LTPP profiler comparison. These data were 

collected by the North Atlantic ICC profiler. The 25-mm (1-inch) data show a crack that is at a 

distance of about 0.9 m (3 ft) as a sharp and narrow downward feature. However, the averaged 

data distorts the shape of the crack and makes the crack appear as a dip that is spread over a 

much wider length. The small variations between the profile data points that are seen in the 

25-mm (1-inch) data are not seen in the averaged data. These variations are smoothed out by the 

application of the moving average. 

62 
Distance (m)

-24

-22

-20

-18

-16

-14

-12

0.5 0.7 0.9 1.1 1.3 1.5

Elevation (mm)

25-mm Data

Averaged 150 mm

25.4 mm = 1 inch 

Figure 47. Profile distortion caused by the application of a moving average onto data collected 

over a crack. 

1 m = 3.28 ft 

Figure 48 shows profile data collected by the Western ICC profiler at site 3, which is a concrete 

section, during the 2003 profiler comparison in Minnesota. This figure shows both the 25-mm 

(1-inch) data and the averaged data after the 25-mm (1-inch) data were processed using ProQual. 

The 25-mm (1-inch) data clearly show the locations of the joints in the concrete pavement as 

downward spikes that occur at regular intervals. However, these features are not seen in the 

averaged profile because the averaging process attenuates the sharp downward features seen at 

the joints. 

63 
0

5

10

15

20

25

30

35

0 50 100 150

Distance (m)

Elevation (mm)

25 mm

Averaged 150 mm

Profiles Offset

25 mm

Averaged 150 mm

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 48. Application of a moving average onto data collected for a concrete pavement. 

Effects of Sharp Upward Features 

Figure 49 shows a portion of the profile data collected by the North Central T-6600 profiler at 

section 3, which is a PCC section, during the 1996 regional verification test. The figure shows 

the 25-mm (1-inch) data and the averaged data after ProQual had processed the 25-mm (1-inch) 

data. The profile contains a sharp upward feature about 2.5 mm (0.1 inch) in height near 62 m 

(203 ft). The application of the moving average eliminates this feature. The application of a 

moving average onto a sharp upward feature that has a greater magnitude than the shown feature 

will cause the feature to appear in the averaged data as a feature that has a much lesser 

magnitude that is spread out over a much greater distance than the actual feature. The profile 

shown in figure 49 also shows a narrow downward feature between 65 and 66 m (213 and 

216 ft). This feature also does not appear in the averaged data. 

Smooth Asphalt Pavement 

Figure 50 shows a plot that contains profile data at 25-mm (1-inch) intervals, and the same data 

after it had been processed using ProQual. The profile shown in figure 50 contains data collected 

by the Western profiler along the left wheelpath at the smooth AC site during the 2003 LTPP 

profiler comparison. This pavement section is a smooth pavement, and there is no distress within 

the limits of the profile plot shown in figure 50. Both the 25-mm (1-inch) data and the averaged 

150-mm (5.9-inch) data overlay well, except that the small spike seen at 16 m (52 ft) does not 

appear in the averaged data. 

64 
Distance (m)

-6

-5

-4

-3

-2

-1

0

60 62 64 66 68

Elevation (mm)

25 mm

Averaged 150 mm

Averaged 150 mm

25 mm

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 49. Application of a moving average onto a profile containing a sharp upward feature. 

-3

-2

-1

0

1

2

3

10 12 14 16 18 20

Distance (m)

Elevation (mm)

25 mm

150 mm

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 50. The 25-mm (1-inch) data and 150-mm (5.9-inch) averaged data from a smooth 

AC section. 

Figure 51 shows the same data, but with the two profiles being offset. The averaged 150-mm 

(5.9-inch) interval profile does not show the profile details that are seen in the 25-mm (1-inch) 

data. The averaged profile shows a smoothened profile, with the profile details that are seen in 

the 25-mm (1-inch) data being eliminated by the moving average. 

65 
SUMMARY OF THE FINDINGS 

Data recorded by inertial profilers do not accurately portray very narrow features such as cracks 

or joints in PCC pavements because of the low-pass filtering that is performed on the data. 

Evaluation of 25-mm (1-inch) data collected by both the T-6600 and ICC profilers over a joint in 

a PCC pavement showed that the joint appeared in the profile as a feature that was spread over a 

distance of 75 mm (3 inches), when the width of the joint was actually closer to 10 mm 

(0.4 inches). Although 25-mm (1-inch) interval data are collected by both the T-6600 and ICC 

profilers, the height sensors in the profilers collect data at much closer intervals and then average 

the data when computing profile data at 25-mm (1-inch) intervals. This causes the magnitude of 

a narrow feature that is recorded in the profile to be less than the actual magnitude, and also 

causes it to be spread out over a much wider distance than the actual feature. The application of 

an anti-aliasing filter onto the profile data can also have the same effect. 

-3

-2

-1

0

1

2

3

4

5

10 12 14 16 18 20

Distance (m)

Elevation (mm)

25 mm

Averaged 150 mm

25 mm

Averaged 150 mm

Profiles Offset

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 51. Offset profile plot of 25-mm (1-inch) data and averaged 150-mm (5.9-inch) data 

collected from a smooth AC pavement. 

Since the DNC 690 profiler recorded profile data at 152.4-mm (6-inch) intervals, when 

comparing data from the T-6600 profiler with the DNC 690 profiler, only the 150-mm (5.9-inch) 

interval ProQual-processed data from the T-6600 profiler can be used to perform a meaningful 

comparison. Comparison of the profile plots for the two profilers showed good agreement, 

although there were some differences between the profiles for sections that had significant longwavelength content. This difference is attributed to the different long-wavelength cutoff filter 

values used for the two profilers (91 m (300 ft) for the DNC 690 profiler and 100 m (328 ft) for 

the T-6600 profiler). An evaluation of the profile data indicated that the long-wavelength cutoff 

filtering technique used in both the DNC 690 and T-6600 profilers appears to be similar. There 

was very good agreement in IRI values for the DNC 690 and T-6600 profilers. 

Since 25-mm (1-inch) interval data were available for both the T-6600 and ICC profilers, a 

comparison of 25-mm (1-inch) interval data for the two profilers could be performed. Evaluation 

66 
of the profile data using PSD plots indicated that there was good agreement in the profile data for 

the two profilers for wavelengths between 1 and 40 m (3 and 131 ft). For wavelengths less than 

1 m (3 ft), the ICC profiler usually showed a higher wavelength content than the T-6600 profiler. 

This is attributed to the smaller footprint of the ICC profiler, which probably caused more texture 

effects and the higher magnitude of narrow features to be recorded. For wavelengths greater than 

40 m (131 ft), the T-6600 profiler recorded more wavelength content than the ICC profiler. This 

is attributed to differences in the long-wavelength filtering techniques that are used by the two 

profilers. When ProQual-processed data for the two profilers were compared using a PSD plot, 

only the differences at the higher wavelengths will be seen, with good agreement being obtained 

between the two profilers for wavelengths less than 1 m (3 ft). This occurs because the 

application of the moving average attenuates the short-wavelength features. Good agreement in 

IRI, which is primarily influenced by wavelengths between 1 and 30 m (3 and 100 ft), was 

obtained for data collected by the ICC and T-6600 profilers. 

In the LTPP program, the 25-mm (1-inch) data obtained from the T-6600 and ICC profilers are 

processed using ProQual. ProQual applies a 300-mm (11.8-inch) moving average onto the 

25-mm (1-inch) data and extracts data at 150-mm (5.9-inch) intervals, and these data are 

uploaded to the LTPP database. The application of the moving average onto the 25-mm (1-inch) 

data attenuates features with wavelengths less than 1 m (3 ft). Detailed profile features cannot be 

observed in the ProQual-processed data because of this effect. The moving average can also 

distort the profile data, with the averaged data showing features that are not actually present in 

the pavement, while eliminating features that are actually present. 

67 

CHAPTER 6: DIFFERENCES BETWEEN DIPSTICK AND PROFILER IRI 

INTRODUCTION 

In this chapter, a description of the factors that cause IRI obtained from the Dipstick data to 

differ from IRI obtained from the profiler data is presented. The range of the difference in IRI 

that can occur between Dipstick and profiler IRI, obtained from an analysis of data from past 

LTPP comparison studies, also is presented in this chapter. 

FACTORS CONTRIBUTING TO DIFFERENCES BETWEEN DIPSTICK AND 

PROFILER IRI 

There are a variety of factors that can cause IRI obtained from the Dipstick data to differ from 

IRI obtained at the same section by an inertial profiler. These factors are: 

• Sampling qualities of Dipstick. 

• Variability in the path followed by a profiler. 

• Features recorded by the profiler that are missed or underestimated by Dipstick. 

• Averaging effects of profiler data. 

• Dipstick data errors. 

• IRI computation procedures for Dipstick data. 

A description of how each of these factors can contribute to differences in the IRI values 

between Dipstick and the profilers is presented in the following sections. 

Sampling Qualities of Dipstick 

The footpad spacing (i.e., the distance between the center of the footpads) for Dipstick and the 

footprint of Dipstick (the area covered by a footpad) affect the measurements obtained by 

Dipstick. 

The diameter of the footpads in the Dipsticks that are used in the LTPP program is 

approximately 32 mm (1.25 inches). The footpad spacing in Dipstick can be adjusted, with the 

maximum spacing usually being 304.8 mm (12 inches) or 300 mm (11.8 inches) for devices with 

a base plate set to metric units or U.S. customary units, respectively. The footpad spacing of the 

Dipsticks that are used in the LTPP program is 304.8 mm (12 inches). In the following 

discussion, the footpad spacing of Dipstick is assumed to be set to this value. 

Gain Characteristics of Dipstick 

Dipstick will have a varying response to sinusoids of different wavelengths. Consider the 

response of Dipstick to a sinusoidal road feature located on a road with zero slope that has a 

wavelength equal to the footpad spacing of Dipstick as shown in figure 52. No matter where 

69 
Dipstick is placed on this sinusoid, Dipstick will give a reading of zero, because the two 

supporting feet will have the same elevation. Dipstick measurements taken along a roadway that 

has such a feature will not capture the feature, and will simply give a straight line for the 

elevation profile. In the case of a road with a slope that has such a feature, the Dipstick 

measurements will only show the slope of the road, and will not show the sinusoidal feature 

present on the road. 

Figure 52. Dipstick response to a sinusoid with a wavelength equal to the footpad spacing of 

Dipstick. 

The response of Dipstick to different wavelengths can be studied by using a gain plot. The 

following procedure is used to generate the gain plot: 

1. Select a sinusoid that has a specific wavelength and specific amplitude. Assume that the 

amplitude of the sinusoid is A. 

2. Simulate placing Dipstick at the start of the sinusoid and compute the reading that would 

be obtained by Dipstick (i.e., the difference in elevation between the front and back 

footpads). 

3. Again place Dipstick on the sinusoid so that the current position of Dipstick is slightly 

ahead (e.g., 25 mm (1 inch)) of the previous location. Compute the reading that would be 

obtained by Dipstick. Repeat this procedure until Dipstick has been placed for all 

positions in the sinusoid, covering an entire wavelength. 

4. After completing this exercise, use the readings that Dipstick obtained for each Dipstick 

position to generate the profile (sinusoid) that was recorded by Dipstick. 

5. Divide the maximum amplitude of the generated profile by the amplitude of the sinusoid 

that was used to generate this profile (i.e., A) to obtain the gain for that wavelength. 

70 
6. Repeat steps 1 through 5 for different wavelengths. 

Note: This is not the procedure that is used when Dipstick is used to obtain measurements. The 

exercise was performed to obtain the gain characteristics of Dipstick. 

The gain plot that was obtained by the previously described procedure is shown in figure 53. As 

discussed previously, the gain is zero for a wavelength of 0.305 m (1 ft). For wavelengths above 

0.305 m (1 ft), the gain gradually increases as the wavelength increases. For a wavelength of 

0.61 m (2 ft), which is approximately twice the footpad spacing of Dipstick, the gain is 0.63. For 

Dipstick to measure an amplitude that is more than 95 percent of the correct amplitude of a 

sinusoid, the wavelength of the sinusoid should be more than 2 m (7 ft). As shown in figure 53, 

the footpad spacing of Dipstick will also cause it to underestimate wavelengths that are shorter 

than the footpad spacing. 

.1 2 5 1 2 5 10

Wavelength (m)

0

0.2

0.4

0.6

0.8

1.0

Gain (1)

1 m = 3.28 ft 

Figure 53. Gain plot of Dipstick. 

Sampling Interval 

Sampling data at discrete intervals limits the range of wavelengths that may be recognized. A 

common rule that is used to characterize the sampling effect is the Nyquist Sampling Theorem. 

This theorem states that the highest frequency that can be accurately represented from discretely 

sampled data is less than one-half of the sampling rate. For this statement of the theorem, the 

sampling rate is expressed in samples per second. When it is restated for road profiles, the 

theorem indicates that the shortest wavelength that can be represented by discretely sampled data 

is longer than twice the sampling interval. Applying this rule to a Dipstick with a footpad 

spacing of 0.305 m (1 ft) indicates that wavelengths shorter than 0.61 m (2 ft) will not be 

represented in the measurements obtained with Dipstick. 

71 
The sampling interval of Dipstick also may cause features with wavelengths shorter than twice 

the sampling interval to contaminate the measurement of longer wavelength features through a 

process called aliasing. Aliasing occurs when short-wavelength features are inadvertently 

interpreted to be longer wavelength features. For example, consider a sinusoid with a wavelength 

that is 11 percent longer than the sampling interval of Dipstick, as shown in figure 54. The first 

sample detects the highest point in the sinusoid; however, each successive point misses the peak 

by a progressively greater distance. The consequence is a gradual transition from the peak level 

to the valley over a long distance. Thus, a Dipstick having a sampling interval of 0.305 m (1 ft) 

that is measuring a sinusoid with a wavelength of 0.339 m (1.1 ft) would obtain a sinusoid of 

roughly equal amplitude, but having a wavelength of 3.05 m (10 ft). 

Figure 54. An example of aliasing. 

In cases where the wavelength of the sinusoid is shorter than the sampling interval of Dipstick, a 

much longer wavelength than that actually present also will appear in the measurement. For 

example, a sinusoid with a wavelength that is 10 percent shorter than the sampling interval 

would also be misinterpreted as a feature with a wavelength of about 3.05 m (10 ft). On the other 

hand, features with wavelengths that are just shorter than double the sampling interval will be 

aliased into features with wavelengths that are just longer than twice the sampling interval. 

The way to avoid aliasing errors is to sample much more often than the shortest wavelength of 

interest, then apply a low-pass filter to remove the contents within the signal that are not of 

interest. However, such a procedure cannot be used with the Dipstick. The overall consequence 

of aliasing by Dipstick is that contents within the profile in a wavelength range shorter than 0.61 

m (2 ft) are “folded” into the longer wavelength range. This causes an upward bias in the IRI 

value by artificially increasing the contents within the profile that fall within the wavelength 

range that affects IRI. The precise level of upward bias depends on the properties of the road 

surface, and this effect is much greater on pavement with a high level of megatexture or coarse 

macrotexture. 

The probable effect of aliasing on IRI for a sampling interval of 0.3 m (1 ft) was estimated in a 

recent study of profile sampling procedures.(1) This study showed that the upward bias in IRI 

because of aliasing is probably on the order of 7 to 9 percent. The probable error level in this 

study was estimated by decimating profiles collected with the FHWA PRORUT profiler at two 

test sections. It should be noted that the treatment of very short road features by the feet of 

72 
Dipstick might be different than the procedure used by the sensor footprint of the profiler. The 

anti-aliasing that had been used for the profile that was used in the study may impact the 

conclusions of that study. It should also be noted that data from only two sites were used in this 

analysis. Therefore, the 7 to 9 percent error level is only a rough estimate. 

Dipstick Footprint 

Dipstick contacts the pavement at two locations (spaced 304.8 mm (12 inches) apart) with rigid 

circular feet that are approximately 32 mm (1.25 inches) in diameter. The feet are attached to the 

base of Dipstick with a ball joint so that the footpads are most likely to rest upon the three 

highest points within the footprint of the footpad. An important property of this type of footprint 

is the ability to bridge over narrow cracks and rest on small pieces of protruding aggregate. If the 

macrotexture is not very deep, the footprint may serve as a mechanical filter that removes the 

potential for aliasing error caused by wavelengths on the order of 32 mm (1.25 inches) and 

shorter. 

Variations in the Path Followed by the Profiler 

During profiler comparison studies, Dipstick measurements at test sections are performed along 

the two wheelpaths. In LTPP comparison experiments, the wheelpaths are laid out so that each 

wheelpath is at a distance of 0.826 m (2.7 ft) from the center of the lane.(6) Where wheelpaths are 

easily identified, the midway point between the two wheelpaths is defined to be the center of the 

lane. If the wheelpaths cannot be clearly identified, but the two lane edges are well defined, the 

center of the travel lane is considered to be midway between the lane edges. Chalk lines are laid 

out along the wheelpaths when performing Dipstick measurements. 

The wheelpaths were marked with paint dots at all sections during the 1991 and 2003 LTPP 

profiler comparisons, while two sections used for the 2000 LTPP profiler comparison had the 

wheelpaths marked. The paint dots provided a guide so that the driver could align the profiler 

along the wheelpaths when profiling the test sections. During other LTPP profiler comparisons, 

the drivers had to judge the location of the wheelpath when profiling the test sections. 

If the profiler driver does not align the profiler along the wheelpaths, the longitudinal path that is 

followed by the sensors in the profiler will be different from the path where Dipstick 

measurements were obtained. Also, if the profiler driver does not consistently follow the 

wheelpaths within the section, but has some lateral wander, this will result in differences in the 

paths measured by the profiler and Dipstick. Both of these conditions can cause IRI obtained 

from profiler measurements to differ from that obtained from Dipstick measurements. A 

discussion of lateral wander and the effects on IRI were presented in chapter 5. As shown in 

table 3, at some sections, lateral wander can have a significant effect on IRI. 

Figure 55 shows the 10-m (33-ft) base-length roughness profiles for the left wheelpath obtained 

for nine repeat runs performed by the Western profiler at test section 1, which is a smooth AC 

section, during the 2003 LTPP profiler comparison in Minnesota. The roughness profiles overlay 

very well, with the IRI for the entire section for the nine runs ranging from 1.29 to 1.33 m/km 

(82 to 84 inches/mi). The roughness profiles imply that either the profiler driver followed the 

73 
same path for all nine runs or that minor variations in the wheelpath during profiling did not 

affect IRI. 

10-m Roughness Profiles

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 80 100 120 140

Distance (m)

IRI (m/km)

1 m = 3.28 ft 

1 m/km = 5.28 ft/mi 

Figure 55. Roughness profiles for nine runs that show good agreement. 

Thus, if the driver was tracking the wheelpath correctly, close agreement between profiler IRI 

and Dipstick IRI is expected at this site. However, the Dipstick IRI for the left wheelpath was 

1.17 m/km (74 inches/mi), which is less than the IRI obtained from the profiler data. A 

comparison of the roughness profiles obtained by the profiler and Dipstick showed that the 

roughness profiles were different in a few localized areas. An evaluation of the profiles indicated 

that the profiler data had a deep narrow feature that was not captured by the Dipstick 

measurements, and omission of this feature in the Dipstick data was the primary reason why the 

IRI from the Dipstick data was lower than the IRI from the profiler data for this section. 

Figure 56 shows the 10-m (33-ft) base-length roughness profiles along the right wheelpath for 

two runs of the Western profiler at test section 1, which is a smooth AC section, during the 2003 

LTPP profiler comparison that was held in Minnesota. The two roughness profiles show good 

agreement, except at three high roughness locations: (1) between 40 and 50 m (131 and 164 ft), 

(2) between 60 and 75 m (197 and 245 ft), and (3) between 85 and 100 m (279 and 328 ft). IRI 

for the two runs shown in this figure are 1.62 m/km (103 inches/mi) for run 4 and 1.79 m/km 

(113 inches/mi) for run 2. The difference in the IRI between the two runs occurs because of 

differences in the pavement features that are measured during the two runs within the three rough 

areas that were described previously. Therefore, variations in the path profiled at this site can 

have a significant influence on the IRI obtained for the right wheelpath. At sites having such 

characteristics, variations in the path profiled by the profiler compared to the path where 

Dipstick data were collected can cause significant differences between profiler IRI and Dipstick 

IRI. 

Figure 57 shows 10-m (33-ft) base-length roughness profiles for a single run by the North 

Atlantic, North Central, and Western profilers along the left wheelpath of section 4, which is a 

PCC section that was used for the 2003 LTPP profiler comparison. 

74 
10-m Roughness Profiles

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 20 40 60 80 100 120 140

Distance (m)

IRI (m/km)

Run 2

Run 4

1 m = 3.28 ft 

1 m/km = 5.28 ft/mi 

Figure 56. Roughness profiles for two profile runs that show variations. 

10-m Roughness Profiles

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 25 50 75 100 125 150

Distance (m)

IRI (m/km)

N. Atlantic

N. Central

Western

N. Central

N. Atlantic

Western

1 m = 3.28 ft 

1 m/km = 5.28 ft/mi 

Figure 57. Roughness profiles along the left wheelpath for three profilers. 

The roughness profiles from the three profilers agree well with each other throughout the section, 

except between 25 and 40 m (82 and 131 ft). This indicates that variations in the path followed 

by the three profilers within these limits are causing differences in the pavement features that are 

measured. This will cause different IRI values to be obtained for this wheelpath by the three 

profilers. Thus, IRI obtained by the three profilers will differ from that obtained by Dipstick. 

Features Recorded by the Profiler That Are Missed or Underestimated by Dipstick 

75 
Dipstick obtains measurements at 304.8-mm (12-inch) intervals and is equipped with footpads 

that are 32 mm (1.25 inches) in diameter. The profilers that have a recording interval of 25 mm 

(1 inch) will record 12 measurements within the distance between the Dipstick footpads. Thus, 

Dipstick can miss features that are measured by a profiler. 

In addition, Dipstick will not measure narrow cracks or joints in a PCC pavement when a 

Dipstick footpad is placed over such a feature, since the footpad will bridge over such features. 

However, a profiler will measure such features. When computing IRI, the IRI algorithm will 

treat a narrow downward feature the same as if that feature were upward. Features missed 

because of the footpad spacing and bridging over of narrow downward features can cause IRI 

obtained from Dipstick measurements to be less than that obtained from profiler measurements. 

Figure 58 shows profile plots for (1) data collected along the left wheelpath at section 5 (chipseal section) during the 2003 LTPP profiler comparison by the North Atlantic ICC profiler at 

25-mm (1-inch) intervals, (2) the same data after being processed using ProQual (i.e., moving 

average applied), and (3) data collected by Dipstick after all data sets have been subjected to a 

3-m (10-ft) high-pass filter. When the 25-mm (1-inch) profile data are processed using ProQual, 

the depth of the cracks in the resulting averaged profile will be attenuated. Also, narrow cracks 

that appear in the 25-mm (1-inch) data will be more spread out, and can appear as a slight dip 

rather than a narrow crack. 

3-m High Pass Filtered

0

5

10

15

20

25

30

110 120 130 140

Distance (m)

Elevation (mm)

Figure 58. Measurement of cracks by a profiler and Dipstick. 

At a distance of 120.5 m (395 ft), there is a crack in the pavement that is seen in the profile data; 

however, this crack is not seen in the Dipstick data, either because the Dipstick footpad bridged 

the crack or the crack was between the contact points of the Dipstick footpad. Sometimes the 

pavement area adjacent to a crack has a slight dip because of settlement close to the crack. If the 

footpad of Dipstick falls within this settled area, the Dipstick profile will show a slight dip at 

such locations. The Dipstick profile shown in figure 58 does show a slight dip close to many 

Top - 25 mm

Middle - 150 mm

Bottom - Dipstick

3-m High-Pass Filtered

25 mm Profiles Offset

150 mm

Dipstick

25.4 mm = 1 inch 

1 m = 3.28 ft 

76 
crack locations. However, as seen at a distance of 135 m (443 ft), the depth of the dip that is seen 

in the Dipstick profile is much less than that seen in the profile data. 

Figure 59 shows 3-m (10-ft) high-pass filtered profile plots for data collected along the right 

wheelpath at section 1 (smooth AC section) during the 2003 LTPP profiler comparison by 

Dipstick, the Western ICC profiler at 25-mm (1-inch) intervals, and the 25-mm (1-inch) data 

after being processed using ProQual (i.e., moving average applied to the data). The 25-mm 

(1-inch) profile data show a narrow downward feature that has an approximate depth of 10 mm 

(0.4 inches). The averaged 150-mm (5.9-inch) data show that applying the moving average onto 

the 25-mm (1-inch) data reduces the depth of this feature, and causes the feature to spread out 

over a much wider distance. The Dipstick data does capture this feature; however, the depth that 

it records for the feature is less than the depth of the feature that is seen in both the 25-mm 

(1-inch) and 150-mm (5.9-inch) data. 

Figure 59. Measurement of a downward feature by a profiler and Dipstick. 

he missing of features by Dipstick as illustrated in figure 58, and the underestimation of the 

veraging Effects of Profiler Data 

 the LTPP profiler comparison studies, the IRI values for profile data obtained from the 

re 

y 

ata 

3-m High Pass Filtered

0

5

10

15

20

25

119 120 121 122 123 124 125

Distance (m)

Elevation (mm)

T

depth of downward features by Dipstick as illustrated in figures 58 and 59, will cause IRI 

obtained from Dipstick data to be lower than IRI obtained from the profiler data. 

A

In

profilers were computed using the ProQual software. Data from the DNC 690 profilers we

recorded at 152.4-mm (6-inch) intervals, and a moving average had been applied to this data b

the profiler software before the data were recorded. When computing IRI values for the 

DNC 690 data, the data available at 152.4-mm (6-inch) intervals was used by ProQual. D

obtained at 25-mm (1-inch) intervals are available for both the T-6600 profilers and the ICC 

profilers. When ProQual computes IRI values for data obtained by these profilers, a 300-mm 

Top - 25 mm

Middle - 150 mm

Bottom - Dipstick

3-m High-Pass Filtered 

Profiles Offset

25.4 mm = 1 inch 

1 m = 3.28 ft 

77 
(11.8-inch) moving average is first applied to the data, then profile data points that are at 

150-mm (5.9-inch) intervals are extracted and the IRI is computed using this averaged data. 

The effects of application of the moving average onto the 25-mm (1-inch) data were discussed in 

chapter 5. The averaged profile obtained from the profiler data that are used for computation of 

IRI by ProQual is an artificial profile, and this profile may not actually exist on the road. 

However, when measurements are performed with Dipstick, readings will be obtained on the 

actual profile of the road. For example, consider the profiles shown in figure 60 that show the 

25-mm (1-inch) profile data, and the profile obtained after ProQual has processed the 25-mm 

(1-inch) data. When computing IRI for profile data, the 150-mm (5.9-inch) interval profile, 

which is an artificial profile, is used. However, when Dipstick obtains readings, the footpads of 

Dipstick will rest on the actual profile of the pavement, and not on the artificial profile that is 

defined by the 150-mm (5.9-inch) interval profile. Thus, this difference in the profiles between 

the profiler data and the Dipstick data can result in differences in the IRI values. 

-20

-15

-10

-5

0

5

136 136.5 137 137.5

Distance (m)

Elevation (mm)

25 mm 

150 mm

25.4 mm = 1 inch 

1 m = 3.28 ft 

Figure 60. Illustration of artificial profile used by ProQual for computing IRI. 

For asphalt-surfaced pavements without distress, the difference between the averaged profile 

computed using ProQual and the profile defined by the 25-mm (1-inch) data will be very minor, 

and the two profiles may actually coincide. However, in new PCC pavements, there could be 

differences between these two profiles at the joints. The 25-mm (1-inch) profile will show the 

joint as a downward feature; however, this feature may not be seen in the averaged profile 

computed using ProQual, since it is smoothed out when the moving average is applied. On 

pavements that have distress, there could be significant differences between the 25-mm (1-inch) 

interval profile and the 150-mm (5.9-inch) interval averaged profile. 

Dipstick Data Errors 

In the LTPP program, when longitudinal Dipstick measurements are performed, the Dipstick 

readings are recorded on a form. Afterwards, these readings are entered into ProQual to compute 

78 
the IRI. After Dipstick measurements are performed in the field, a closure error computation is 

performed as a data quality check.(6) Although this procedure does provide a check on the data 

quality, it is always possible for errors during measurement to occur, yet the closure error may be 

within the acceptable value. Also, multiple errors may occur that compensate for each other and 

cause the closure error to be within the acceptable value. 

Figure 61 shows the 10-m (33-ft) base-length roughness profiles obtained along the left 

wheelpath at site 5 (chip-seal section) during the 2003 LTPP profiler comparison for a run by the 

Western profiler and Dipstick. There are differences between the two roughness profiles, with 

Dipstick showing much higher roughness than the profiler between 20 and 30 m (66 and 100 ft). 

This indicates that Dipstick is capturing a feature between these limits that is causing a high 

roughness, and this feature is not appearing in the profile data collected by the profiler. 

10-m Roughness Profiles

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 20 40 60 80 100 120 140

Distance (m)

IRI (m/km)

Profiler

Dipstick

Dipstick Profiler

1 m = 3.28 ft 

1 m/km = 5.28 ft/mi 

Figure 61. Left-wheelpath 10-m (33-ft) base-length roughness profiles for profiler and 

Dipstick at site 5. 

Figure 62 shows the 10-m (33-ft) high-pass filtered profiles of the profiler and Dipstick at this 

site from the start of the section to a distance of 50 m (164 ft). This figure shows that at an 

approximate distance of 22 m (72 ft), a sharp upward feature is seen in the Dipstick profile; 

however, this feature does not appear in the profile recorded by the profiler. This is the feature 

that caused the Dipstick roughness profile to show a higher value than the profiler roughness 

profile between the limits of 20 and 30 m (66 and 100 ft). This particular feature did not appear 

in any of the profile data collected by all of the profilers at this site. 

Since this feature did not appear in the profile data collected by any profiler, the sharp upward 

nature of the feature indicates that it is most likely an erroneous data point. The possible reason 

for this feature appearing in the Dipstick data is either: (1) an incorrect reading being recorded at 

that location in the field, (2) an incorrect reading at that location being entered into ProQual, or 

(3) a Dipstick malfunction occurring at that location. 

79 
Figure 63 shows a portion of the right-wheelpath profile that was recorded by Dipstick and a 

profiler at site 1 (smooth AC section) during the 2003 LTPP profiler comparison. Both profiles 

have been subjected to a 10-m (33-ft) high-pass filter, and the profiles have been offset for 

clarity. At a distance of about 145 m (476 ft), the Dipstick profile shows a sudden drop in 

elevation; however, this feature is not seen in the profile recorded by the profiler. None of the 

profile runs for any of the profilers showed this feature in the profile. It is most likely that this 

feature in the Dipstick profile was caused by one of the factors that were described previously for 

the previous example. The inclusion of this feature in the Dipstick profile will cause an increase 

in the IRI for the Dipstick data. 

Figure 62. Left-wheelpath profiles for profiler and Dipstick at site 5. 

Figure 63. Right-wheelpath profiles for profiler and Dipstick at site 1. 

10-m High Pass Filtered

0

5

10

15

20

25

30

0 10 20 30 40 50

Distance (m)

Elevation (mm)

Top - Profiler

Bottom - Dipstick

Profiles Offset

Off t

Profiler

Dipstick

10-m High Pass Filtered

0

5

10

15

20

25

130 135 140 145 150

Distance (m)

Elevation (mm)

25.4 mm = 1 inch 

1 m = 3.28 ft 

25.4 mm = 1 inch 

1 m

Dipstick

ICC Profiler

Profiles Offset

1 m = 3.28 ft 

1 inch = 25.4 mm

 = 3.28 ft 

80 
At the start of the LTPP program, the data collection procedures that were employed for Dipstick 

. 

e 

red 

he two examples shown in this section illustrate errors that can occur during Dipstick data 

, or 

I Computation Procedure for Dipstick Data 

 the LTPP program, the IRI values of a profile obtained from Dipstick measurements are 

ed in 

he correct method to accurately compute the IRI from the Dipstick data is to apply the IRI 

ed 

are 

s 

est 

s 

16 

re 

calculates IRI. 

data collection were different from the current procedures. In those procedures, measurements 

along a wheelpath were performed from the start of the section to the end of the section and, 

thereafter, measurements were made back along the same path to end at the start of the section

These procedures provided two profiles for a wheelpath, and if an error was suspected, the 

forward and return runs could be compared to determine whether there were differences. Th

current Dipstick data collection procedures where measurements are performed along a loop 

were adopted to save the time required to perform Dipstick measurements (and the time requi

was cut in half). However, the disadvantage of these procedures is that since only one profile is 

available for a wheelpath, there is no way to check whether a potentially incorrect data point is 

indeed incorrect. 

T

collection. Such errors can occur because of a data recording error, an error during data entry

a malfunction of Dipstick. If such errors occur, they will cause an upward bias in the IRI 

computed from the Dipstick data. 

IR

In

computed using the ProQual software.(7,8) The procedure used by the ProQual software to 

compute the IRI values from Dipstick elevation data was described in chapter 2. As describ

that chapter, the Dipstick elevation profile is rotated to obtain an additional distance of 152.4 m 

(500 ft) before the section, the entire profile is filtered with the upper-wavelength cutoff filter 

used in the profiler, and then the portion of the profile corresponding to the test section is 

extracted from the filtered profile and the IRI is computed using this extracted profile. 

T

algorithm to the actual elevation profile that is obtained from the Dipstick data, and not to a 

filtered Dipstick profile. The analysis of the data from the LTPP profiler comparison conduct

in 2003 indicated that there was a slight difference in IRI values obtained when the same 

Dipstick elevation profile was processed using ProQual and RoadRuf. The RoadRuf softw

uses the IRI computation procedure documented in American Society for Testing and Material

(ASTM) Standard E1926-98 (2003).(29) RoadRuf does not perform any prefiltering of the 

Dipstick data before computing the IRI. Table 8 presents the IRI values obtained for four t

sections in the 2003 LTPP comparison using ProQual and RoadRuf. The IRI computed using 

ProQual were slightly higher than that computed using RoadRuf for all of the cases by amount

varying from 0.02 to 0.08 m/km (1.3 to 5.1 inches/mi). For all practical purposes, differences of 

these magnitudes can be considered to be negligible. However, for LTPP comparison studies 

where one criterion that is being evaluated is to determine whether the profiler IRI is within 0.

m/km (10 inches/mi) of the IRI obtained from Dipstick, the magnitude of the IRI differences 

shown in table 8 can make the difference between either satisfying or failing the criterion. The

was perfect agreement in the IRI values that were computed for the profiler data using ProQual 

and RoadRuf. Therefore, a possible reason for the discrepancy in Dipstick IRI values between 

RoadRuf and ProQual may be the filtering that is performed on the profile data before ProQual

81 
Table 8. IRI values for Dipstick data computed using ProQual and RoadRuf. 

Section Wheelpath Dipstick IRI (m/km) Difference in IRI1

 ProQual RoadRuf (m/km) 

2 Left 2.88 2.80 0.08 

3 Left 0.90 0.88 0.02 

4 Left 1.35 1.32 0.03 

5 Left 2.29 2.24 0.06 

2 Right 2.87 2.79 0.08 

3 Right 1.00 0.99 0.00 

4 Right 1.68 1.64 0.03 

5 Right 2.70 2.63 0.07 

1

 Difference in IRI = ProQual IRI – RoadRuf IRI 

DISCUSSION OF DIFFERENCES IN IRI BETWEEN DIPSTICK AND THE 

PROFILERS 

As described in the previous section, differences between profiler IRI and Dipstick IRI can occur 

because of a variety of factors. In some cases, the different errors compensate for each other and 

cause the profiler IRI and Dipstick IRI to agree well with each other. 

Figure 64 shows the 10-m (33-ft) base-length roughness profiles along the right wheelpath at 

section 2 (rough AC section) used in the 2003 LTPP profiler comparison for Dipstick and one 

run from the Western profiler. IRI from the profiler run and the Dipstick measurements were 

2.77 and 2.79 m/km (176 and 177 inches/mi), respectively. When the overall IRI value for the 

section is considered, the profiler IRI and Dipstick IRI are virtually identical. As seen in 

figure 64, the roughness profiles for the profiler and the Dipstick overlay very well, with only 

some minor deviations noted at some localized area. The roughness profiles show that both the 

profiler and Dipstick are sensing the same roughness in terms of IRI throughout the section. 

1 m/km = 5.28 ft/mi 

82 
10-m Roughness Profiles

0.

6.0

0

1.

2.

3.

4.

5.0

40 6 80 120 140

nce (m)

IRI (m/km)

0

0

0

0

0 20 0 100

Dista

Profiler

Dipstick

Profiler

Dipstick

Figure 64. Roughness profiles for a profiler and Dipstick showing good agreement in 

igure 65 shows the 10-m (33-ft) base-length roughness profiles for the same wheelpath of the 

I and Dipstick IRI agree 

ery well with each other. However, the agreement in the roughness profiles for the profiler and 

ees 

ured 

1 m/km = 5.28

1 m =

 ft/mi 

 3.28 ft 

roughness distribution. 

F

same section for Dipstick and one run for the Southern profiler. The IRI for the profiler run and 

the Dipstick measurements were 2.70 and 2.79 m/km (171 and 177 inches/mi), respectively. 

When the overall IRI value for the section is considered, the profiler IR

v

Dipstick is not as good as the agreement that was seen for the previous example. There are 

several locations where noticeable differences in the roughness profiles are seen. At some 

locations, the profiler sees higher roughness than Dipstick, while at other locations, Dipstick s

higher roughness than the profiler. However, when the overall roughness for the section is 

evaluated, these differences compensate for each other, and the overall roughness as meas

by the profiler and Dipstick agree very well with each other. 

83 
The cross-correlation technique is a method that can be used to judge the agreement in IRI and 

the agreement in spatial distribution of IRI between two devices. The data collected for the 2003 

LTPP profiler comparison by the North Atlantic and Western profilers were used with the 

Dipstick data to calculate cross correlation for IRI between the profilers and Dipstick. In this 

analysis, Dipstick was considered to be the correct device, and the analysis will indicate how 

well each of these profilers was able to reproduce the IRI obtained from the Dipstick 

measurements. The five ProQual-processed profile runs submitted by the North Atlantic and 

Western regions were used to calculate the cross correlation with Dipstick. The results of this 

analysis are presented in tables 9 and 10 for the left and right wheelpaths, respectively. 

 in 

: Left wheelpath. 

Site Profiler Cross Correlation With Dipstick 

10-m Roughness Profiles

1.0

5.0

6.0

Figure 65. Roughness profiles for a profiler and Dipstick showing moderate agreement

roughness distribution. 

Table 9. Results of cross-correlation analysis

 Run Number Minimum Maximum Average 

 1 2 3 4 5 

1: Smooth AC North Atlantic 0.84 0.84 0.86 0.86 0.88 0.84 0.88 0.86 

 Western 0.82 0.83 0.83 0.84 0.84 0.82 0.84 0.84 

2: Rough AC North Atlantic 0.82 0.82 0.82 0.83 0.87 0.82 0.87 0.83 

 Western 0.85 0.89 0.89 0.89 0.90 0.85 0.90 0.89 

3: Smooth PCC North Atlantic 0.78 0.78 0.79 0.80 0.82 0.78 0.82 0.79 

 Western 0.82 0.82 0.83 0.84 0.84 0.82 0.84 0.83 

4: Medium PCC North Atlantic 0.79 0.80 0.86 0.87 0.89 0.79 0.89 0.84 

 Western 0.82 0.83 0.84 0.84 0.84 0.82 0.84 0.83 

5: Chip Seal North Atlantic 0.68 0.70 0.71 0.72 0.72 0.68 0.72 0.71 

 Western 0.69 0.70 0.72 0.73 0.73 0.69 0.73 0.71 

0.0

0 20 40 60 80 100 120 140

Distance (m)

IRI (

2.0

m/k

3.0

m)

4.0 Profiler

Dipstick

Dipstick Profiler

1 m/km = 5.28 ft/mi 

1 m = 3.28 ft 

84 
Table 10. Results of cross-correlation analysis: Right wheelpath. 

Site Profiler Cross Correlation With Dipstick 

 Run Number Minimum Maximum Average 

 1 2 3 4 5 

1: Smooth AC North Atlantic 0.68 0.68 0.69 0.74 0.74 0.68 0.74 0.70 

 Western 0.62 0.65 0.70 0.72 0.77 0.62 0.77 0.69 

2: Rough AC North Atlantic 0.76 0.82 0.82 0.86 0.87 0.76 0.87 0.83 

 Western 0.67 0.70 0.70 0.74 0.75 0.67 0.75 0.71 

3: Smooth PCC North Atlantic 0.80 0.81 0.81 0.82 0.82 0.80 0.82 0.81 

 Western 0.80 0.81 0.81 0.81 0.83 0.80 0.83 0.81 

4: Medium PCC North Atlantic 0.85 0.85 0.85 0.87 0.87 0.85 0.87 0.86 

 Western 0.87 0.88 0.88 0.89 0.90 0.87 0.90 0.88 

5: Chip Seal North Atlantic 0.75 0.77 0.79 0.79 0.79 0.75 0.79 0.78 

 Western 0.77 0.77 0.78 0.78 0.79 0.77 0.79 0.78 

 0.71 

the maxim

indicates tha

istribution wever, this does not necessarily mean that the runs of the profiler 

llowed the as measured by Dipstick. 

Alo

.69 (site 1) to 0.88 (site 3). For a sp fic wheelpath, the difference 

etween the maximum and minimum cross-correlation values for the different runs was within 

.05 for six cases,

two cases where t a difference of 

0.15) and the North Atlantic profiler at site 2 (a tion values 

along the right wheelpath also general the prof ki n

during the repeat runs or that minor variati

n u o g s. 

ow e lpath ction 1 f stick a

e r o r the profile 0.62, w

was the lowest elatio ed in tudy. Th values 

a km (100 and 115 inches/mi), 

respectively. M rough

i the f s within these limits 

differently, and lts in a low c or th devices

Figure 67 show hness e ath ion 2 fo tick an

run 5 for the Western profiler. The IRI cross correlation for this case was 0.90, which was the 

Along the left wheelpath, the average cross-correlation values for the five sites ranged from

(site 1) to 0.89 (site 2). For a specific profiler and a specific wheelpath, the difference between 

um and minimum cross-correlation values for the different runs was within 0.05 for all 

of the cases, except for the North Atlantic profiler at site 4, which had a difference of 0.10. This 

t the profilers are tracking a consistent path during the repeat runs or that minor 

variations in the wheelpaths for repeat runs are not significantly influencing the spatial 

d of roughness. Ho

fo path that w

ng the right wheelpath, the average cross-correlation values for the five sites ranged from 

0 ecific profiler and a speci

b

0 between 0.05 and 0.10 for two cases, and exceeded 0.10 for two cases. The 

he value exceeded 0.10 were for the Western profiler at site 1 (

 difference of 0.11). The cross-correla

ly indicate that 

ons in the wheelpaths for repeat runs are not 

ilers are trac ng a consiste t path 

significantly i fluencing the spatial distrib tion f rou hnes

Figure 66 sh

run 1 from th

s the roughness

 Western profiler

 profil

. The 

es alo

IRI c

ng th

oss c

 right

rrelat

 whee

ion fo

 at se

se two

or Dip

s was 

nd 

hich 

 cross corr n for all of the cases consider this s e IRI for 

the profiler and Dipstick for this case were 1.57 nd 1.81 m/

ost of the ness at this site occurs at four localized areas, as shown in 

figure 66. However, the profiler and Dipst ck are measuring eature

 this resu ross-correlation value f e two . 

s the roug profiles along th left wheelp at sect r Dips d 

85 
highest cross correlation for all of the cases considered in this study. The IRI from the profiler 

and Dipstick for ctively. 

Rou profiles o devices overlay w e localized 

area ese c profiler an

ess 

or 

 this case were 2.76 and 2.80 m/km (175 and 178 inches/mi), respe

ghness

s. t th

 for the tw

ations, the

ell within the section, except for som

A lo d Dipstick recorded dif res ferent featu . 

10-m s iles

0 20 40 60 80 100 120 140

Roughnes Prof

4.0

3.5

Figure 66. Roughness profiles for the case with the lowest cross correlation. 

Figure 67. Roughness profiles for the case with the highest cross correlation. 

As shown in the cross-correlation study, perfect agreement in the magnitude of the roughn

and spatial distribution of the roughness did not occur between the profiler and Dipstick. 

However, a fairly good correlation (i.e., a correlation of greater than 0.80) was observed for 

many of the cases. The factors described previously in this chapter can contribute to a lowering 

of the cross-correlation values for the profiler and Dipstick. The IRI cross-correlation values f

0.0

0.5

1.0

1.5

2.0

 (m

2.5

3.0 )

Distance (m)

IRI /km

Di

Pro

pstick

filer

1 m/km = 5.28 ft/mi 

1 m = 3.28 ft 

10-m Roughness Profiles

5.0

6.0

0.0

1.0

2.0

3.0

4.0

0 20 40 60 80 100 120 140

Distance (m)

IRI (m/km)

Dipstick

Profiler

1 m/km = 5.28 ft/mi 

1 m = 3.28 ft 

86 
a profiler and Dipstick are not necessarily a function of the roughness, but rather will depend on

the type of features within the section that contribute to the roughness. 

A cross-correlation analysis was performed for the North Atlantic and Western profilers to 

determine how well they reproduced their IRI results. The data collected during the 2003 LTPP 

profiler comparison were used in this analysis. One representative run for each profiler was 

obtained at each section to perform this analysis. Results of this analysis are shown in table 11. 

Table 11. Cross correlation of IRI for North Atlantic and Western profilers. 

Wheelpath Site 

 1 2 3 4 5 

Left 0.98 0.94 0.91 0.92 0.94 

Right 0.95 0.84 0.91 0.97 0.96 

The crossfollowed by the two profilers. The results shown in table 11 indicate that the two profilers are 

picking up d for the 

two profilers are m

te 

the accuracy

e 

the lim

for m

smooth pavem

obtaining measurem

overcom

time consum

urrent proced PP profiler comparisons use the average profiler IRI obtained from 

five error- h out 

me of th k, data 

ck 

correlation values shown in table 11 indicate that there is excellent reproducibility 

between the two profilers in their ability to obtain IRI values. All cross-correlation values were 

greater than 0.90, except along the right wheelpath at site 2. There were significant distresses 

along this wheelpath, and the lower cross correlation is attributed to lateral variations in the path 

 similar features within the test sections. The cross-correlation values obtaine

uch higher than those obtained for each of these profilers and Dipstick. 

The IRI obtained from Dipstick measurements usually is considered as the reference to evalua

 of the profilers. However, because of the deficiencies in the device that were 

described previously in this chapter, Dipstick cannot be considered as a device for measuring the 

reference profile of a pavement. There is currently no other device available that can overcom

itations of Dipstick. Some agencies use the ARRB walking profiler as a reference device 

easuring reference profiles; however, this device is subject to the same limitations as 

Dipstick. The rod and level has similar limitations. It has been shown that measurement errors 

are possible with the rod and level that can cause errors in the computed roughness indices for 

ents.(12) Obtaining rod-and-level measurements at a closer sampling interval or 

ents with Dipstick with the footpads set to a smaller sampling interval can 

e some of the errors that are introduced because of the 304.8-mm (1-ft) sampling 

interval of Dipstick. However, obtaining measurements at shorter sampling intervals can be very 

ing. 

C ures for LT

free runs for comparison with the Dipstick IRI. This procedure helps to smoot

so e variability in the profiler runs. In spite of all of these limitations with Dipstic

from past LTPP comparisons have shown that good agreement between profiler IRI and Dipsti

IRI, typically within ±0.16 m/km (±10 inches/mi), can be obtained at many sections (see 

chapter 3). Usually, agreement between profiler IRI and Dipstick IRI that is within ±0.16 m/km

(±10 inches/mi) is possible on AC and PCC pavements that do not have distress. However, 

significant differences in IRI between Dipstick and the profiler can occur on rough pavements 

that have distress. The magnitude of the difference in IRI is not necessarily a function of the IRI 

87 
from the section, but rather it depends on the type of roughness features that are present in the 

section. 

In spite of all of the limitations with Dipstick, it still can be used as a device for checking th

obtained from the profilers. However, it cannot be considered as a device for checking the 

accuracy of the profilers on pavements that have rough features or distress. The current LTPP

comparison procedure uses an IRI difference of ±0.16 m/km (±10 inches/mi) between the 

e IRI 

rofiler IRI and the Dipstick IRI to judge the accuracy of LTPP’s profilers. If differences outside 

of this lim rofiler. 

Such difference can occur because of one or more of the causes that were described previously in 

this chapter. If su re e countere a more detailed analysis of the data should be 

performed to identify the cause o diffe in IR

CTED DIF NCES B EE PSTICK AND PROFILER IRI 

, 

en 

s ranges. This is because the differences in IRI between the profiler and 

ipstick are not necessarily a function of the roughness, but rather will depend to a great extent 

p

it are obtained, it does not necessarily mean that there is a problem with the p

ch situations a n

the

d,

f re e nc I. 

EXPE FERE ETW N DI

An analysis was performed to identify the range of differences in IRI that can be expected 

between the Dipstick IRI and the profiler IRI. This analysis was performed for the DNC 690 

(using data from the 1992 LTPP profiler comparison), T-6600 (using data from the 1998 LTPP 

comparison), and ICC profilers (using data from the 2003 LTPP comparison). 

The following procedures were used to perform the analysis for each case: 

1. For each wheelpath at each site, use the IRI values for the repeat runs of each profiler to 

compute the differences between the IRI from each profiler run and the IRI from the 

Dipstick measurements (i.e., profiler IRI – Dipstick IRI). 

2. Use the computed differences for that wheelpath from all of the runs for all of the 

profilers to compute the 15th percentile, 85th percentile, and median of the differences in 

IRI between the profiler IRI and the Dipstick IRI. 

The computed values are presented for the DNC 690, T-6600, and ICC profilers in tables 12, 13

and 14, respectively. In each table, the results are grouped according to the surface type, and th

under each surface type, the results are sorted according to the Dipstick IRI. 

The data presented in these tables were not used to compute expected IRI differences for 

different roughnes

D

on the profile features within the section that contribute to the roughness. 

88 
Table 12. Differences between the K.J. Law Engineers DNC 690 profiler IRI and Dipstick IRI

Surface Wheelpath

. 

 Test IRI from Profiler IRI – Dipstick IRI (m/km) 

Type Section Dipstick 15th 85th Median 

 (m/km) Percentile Percentile 

Asphalt Left 6 0.66 −0.07 −0.01 −0.05 

Asphalt Right 6 0.76 −0.02 0.03 −0.01 

Asphalt Right 5 0.93 −0.08 0.01 -0.04 

Asphalt Left 5 1.31 −0.14 −0.07 −0.09 

Asphalt Right 3 1.37 −0.04 0.08 0.00 

Asphalt Left 3 1.47 −0.09 −0.05 −0.07 

Asphalt Left 7 1.85 −0.37 −0.09 −0.31 

Asphalt Right 7 3.52 −0.45 0.17 0.00 

 

Concrete Right 8 1.10 −0.03 0.02 0.00 

Concrete Left 8 1.39 −0.11 0.09 0.05 

Concrete Right 2 1.69 −0.11 −0.04 −0.08 

Concrete Left 1 1.81 −0.20 −0.06 −0.13 

Concrete Right 1 2.13 −0.18 −0.10 −0.14 

Concrete Left 2 2.32 −0.19 0.02 −0.05 

Concrete Left 4 4.12 0.16 0.44 0.26 

Concrete Right 4 5.63 −0.06 0.11 0.06 

Table 13. Differences between the K.J. Law Engineers T-6600 profiler IRI and Dipstick IRI. 

Surface Wheelpath Test IRI from Profiler IRI – Dipstick IRI (m/km) 

Type Section Dipstick 15th 85th Median 

 (m/km) Percentile Percentile 

Asphalt Left 1 0.95 0.06 0.09 0.07 

Asphalt Right 1 0.96 0.07 0.11 0.09

Asphalt Right 2 2.46 −0.09 0.15 −0.01 

Asphalt Left 2 2.52 −0.01 0.06 0.03 

 

Concrete Right 3 1.13 0.10 0.13 0.11 

Concrete Left 3 1.17 0.09 0.13 0.12 

Concrete Left 4 2.87 −0.09 0.03 −0.01 

Concrete Right 4 3.17 −0.13 0.02 −0.05 

1 m

1 m/km = 5.28 ft/mi 

/km = 5.28 ft/mi 

89 
Table 14. Differences between the ICC profiler IRI and Dipstick IRI. 

Surface Wheelpath Test IRI From Profiler IRI – Dipstick IRI (m/km) 

Type Section Dipstick 15th 85th Median 

 (m/km) Percentile Percentile 

Asphalt Left 1 1.17 0.08 0.12 0.11 

Asphalt Right 1 1.81 −0.18 −0.06 −0.09 

Asphalt Right 2 2.79 −0.25 0.23 −0.04 

Asphalt Left 2 2.80 −0.07 −0.01 −0.04 

 

Concrete Left 3 0.88 0.02 0.00 0.04 

Concrete Right 3 0.99 −0.03 −0.05 0.00 

Concrete Left 4 1.32 0.11 0.22 0.12 

Concrete Right 4 1.64 0.04 0.08 0.06 

 

Chip Seal Left 5 2.24 −0.11 0.02 −0.05 

Chip Seal Right 5 2.63 −0.13 −0.06 −0.09 

SUMMARY OF THE FINDINGS 

Seve th n cause IR btained the Dipstick data to dif IRI obtained 

from er hese factors are: 

n ties of D tick: A theoretical analysis indicated that Dipstick does not 

lengths less than 2 m (7 ft) accurately. The sampling interval of Dipstick 

• tions in he pa ed by the profiler: ions in fol the 

r from path ip urem bt u ces 

in IRI between the two devices. S dif RI ecause of this 

 in ents with distress. 

• res ed by th profiler are mis Dipstic spacin een 

o f ds of Dips is 0.30 ft). A with a 25-mm (1-inch) recording 

l o s 12 readin within stance. Thus, Dipstick can miss featu

measured by the profiler. Dipstick has ately 32 mm 

c diameter. The footpads can bridge over narrow downward features such 

s an cause t IRI from stick to be lower than that obtained fr e 

. 

ted by Dipstick: S s 

ent close to the cracks in the pavement. Although Dipstick can 

record a dip at such features, the magnitude of the depth of the dip measured can be less 

1 m/km = 5.28 ft/mi 

ral factors at ca I o from fer from

 the profil data. T

• Sampli g quali ips

measure wave

also may cause features with wavelengths shorter than twice the sampling interval of 

Dipstick to contaminate the measurement of longer wavelengths because of aliasing. 

Varia

pr file

 t th follow

 wh e D

 Variat the path lowed by 

o the er stick meas se di eren

ignificant

ents were o

ferences in I

ained can ca

 can occur b

ff

factor pavem

Featu record e that sed by k: The g betw

the tw

interva

ootpa

btain

tick

gs

5 m (1

this di

profiler

res 

footpads that are approxim

(1.25 in hes) in 

as crack and c he Dip om th

profiler

• Features recorded by the profiler that are underestima ometime

there is some settlem

90 
than th sampling 

the deepest part of

• ging eff cts of da LTPP , profi obta

25 m (1-inc intervals e proc ng P ch 0-mm

inc ving aver onto th m (1-in ta and th tracts da

m ch) interv s. The I ompute this av profile is an 

ial When Dipstick measurements a rmed, r s will b ned 

 ac avement and not on this artificial profile. 

• k errors: E easure can occur because of incorrect 

s ecorded, alfunction of Dipstick, or data entry errors during data input 

u These er s can ca bias in the IRI computed from the Dipstick data. 

• n proce es for ck data: LTPP program, IRI values are 

d g the ProQ l softwa hen ProQual computes I valu the 

 e data ar anipulated and filter re com the IRI

is RI value btained oQua oadRu d that 

g ProQual had a slight upward bias. This is attributed to the filtering that is 

fore ProQual computes the IRI. 

ipstick IRI and the profiler IRI in both magnitude and 

atial d

Despite t 

good ag

±10 in

omparisons use the average profiler IRI obtained from five error-free runs for comparison with 

Althou

conside

distress e an IRI difference of ±0.16 m/km (±10 inches/mi) 

etween the profiler IRI and the Dipstick IRI to judge the accuracy of LTPP’s profilers. If 

feren

necessa

at recorded by the profilers because the profilers have a much smaller 

interval and, thus, can capture the dip. 

Avera e profiler ta: In the program le data ined at 

-m h) ar essed usi roQual, whi applies a 30

(11.8- h) mo age e 25-m ch) da en ex ta at 

150-m (5.9-in al RI is c d using eraged , which

artific profile. re perfo eading e obtai

on the tual p

Dipstic data rrors in Dipstick m ments

reading being r

.

 a m

to comp te IRI ror use a 

IRI computatio d ru Di sti p In the

compute usin ua re. W the IR es from

Dipstick

Compar

data, th

on of I

e m

s o

ed befo

l and R

puting 

f showe

. 

 from Pr the IRI 

computed usin

performed on the Dipstick data be

It is possible to have good agreement between profiler IRI and Dipstick IRI because the various 

errors compensate for each other. The cross-correlation technique can be used to determine 

whether there is agreement between the D

sp istribution. 

 all of these limitations with Dipstick, data from past LTPP comparisons have shown tha

reement between profiler IRI and Dipstick IRI, typically within ±0.16 m/km 

( ches/mi), can be obtained at many sections. Current procedures for LTPP profiler 

c

the Dipstick IRI. This procedure helps to smooth out some of the variability in the profiler runs. 

gh Dipstick can be used to check the IRI obtained from the profilers, it cannot be 

red for checking the accuracy of profilers on pavements that have rough features or 

. The current LTPP comparisons us

b

dif ces outside of this limit are obtained on pavements having distress, it does not 

rily mean that there is a problem with the profiler. 

91 

CHAPTER 7: OTHER FINDINGS FROM ANALYSIS OF THE DATA 

INTRODUCTION 

This chapter describes other findings that were observed during the analyses that were performed 

for this research project. The specific findings that are described in this chapter are: 

• IRI values computed using ProQual. 

• Accelerometer effects on the T-6600 profiler data. 

• Observations on short-wavelength data collected by the T-6600 profiler. 

• IRI differences for the Southern profiler during the 1991 comparison. 

IRI VALUES COMPUTED USING PROQUAL 

The IRI values in the LTPP database have been computed using ProQual. The data collected by 

the DNC 690 profiler were at 152.4-mm (6-inch) intervals, and the IRI was computed using the 

collected data. Data at 25-mm (1-inch) intervals were available for the T-6600 and ICC profilers. 

ProQual applies a 300-mm (11.8-inch) moving average onto this data, extracts the data at 150-

mm (5.9-inch) intervals, then computes the IRI using this data. 

The IRI computation algorithm contained in ProQual is the algorithm that is described in World 

Bank Technical Report 46.(9) This report indicates that the IRI algorithm will apply a 250-mm 

(9.8-inch) moving average onto the profile data before computing the IRI when the data 

recording interval is less than 250 mm (9.8 inches). Subsequent literature on IRI have indicated 

that the moving average applied by the IRI algorithm should be omitted if a moving average has 

already been applied to the profile data.(2,30) The World Bank report did not specifically describe 

such a criterion. 

A study was conducted to compare the IRI values computed using ProQual with those computed 

using RoadRuf. In this study, RoadRuf was used for computing the IRI with and without the 

moving average being enabled in the IRI algorithm. The five profile runs collected by the North 

Atlantic ICC profiler at the test site used in the 2003 LTPP profiler comparison were used for 

this study. The profile data used for this study were the ProQual-processed data, which were at 

150-mm (5.9-inch) intervals. The average IRI values computed from the five runs for each 

wheelpath at all five sites are presented in table 15. 

There was excellent agreement among IRI values computed using ProQual and RoadRuf when 

the moving average was applied by the IRI algorithm in RoadRuf. (Note: The IRI algorithm 

automatically applies the moving average.) However, the IRI values computed using RoadRuf 

with the moving average omitted were slightly higher than those computed using ProQual. The 

IRI for the individual wheelpaths considered in this study ranged from 0.92 to 2.81 m/km (58 to 

178 inches/mi), and the percent difference between IRI from ProQual and IRI from RoadRuf 

with the moving average omitted in the IRI algorithm ranged from 0.7 to 2.3 percent. A 

researcher who obtains 150-mm (5.9-inch) interval profile data from the LTPP database and 

93 
computes IRI values will obtain slightly higher values if the moving average is omitted in the IRI 

algo

I values computed using ProQual and RoadRuf. 

Percent Difference in 

rithm. 

Table 15. IR

Site Wheelpath Average IRI (m/km) IRI Between 

 ProQual RoadRuf RoadRuf: No ProQual 

 Averaged1 Averaging2 and RoadRuf3

1 Left 1.269 1.268 1.290 1.7 

2 Left 2.762 2.762 2.820 2.1 

3 Left 0.925 0.926 0.938 1.4 

4 Left 1.451 1.450 1.466 1.0 

5 Left 2.249 2.250 2.284 1.6 

 

1 Right 1.682 1.684 1.706 1.4 

2 Right 2.814 2.814 2.880 2.3 

3 Right 0.982 0.978 0.994 1.3 

4 Right 1.699 1.698 1.710 0.7 

5 Right 2.540 2.540 2.586 1.8 

1

 Moving average is applied in the IRI algorithm. 

2

 Moving average is not applied in the IRI algorithm. 

3

 Percent Difference = 100 x (IRI from RoadRuf Without Moving Average – IRI fro

ProQual) / IRI from ProQual 

m 

There is a slight d

1 m/km = 5.28 ft/mi 

ifference in the IRI computed using the 25-mm (1-inch) data and that 

omputed using ProQual. Table 16 shows the average IRI values (from five runs) computed 

e 

in the 

to 5.0 percent. Thus, a researcher who 

btains 25-mm (1-inch) interval profile data on LTPP sections and computes IRI values will 

0 

n, 

ble 

than the data collected by the DNC 690 profiler.

c

using 25-mm (1-inch) data and ProQual along each wheelpath for the data collected by the North 

Atlantic ICC profiler at the test sites used in the 2003 LTPP profiler comparison. These are th

same profile runs used for the previously described analysis. IRI for 25-mm (1-inch) data were 

computed using RoadRuf. The IRI for the 25-mm (1-inch) data were higher than the IRI 

computed using ProQual for all of the cases. The IRI for the individual wheelpaths considered in

this study ranged from 0.92 to 2.81 m/km (58 to 178 inches/mi), and the percent difference 

IRI between RoadRuf and ProQual ranged from 2.1 

o

obtain higher IRI values than those stored in the LTPP database for the corresponding data set. 

ACCELEROMETER EFFECTS ON THE K.J. LAW ENGINEERS T-6600 

PROFILER DATA 

During the 1996 regional verification test, each region performed a comparison of the DNC 69

and T-6600 profilers. The report on this comparison, prepared by the North Atlantic regio

indicated that the profile data collected by the T-6600 profiler at rough sites were less repeata

(25)

94 
Table 16. Comparison of the IRI from the 25-mm (1-inch) data with the IRI from ProQual. 

S

Percent 

ite Wheelpath Average IRI (m/km) Difference 

 ProQual 25-mm Data in IRI1

1 Left 1.27 1.32 3.9 

2 Left 2.76 2.89 4.7 

3 Left 0.92 0.95 3.3 

4 Left 1.45 1.48 2.1 

5 Left 2.25 2.33 3.6 

 

1 Right 1.68 1.74 3.6 

2 Right 2.81 2.95 5.0 

3 Right 0.98 1.01 3.1 

4 Right 1.70 1.73 1.8 

5 Right 2.54 2.64 3.9 

1

 Percent Difference = 100 x (25-mm IRI – P l IRI)/ProQual IRI roQua

Figure 68 shows the overlaid repeat r profile data collected by the 

North ite (section 4) during the regional 

verif n in this figure. The data show 

excel

laid right-sensor profiles of the K.J. Law Engineers DNC 690 profiler. 

1 m/km = 5.28 ft/mi 

profile plots of the right-senso

 Atlantic DNC 690 profiler at the rough concrete s

ication test. Profile data collected for five runs are show

lent repeatability. 

25.4 mm = 1 inch 

1 m = 3.28 ft 

15

-20

-15

-10

-5

0

5

10

0 25 50 75 100 125 150

Distance (m)

Elevation (mm)

Figure 68. Over

Figure 69 shows the overlaid repeat profile plots of the right-sensor profile data collected by the 

T-6000 profiler at the same site. This figure also contains data collected for five profile runs. The 

95 
pro

profiles show poor repeatability. 

Figure ofiler. 

igure 70 show or profile of one profile from the DNC 690 and T-6600 profilers. 

ty of 

and 

r immediately after this feature. 

was not known when the data from the 1996 verification test were analyzed. However, 

files show good repeatability up to a distance of 50 m (164 ft); however, thereafter, the 

-25

-20

-15

-10

-5

0

5

10

15

20

25

Run 1

 69. Overlaid right-sensor profiles of the K.J. Law Engineers T-6600 pr

F

T

s the right-sens

he two profiles agree very well with each other to a distance of about 50 m (164 ft); however, 

thereafter, they diverge from each other and converge again near the end of the section. An 

extremely rough feature is present on the pavement at 50 m (164 ft), and the poor repeatabili

the data collected by the T-6600 profiler and the differences in the profiles for the DNC 690 

the T-6600 profilers occu

Figure 70. Overlaid right-sensor profiles from the K.J. Law Engineers DNC 690 

and T-6600 profilers. 

The cause of the poor repeatability in the profiles after this feature is that the accelerometer on 

the T-6600 profiler reached its upper limit when the rough feature was encountered. This fact 

0 25 50 100 25 150 Dis m)

Elevation (mm)

Run 2

Run 3

Run 4

Run 5

75 1 tances (

-25

-20

-15

-10

-5

0

5

10

15

20

25

0 25 50 75 100 125

Distance (m)

Elevation (mm)

150

25.4 mm = ch 

1 m = 3.28

25.4 mm = 1 inch 

1 m

 1 in

 ft 

T-6600

DNC 690

 = 3.28 ft 

96 
subsequent discussions with K.J. Law Engineers indicated that this indeed was the cause of

poor profile repeatability. Since th

 the 

e sensors on the DNC 690 profiler are housed on the vehicle 

ody between the two axles, the accelerometers on the DNC 690 profiler are subjected to much 

ss acceleration than that felt by the accelerometers on the T-6600 profiler, which are housed in 

e sensor bar located on the front of the vehicle. 

he IRI values for the five profiles shown in figure 68 for the DNC 690 profiler ranged from 

.82 to 1.90 m/km (115 to 120 inches/mi). The IRI values for the five profiles shown in figure 69 

r the T-6600 profiler ranged from 1.87 to 1.91 m/km (119 to 121 inches/mi). The IRI for the 

o profiles shown in figure 70 were 1.85 m/km (117 inches/mi) for the DNC 690 profiler and 

.87 m/km (119 inches/mi) for the T-6600 profiler. Although there was poor repeatability of the 

rofile data collected by the T-6600 profiler at this site, the IRI values for the five runs were 

milar. Also, the IRI values for the DNC 690 and T-6600 profilers were similar. Since IRI is 

rimarily influenced by wavelengths between 1 and 30 m (3 and 100 ft), the previously described 

bservations indicate that the poor repeatability of the profiles for the T-6600 profiler is caused 

y wavele an 30 m (100 ft). 

The N d in 

e 19 test 

com is 

ma

accelerom

increased from

collected by the W

hree LTPP profilers.(14) 

igure 71 shows an example of the data collected along the right wheelpath at section 2 by the 

Western sent 

in the Western profiler data, while thi r in the data collected by the North 

entral profiler. 

b

le

th

T

1

fo

tw

1

p

si

p

o

b ngths greater th

orth Atlantic profiler encountered a similar situation at another rough site that was use

th 96 regional verification test. However, this situation was not encountered at any of the

sites that were used in the other three regions. It appears that when the T-6600 profiler 

encounters an extremely rough pavement feature, the accelerometer can go out of range, and this

causes some contamination of the long-wavelength data that are collected after that event. 

However, this contamination appears to disappear after the profiler has traveled some distance 

after encountering the very rough spot. 

“noisy” when compared to the data collected by the other t

In 2000, all four of LTPP’s profilers visited the K.J. Law Engineers facility in Michigan for 

puter maintenance and upgrades. The range of the accelerometers was increased during th

intenance by modifying the data collection software and by making modifications to the 

eter cards in the computer. After this upgrade, the range of the accelerometers 

 ±2 gigabyte (GB) to ±8 GB, and the regional contractors indicated that they were 

able to obtain profiles that had better repeatability at rough sites. 

OBSERVATIONS ON SHORT-WAVELENGTH DATA COLLECTED BY THE 

K.J. LAW ENGINEERS T-6600 PROFILERS 

Data from the 2000 Profiler Comparison 

The report prepared for the 2000 LTPP profiler comparison indicated that the profile data 

estern profiler at test sections 1 and 2, which were both AC pavements, were 

F

 and North Central profilers. The figure shows that a cyclic repetitive pattern is pre

s pattern does not occu

C

97 
Figure 72 shows PSD plots of profile data collected along the right wheelpath at section

North Central and Western profilers. Two dominant peaks occur in the PSD plot for the Western

profiler data at wave numbers of approximately 9 and 15 cycles/m (2.7 and 4.6 cycles/ft), which

correspond to a wavelength of 0.11 and 0.07 m (0.36 and 0.23 ft), respectively. These peaks wer

not observed in the data collected by the North Central profiler. 

 2 by the 

e 

yclic pattern seen in the profile data collected by the Western profiler is 

contamination occurring in the profile data at these two wave numbers. Examination of the rightFigure 71. Right-sensor profile data collected by the Western and North Central profilers. 

-8

-6

-4

-2

0

2

4

6

Elevation (mm)

Figure 72. PSD plots of the right-sensor data collected by the North Central and Western 

profilers at site 2. 

The cause of the c

25 25.5 26 26.5 27 27.5 28

Distance (m)

Western

Profiles Offset North Central

25.4 mm = 1 inch 

1 m = 3.28 ft 

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

98 
wheelpath data collected at this site by the Western profiler using a PSD plot indicated that this 

es/m (2.7 and 4.6 cycles/ft). For 

ll repeat runs along a specific path, the spikes in the PSD plots occurred at the same wave 

umbers. 

n evaluation of the PSD plots of the profile data collected by the other three profilers at sites 1 

nd 2 showed similar spikes in the PSD plots for the following cases: 

Data collected by the North Atlantic profiler along both wheelpaths at sites 1 and 2. 

Data collected by the North Central profiler along both wheelpaths at site 1, and along the 

left wheelpath at site 2. 

Data collected by the Southern profiler along both wheelpaths at site 1. 

or the North Atlantic profiler, two spikes in the PSD plot were similar to those seen for the 

estern p other two profilers, only a single spike was observed. For each profiler, 

the wave n hich these spikes appeare e for both sites, and the spikes 

also

iff

agnitude of the spikes observed in the PSD plots was highest for the Western profiler. This 

henomenon of spikes in the PSD plots for wave numbers greater than 1 cycle/m (0.3 cycle/ft) 

as not detected in the data collected by all of the profilers at the other three sites used in the 

000 comparison. 

he effects of the contamination were apparent in the Western profiler at sites 1 and 2, since the 

rofiles appeared to be noisy when compared to those obtained by the other three profilers. 

lthough evidence of contamination was not apparent in the data collected by the other profilers, 

e PSD plots indicated that the data were affected for several cases as described previously. 

he cause of this contamination is not known. As this contamination was seen only at sites 1 and 

, it appears that an environmental condition during data collection, or some equipment factor, or 

a combination of both, caused the contamination of the profile data. 

When the 25-mm (1-inch) profile data are proces ed using ProQual, the short wavelengths will 

become attenuated. Thus, the PSD plots of the 150-mm (5.9-inch) averaged data will not show 

spikes. The contamin le data occurs at very low wavelengths that are outside the 

wavelength range influencing the IRI. The IRI values computed for the Western profiler at sites 

1 and 2 showed excellent agreem

phenomenon also was occurring along the right wheelpath. Evaluation of the data collected by 

the Western profiler at site 1 indicated that this phenomenon also was occurring along both 

wheelpaths. At sites 1 and 2, the two spikes in the left-sensor PSD plot occurred approximately 

at wave numbers of 5 and 14 cycles/m (1.5 and 4.3 cycles/ft), while for the right-sensor data, the 

spikes occurred approximately at wave numbers of 9 and 15 cycl

a

n

A

a

•

•

•

F

W rofiler. For the 

umbers at w d were the sam

 appeared at the same wave numbers for the repeat runs. However, there were slight 

d erences in the wave numbers at which the spikes appeared between the profilers. The 

m

p

w

2

T

p

A

th

 

T

2

s

ation of the profi

ent with the IRI values obtained for the other three profilers. 

99 
Evaluation of Data from the 1998 LTPP Profiler Comparison 

Spikes in PSD Plots for High Wave Numbers 

An evaluation of the 25-mm (1-inch) data collected by the T-6600 profilers during the 1998 

LTPP profiler comparison indicated that there was some contamination in the left-sensor profile 

data collected by the North Atlantic, North Central, and Western profilers. Figure 73 shows the 

PSD plot of the left-sensor profile data collected by the North Atlantic profiler during this 

comparison at site 1. 

ch correspond to wavelengths of 0.2 and 0.14 m (0.7 and 0.46 ft), 

e 

Figure 73. PSD plot of the left-sensor profile data from the North Atlantic profiler. 

The PSD plot shows two distinct peaks for wave numbers of 5 and 7.1 cycles/m (1.52 and 

2.16 cycles/ft), whi

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

respectively. These two spikes appear in the left-sensor profile data collected by the North 

Atlantic profiler at all four sites. The North Central profiler also exhibited similar spikes in the 

PSD plot for left-sensor data at wave numbers of 5 and 10 cycles/m (1.52 and 3.05 cycles/ft), 

which correspond to wavelengths of 0.2 and 0.1 m (0.7 and 0.3 ft), respectively. The PSD plots 

of the left-sensor data from the Western profiler at all of the sites had a single spike close to th

wave number of 9 cycles/m (2.74 cycles/ft), which corresponds to a wavelength of 0.11 m 

(0.4 ft). The left-sensor data from the Southern profiler did not show such spikes in the PSD 

plots at any site. None of the profilers showed this phenomenon for data collected by the right 

sensor. 

The appearance of spikes in the PSD plot indicates that there is high spectral content at that 

specific wavelength in the profile data. Since these peaks were noted at all of the test sites, they 

obviously were not caused by a pavement feature. With the exception of the Southern profiler, 

some interference was being captured in the left-sensor data collected by the profilers at all of the

test sections. The cause of this interference is unknown. 

100 
These spikes appear at wavelengths that are outside the wavelength range influencing the IRI 

m (1-inch) data was seen 

ith the Southern profiler and the other three profilers. The RN is another index that is 

m (1.6 and 36 ft). This index gives much more 

eighting to short wavelengths than the IRI. A comparison of left-wheelpath RN values for the 

hus, 

 have been processed using ProQual were examined using PSD plots, the 

ikes 

 because the application of the 300-mm (11.8-inch) moving average by ProQual attenuates 

ort wavelengths. 

ontamination of Right-Sensor Signals in the Western Profiler 

n evaluation of the 25-mm (1-inch) interval profile data collected by the right sensor of the 

rofilers indicated that the data collected by the Western profiler had some contamination at 

avelengths of less than 1 m (3 ft). This phenomenon was seen in the right-sensor data collected 

y the Western profiler at all of the sites. The other three profilers showed good agreement for 

avelengths in this range. 

igure 74 shows PSD plots of the 25-mm (1-inch) right-sensor data from a test section for both 

the North Central and Western profilers. The PS ots show that there is very good agreement 

and, thus, no noticeable bias in IRI values computed using the 25-m

w

influenced by wavelengths between 0.5 and 11 

w

Southern profiler and the other three profilers did not show a noticeable bias in RN values. T

it appears that the short-wavelength interferences observed in the left-sensor data collected by 

the North Atlantic, North Central, and Western profilers are also not influencing the RN. 

When profile data that

sp observed in the 25-mm (1-inch) data for the high wave numbers were not observed. This 

is

sh

C

A

p

w

b

w

F

D pl

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 74. PSD plots of 25-mm (1-inch) right-sensor data from the North Central and 

Western profilers. 

101 
between the two profilers, except for the spectral content that has wave numbers greater than 

1 cycle/m (0.3 cycle/ft), which corresponds to wavelengths of less than 1 m (3 ft). This 

phenomenon was observed for data collected by the Western profiler at all of the sites. 

Figure 75 shows the PSD plots for the ProQual-processed profile data for the two profile runs 

shown in figure 74. The application of the 300-mm (11.8-inch) moving average by ProQual 

attenuates profile features that have wave numbers greater than 1 cycle/m (0.3 cycle/ft), which 

orrespond to wavelengths of less than 1 m (3 ft). Thus, the difference between the two profilers 

or 

een 1 and 0.03 cycle/m (0.3 and 0.009 cycle/ft), which correspond to 

avelengths between 1 and 30 m (3 and 100 ft). This is the wavelength range that is primarily 

n the two profiles. 

data indi values computed for the Western profiler were lower than the values 

obtain

wavel

c

for wave numbers greater than 1 cycle/m (0.3 cycle/ft) that was seen in figure 74 is not 

noticeable in figure 75. The PSD plots for the two profilers shown in figure 75 agree fairly well 

for wave numbers greater than 1 cycle/m (0.3 cycle/ft). The plots show excellent agreement f

wave numbers betw

w

influencing the IRI. Thus, there was good agreement in IRI betwee

1 cycle/m = 0.3 cycle/ft 

1 m/cycle = 3 ft/cycle 

Figure 75. PSD plots of ProQual-processed right-sensor data from the North Central and 

Western profilers. 

The contamination of the right-sensor signal in the Western profiler occurs for short wavelengths 

that do not influence the IRI. Therefore, when an analysis of the IRI values from this comparison 

was performed, no bias in the IRI values for the Western profiler was seen. An evaluation of the 

RN values for the right-sensor data that were computed using the 25-mm (1-inch) interval profile 

cated that the RN

ed for the other three profilers at all four of the test sites. This indicates that the shortength contamination that is present in the Western profiler data is affecting the RN. 

102 
When overlaid left- and right-sensor data PSD plots of data collected by the Western profiler at 

f the 

ata 

ollected during the 2000 LTPP profiler comparison did not show such a trend. Therefore, it 

d 

t 

much higher that those computed from data collected by the other three profilers. The IRI 

values obtained for all four of the profilers for this comparison are presented in appendix A. The 

profile data collected by the Southern profiler at the eight test sections used in this comparison 

were compared to the data collected by the Western profiler to identify the cause of the high IRI 

values. An evaluation of the PSD plots of the Southern profiler data did not indicate any 

evidence of contamination at a specific wavelength. However, the PSD plots indicated that over 

a wavelength range of between 1 and 10 m (3 and 33 ft), the left-sensor data collected by the 

Southern profiler had slightly more spectral content than the data collected by the Western 

profiler. The cause of this phenomenon is unknown. This phenomenon resulted in higher IRI 

values being computed for the data collected by the left sensor in the Southern profiler. A 

comparison of the profile data collected by the two profilers using filtering techniques did not 

show the cause of the high IRI values of the left-sensor data from the Southern profiler. The data 

collected by the Southern profiler during the 1992 comparison did not show this phenomenon. 

SUMMARY OF THE FINDINGS 

ProQual applies a 300-mm (11.8-inch) moving average onto the 25-mm (1-inch) profile data 

collected by the T-6600 and ICC profilers and extracts the data points at 150-mm (5.9-inch) 

tervals. The IRI values are computed using this averaged data. ProQual does not apply a 

are a

uses the IR ented in the World Bank report.(9) The 

cally 

ll be 

ightly higher than the IRI values for the corresponding section in the LTPP database. 

the test sections were evaluated, the difference between the two sensors for wave numbers 

greater than 1 cycle/m (0.3 cycle/ft) was seen at all four of the test sites. An evaluation o

data collected by the Western profiler during the 1996 regional verification test and the d

c

appears that this problem in the right sensor occurred at some point between 1996 and 1998, an

was fixed at some point between 1998 and 2000. This also could have been an intermittent 

problem with the sensor that occurred when testing was performed for the 1998 profiler 

comparison. 

IRI DIFFERENCES FOR THE SOUTHERN PROFILER DURING THE 

1991 PROFILER COMPARISON 

The report prepared for the LTPP profiler comparison that was conducted in 1991 indicated tha

the data collected by the left sensor in the Southern profiler was giving IRI values that were 

(11)

in

moving average before computing the IRI for the data collected by the DNC 690 profiler, which 

t 152.4-mm (6-inch) intervals, with a moving average having already been applied to this

data by the profiler. ProQual I algorithm docum

IRI algorithm applies a 250-mm (9.8-inch) moving average onto the profile data before 

computing IRI if the data recording interval is less than 250 mm (9.8 inches). Literature 

published after this report have indicated that the moving average applied by the IRI algorithm 

should be omitted if the profile data have already been subjected to a moving average.(2,30)

However, the IRI values that are currently in the LTPP database have been computed by 

subjecting the profile data to two moving averages, one applied by ProQual and the other applied

by the IRI algorithm. If a researcher obtains profile data from the LTPP database and specifi

omits the moving average that is applied by the IRI algorithm, the computed IRI values wi

sl

103 
If a researcher obtains 25-mm (1-inch) interval LTPP profile data and computes the IRI, the

resulting IRI values will be slightly higher than the corresponding IRI values that are in the 

LTPP database. This is because the IRI in the LTPP database have been computed after the 

25-mm (1-inch) profile data have been processed using ProQual, which applies a 300-mm 

(11.8-inch) moving average onto the 25-mm (1-inch) data, extracts data at 150-mm (5.9-inch) 

intervals, and then uses that data to compute IRI. 

The profile data for the repeat runs conducted by the T-6600 profiler before June 2000 may show 

oor repeatability at sections where an extremely rough localized feature is present on the 

celerometer(s) in 

er range and contaminate the long wavelengths after that 

vent. This occurrence will not affect the IRI computed from the profiles, since the contaminated 

ta 

tion in 

urs 

he 1998 LTPP profiler comparison indicated that some 

ontamination was present in the data collected by the right sensor of the Western profiler at 

han 

es 

. 

nd was 

ssed 

on the 

p

pavement. This is because an extremely rough localized feature can cause the ac

the profiler to exceed the acceleromet

e

wavelengths are outside of the wavelength range influencing the IRI. The range of the 

accelerometers in LTPP’s T-6600 profilers was increased during May–June 2000, and the da

collected after that were not expected to show such behavior. 

In some instances, PSD plots of data collected by T-6600 profilers showed either one or two 

spikes for wave numbers greater than 5 cycles/m (1.52 cycles/ft), which corresponds to 

wavelengths of less than 0.2 m (0.7 ft). This is an indication that there is some contamina

the profile data in the short wavelengths. This phenomenon will not be seen in the profile data 

that is in the LTPP database, since wavelengths of less than 1 m (3 ft) have been attenuated in

these profiles by the application of the moving average by ProQual. The cause of this 

contamination is not known. This contamination affects neither the IRI or the RN, since it occ

at wavelengths outside of the wavelength range influencing these two smoothness indices. 

Currently in the LTPP program, quality control of the profile data is performed using the 

ProQual-processed data. To detect contamination such as this, quality control procedures must be 

performed on the 25-mm (1-inch) data. 

An analysis of the data collected for t

c

wave numbers greater than 1 cycle/m (0.3 cycle/ft), which corresponded to wavelengths less t

1 m (3 ft). This phenomenon will not be seen in the ProQual-processed data, since short 

wavelengths are attenuated by the application of the moving average. The contamination do

not affect the IRI because it occurs at wavelengths that are outside of the range of the 

wavelengths influencing the IRI. However, it is affecting the RN computed from the 25-mm (1-

inch) data. This phenomenon was not noted in the data collected by the Western profiler during 

the 1996 verification study nor in the data collected during the 2000 profiler comparison

Therefore, it appears that this phenomenon occurred sometime between 1996 and 1998, a

fixed sometime between 1998 and 2000, or perhaps this is an intermittent problem. Currently, in

the LTPP program, quality control of the profile data is performed using the ProQual-proce

data. To detect contamination such as this, quality control procedures must be performed 

25-mm (1-inch) data. 

104 
CHAPTER 8: CONCLUSIONS AND RECOMMENDATIONS 

High-quality longitudinal profile data have been collected with all three inertial profilers th

have been used in the LTPP program—the DNC 690, T-6600, and ICC profilers. The data 

collected by these profilers provide a valuable reso

at 

urce for researchers. Good agreement in IRI 

alues between the DNC 690 and T-6600 profilers was observed. Also, good agreement in IRI 

t any 

es 

n error, 

 (0.4 

e 

ctual 

FFECT OF APPLYING A MOVING AVERAGE ONTO PROFILE DATA 

CC 

ervals. 

tes 

e 

e 

or for those who are interested in examining minute details in the profile 

.g., pavement distresses and joints in PCC pavements). Therefore, it is recommended that a 

rocedure be put in place where the 25-mm (1-inch) interval data are easily available to 

researchers. 

v

values between the T-6600 and ICC profilers was observed. This indicates that the IRI values in 

the LTPP database can be used for analysis of roughness progression at test sections withou

adjustments being made to the IRI values obtained by the different profilers. The research also 

indicated that using IRI to evaluate profiler repeatability, accuracy, and reproducibility is not 

sufficient. The following are conclusions and recommendations based on the analyses that were 

conducted for this project. 

DATA COLLECTED BY INERTIAL PROFILERS 

Conclusions: Data collected by inertial profilers do not accurately portray very narrow featur

such as cracks in either AC or PCC pavements, or joints in PCC pavements. This is not a

but rather an effect of the low-pass filtering that is performed on the profile data. Evaluation of 

25-mm (1-inch) data collected by both the T-6600 and ICC profilers over a joint in a PCC 

pavement showed that the joint appeared in the profile as a feature that was spread over a 

distance of 75 mm (3 inches), when the width of the joint was actually closer to 10 mm

inches). This happens because of the low-pass filter that is applied to the profile data. The lowpass filter applied to the profile data will distort narrow downward features by attenuating th

depth of the feature, and will also spread the feature over a distance that is more than the a

width of the feature. 

E

Conclusions: In the LTPP program, the 25-mm (1-inch) data obtained from the T-6600 and I

profilers are processed using the ProQual software. ProQual applies a 300-mm (11.8-inch) 

moving average onto the 25-mm (1-inch) data and extracts data at 150-mm (5.9-inch) int

These data are used to compute IRI values, and this averaged data are uploaded to the LTPP 

database. The application of the moving average onto the 25-mm (1-inch) data attenua

features with wavelengths less than 1 m (3 ft). Detailed profile features cannot be observed in th

ProQual-processed data because of this effect. The moving average also can cause distortion of 

the profile data, and the averaged data can show features that are not actually present in th

pavement while eliminating features that are present. 

Recommendations: Although the profile data that are currently in the LTPP database can be 

used for many research purposes, these data are not useful for researchers who are interested in 

short-wavelength data 

(e

p

105 
COMPA ERS 

Conclusions: Since the DNC 690 profiler recorded profile data at 152.4-mm (6-inch) intervals, 

0 

OMPARISON OF THE K.J. LAW ENGINEERS T-6600 AND ICC PROFILERS 

onclusions: Since the 25-mm (1-inch) interval data were available for both the T-6600 and 

tent 

ed by the two profilers, although both profilers apply an upper-wavelength 

utoff filter of 100 m (328 ft). When the ProQual-processed data for the two profilers are 

een, and good 

greement between the two profilers will be seen for wavelengths less than 1 m (3 ft) since the 

a before computing IRI if the data 

cording interval is less than 250 mm (9.8 inches). Literature published after the World Bank 

if 

 has 

d to a 304.8-mm (12-inch) moving average. When ProQual computes IRI from 

RISON OF THE K.J. LAW ENGINEERS DNC 690 AND T-6600 PROFIL

when comparing data from the T-6600 profiler with the DNC 690 profiler, only the 150-mm 

(5.9-inch) interval ProQual-processed data from the T-6600 profiler can be compared. 

Comparison of profile data for the two profilers showed good agreement, although there were 

some differences in the profiles for sections that had significant long-wavelength content. These

differences are attributed to the different long-wavelength cutoff filter values used with the two 

profilers (91 m (300 ft) for the DNC 690 profiler, and 100 m (328 ft) for the T-6600 profiler). An 

evaluation of the profile data indicated that the long-wavelength cutoff filtering technique used 

in both of these profilers appeared to be similar. Good agreement in IRI values for the DNC 69

and T-6600 profilers was observed. 

C

C

ICC profilers, a comparison of the 25-mm (1-inch) interval data for the two profilers could be 

performed. Comparison of profile data for the two profilers using PSD plots indicated that there 

were differences in both the short and long wavelengths. There was good agreement in the 

profile data for the two profilers for wavelengths between 1 and 40 m (3 and 131 ft). For 

wavelengths less than 1 m (3 ft), the ICC profiler usually showed a higher wavelength con

than the T-6600 profiler. This is attributed to the smaller footprint of the ICC profiler, which 

most likely caused texture effects and the higher magnitude of narrow features to be recorded. 

For wavelengths greater than 40 m (131 ft), the T-6600 profiler recorded more wavelength 

content than the ICC profiler. This is attributed to differences in the long-wavelength filtering 

techniques that are us

c

compared using PSD plots, only the differences at the higher wavelengths will be s

a

application of the moving average attenuates the short-wavelength features. Good agreement in 

IRI, which is primarily influenced by wavelengths between 1 and 30 m (3 and 100 ft), was 

obtained for data collected by these two profilers. 

IRI VALUES 

Conclusions: In the LTPP program, IRI values are computed using ProQual, which uses the IRI

algorithm documented in World Bank Technical Report 46.(9) The IRI algorithm applies a 

250-mm (9.8-inch) moving average onto the profile dat

re

report have indicated that the moving average applied by the IRI algorithm should be omitted 

the profile data have already been subjected to a moving average.(2,30) Before computing IRI, 

ProQual applies a 300-mm (11.8-inch) moving average onto the 25-mm (1-inch) profile data 

collected by the T-6600 and ICC profilers; however, this moving average is not applied to the 

152.4-mm (6-inch) interval profile data collected by the DNC 690 profiler, which already

been subjecte

106 
such data, the moving average in the IRI algorithm is applied to the data. Thus, during 

roQual and the other applied by the IRI algorithm. This will cause a slight downward bias in the 

rvals and uses these data to compute IRI. 

nderestimates the IRI value. The procedure described in the World Bank report on which the 

sing 

ge 

g 

Variations in the path followed by the profiler from the path where Dipstick 

ments are performed. 

Features recorded by the profiler that are missed by Dipstick. 

d 

computation of IRI, the profile data are subjected to two moving averages, one applied by 

P

IRI values. A limited analysis using a set of sections with IRI ranging from 0.9 to 2.8 m/km 

(57 to 178 inches/mi) showed that the current LTPP procedures for computing IRI result in a 

downward bias in IRI ranging from 0.7 to 2.3 percent. 

The IRI is influenced by the sampling interval. Thus, if a researcher obtains 25-mm (1-inch) 

interval LTPP profile data and computes IRI, the resulting IRI values will be slightly higher than

the corresponding IRI values that are stored in the LTPP database. This is because IRI in the 

LTPP database have been computed after the 25-mm (1-inch) profile data have been processed 

using ProQual, which applies a 300-mm (11.8-inch) moving average onto the data, then extracts 

data points at 150-mm (5.9-inch) inte

Recommendations: The current procedure used by ProQual to compute IRI slightly 

u

IRI computation procedure used in ProQual is based does not specifically indicate that the 

moving average applied by the IRI algorithm should be omitted if the profile data have already

been subjected to a moving average.(9) Since all time-sequence IRI values for a test section have 

this bias, the bias in the IRI values will not affect roughness progression studies performed u

these data. Reprocessing profile data to compute IRI values by omitting the moving avera

applied by the IRI algorithm will be a major undertaking that will require a vast amount of 

resources. The very slight bias in IRI is not a major error that justifies reprocessing the IRI 

values. Thus, it is recommended that no changes be made to the current procedure for computin

IRI values. 

DIFFERENCES IN IRI BETWEEN PROFILERS AND DIPSTICK 

Conclusions: A variety of factors can cause the IRI obtained from the Dipstick data to differ 

from the IRI obtained from the profiler data. The factors that contribute to the differences 

between the Dipstick IRI and the profiler IRI are: 

• Sampling qualities of Dipstick. 

•

measure

•

• The footpads of Dipstick bridging over narrow downward features that are measured by 

the profiler. 

• Features recorded by the profiler that are underestimated by Dipstick because the footpa

of Dipstick may not rest in the deepest part of a feature. 

• Differences between the profiles used for computing the difference in IRI between 

profilers and Dipstick because of the application of the moving average onto profiler 

data. 

• Errors in Dipstick measurements. 

• IRI computational procedure for Dipstick data used in the ProQual software. 

107 
Despite these limitations, data from past LTPP comparisons have shown that good agre

between profiler IRI and Dipstick IRI, typically within ±0.16 m/km (±10 inches/mi), can us

be obtained at sections that do not have significant distress. Current procedures for LTPP profiler 

comparisons use the average profiler IRI obtained from five error-free runs for comparison w

the Dipstick IRI. This procedure helps smooth out some of the variability in the profiler runs. 

ement 

ually 

ith 

ecommendations: One of the tasks in this project was to provide recommendations on 

I 

onclusions: The profile data for repeat runs that were collected by the T-6600 profiler before 

calized 

) 

pikes in PSD Plots 

onclusions: In some instances, PSD plots of data collected by T-6600 profilers showed either 

s 

he 

he profile data that are in the LTPP database, 

nce wavelengths of less than 1 m (3 ft) have been attenuated in these profiles by applying the 

mination is not known. This contamination does not 

e 

influen presence of this contamination will not affect many of the 

Recom Researchers who are using the 25-mm (1-inch) data and performing 

ould 

n to 

 with

R

recalculating IRI because of differences between profiler IRI and Dipstick IRI. The current IR

values in the LTPP database that were computed from profiler data are considered to be accurate, 

and no recalculation of IRI is necessary. 

REPEATABILITY OF THE K.J. LAW ENGINEERS T-6600 PROFILER 

C

June 2000 may show poor repeatability at a few sections where an extremely rough lo

feature is present on the pavement. This is because such a feature can cause the accelerometer(s

in the profiler to exceed the range and contaminate the long wavelengths in the profile data 

collected after such an event. However, IRI computed from such profiles still will be accurate, 

since the contaminated wavelengths are outside of the wavelength range influencing the IRI. The 

range of the accelerometers in LTPP’s T-6600 profilers was increased during May–June 2000, 

and data collected after that were not expected to show such behavior. 

SHORT-WAVELENGTH ERRORS IN DATA COLLECTED BY THE K.J. LAW 

ENGINEERS T-6600 PROFILER 

S

C

one or two spikes for wave numbers greater than 5 cycles/m (1.52 cycles/ft), which correspond

to wavelengths less than 0.2 m (0.7 ft). This indicates that there is some contamination in t

profile data. This phenomenon will not be seen in t

si

moving average. The cause of this conta

affect the IRI or RN because it occurs at wavelengths outside of the wavelength rang

cing both of these indices. The 

analyses that can be performed using the 25-mm (1-inch) profile data.

mendations:

research that make use of the extremely short wavelengths should be aware of this issue. They 

sh evaluate the data and determine whether any contamination is present in the short 

wavelengths and, if it is present, determine whether it will impact the research that they pla

do that data. 

108 
Western K.J. Law Engineers T-6600 Profiler Data 

Conclusions: Data obtained from the 1998 LTPP profiler comparison indicated that some 

contamination was present in the data collected by the right sensor of the Western profiler for 

wave numbers greater than 1 cycle/m (0.3 cycle/ft), which corresponds to wavelengths less than 

d to the 

. 

However, the contamination is affecting the RN 

omputed from 25-mm (1-inch) data. The presence of this contamination does not mean that the 

(1-inch) data 

ith this contamination cannot be used for any analyses that make use of wavelengths of less 

n. 

o 

as fixed, and to identify 

e cause of the problem. Overlaid PSD plots of left- and right-sensor data for the set of profiles 

ct 

 should be aware of this issue, since this contamination can 

e interpreted as a pavement effect. 

ECOMMENDATIONS FOR IMPROVING CURRENT LTPP DATA COLLECTION 

data 

s can cancel out errors. In 

the field during data collection, particular attention should be paid before recording high 

m repeat runs to evaluate the repeatability of the data, (2) overlay of the data 

collected during the current data collection with the data collected during the previous 

visit to the site to determine whether or not they agree, (3) verification that spikes noted 

in the profile are caused by pavement features and not by any other factor, (4) evaluation 

of the repeatability of the IRI values, and (5) comparison of IRI obtained during the 

1 m (3 ft). This phenomenon will not be seen for ProQual-processed data that are uploade

LTPP database, since short wavelengths are attenuated by the application of the moving average

The contamination does not affect IRI because it occurs at wavelengths that are outside of the 

range of wavelengths influencing the IRI. 

c

25-mm (1-inch) data obtained from this sensor are not usable. However, 25-mm 

w

than 1 m (3 ft). This phenomenon was not noted in the data collected by the Western profiler 

during the 1996 verification study nor in the data collected during the 2000 profiler compariso

Therefore, it appears that this phenomenon occurred sometime between 1996 and 1998, and was 

fixed sometime between 1998 and 2000, or perhaps this is an intermittent problem. 

Recommendations: It is recommended that a set of 25-mm (1-inch) interval data collected by 

the Western profiler at regular time periods (e.g., 6-month intervals) be obtained and reviewed t

pinpoint when the problem with the sensor began, when the problem w

th

can be used to investigate this issue. Researchers who use 25-mm (1-inch) data and condu

research involving short wavelengths

b

R

AND DATA PROCESSING PROCEDURES 

• The current field procedures for collecting profile data using inertial profilers are 

considered to be adequate, and no changes to current procedures are required. 

• Errors during Dipstick data collection can occur because of data recording errors and 

equipment problems. A closure error check is performed at the end of Dipstick 

collection as a quality control procedure. However, it still is possible to pass the closure 

error criterion even with erroneous data, as compensation effect

data values to ensure that such readings are indeed correct. 

• Currently, in the LTPP program, there are several procedures in place to ensure the

quality of the profile data.(6) These procedures include: (1) overlay of the profile data 

obtained fro

109 
current data collection for each wheelpath with IRI obtained during the previous visit to 

ks are performed on the ProQualprocessed averaged data and will not detect problems that may occur in wavelengths less 

oving 

n 

rs 

 1 m (3 ft)), but yet the IRI values can show very good 

agreement because these short wavelengths do not influence the IRI. Thus, it is 

h) 

d using PSD plots during future comparisons so 

that the short-wavelength data collection capabilities of the profilers also can be 

Although Dipstick can be used to check IRI obtained from the profilers, it cannot be used 

s. 

ormed to identify the cause 

of the difference in IRI. 

•

the site. However, all these quality control chec

than 1 m (3 ft). These short wavelengths are attenuated when ProQual applies the m

average onto the profile data. Thus, short-wavelength data can be evaluated only by 

analyzing 25-mm (1-inch) data. It is possible to satisfy all the quality control checks that 

are currently performed on the LTPP data and yet have data errors in the short 

wavelengths. It is recommended that a quality control procedure be adopted to check the 

25-mm (1-inch) data collected by the profilers. The recommended procedure is to obtai

a set of data collected by each of LTPP’s profilers at regular intervals (e.g., 6-month

intervals) and compare the data with the data collected at the same sites during the 

previous visit to the site by using PSD plots. A consistent difference in the short 

wavelengths for the two data sets or any sharp spikes in the PSD plots for wave numbe

greater than 1 cycle/m (0.3 cycle/ft) will indicate potential problems in the shortwavelength data collected by a profiler. 

RECOMMENDATIONS ON LTPP PROFILER COMPARISONS 

• It is recommended that LTPP profiler comparisons be conducted at regular intervals to 

compare the outputs obtained from the four profilers. The data obtained from such 

comparisons will verify whether all four of the profilers are collecting similar data. 

• In the current profiler comparison procedures, emphasis is placed on comparing IRI. 

However, there can be differences in the short-wavelength data collected by the profilers 

(i.e., for wavelengths less than

recommended that in addition to the current data analysis procedures, the 25-mm (1-inc

data collected by the profilers be evaluate

compared. 

•

to check the accuracy of the profilers on pavements that have rough features or distres

The current LTPP comparisons use an IRI difference of ±0.16 m/km (±10 inches/mi)

between the profiler IRI and Dipstick IRI to judge the accuracy of LTPP’s profilers. If 

differences outside of this limit are obtained on pavements with distress or on rough 

pavements, it does not necessarily mean that there is a problem with the profiler. If such 

occurrences are encountered, an investigation should be perf

Research performed for this project showed that agreement in IRI values at a section 

between the profilers can occur because the errors compensate for each other. Roughness 

profiles or cross correlations are techniques that can be used to compare spatial 

distribution of IRI within a section between the profilers. It is recommended that such 

procedures be used in addition to current procedures when analyzing data from LTPP 

comparison studies. 

110 
APPENDIX A: LTPP PROFILER COMPARISON STUDIES 

OMPARISON: AUSTIN, TX 

mparison was conducted in February 1990. The information presented in this section was 

d from a paper written about this comparison.

1990 C

This co

obtaine

The fou m 

(1056 f

wheelp

runs at was 

marked s were asked to align the profiler along this 

ath. Each test section was divided into two sections for analysis, resulting in the availability of 

re the main findings 

om this study: 

• for 

wheelpath usually was higher than that for the left wheelpath. Poor agreement between 

•

 cases 

a, which 

e of the high IRI value. It was concluded that the side-to-side rocking motion 

induced on the profiler when traversing these rough sections caused sunlight to seep 

•

•

o Left Wheelpath: Seven sections had IRI values between 1.2 and 2.4 m/km 

m 

ches/mi), five sections had IRI values between 2.4 and 4.7 m/km 

(10)

r LTPP DNC 690 profilers were used in this comparison. Six test sections, each 322

t) long were established for testing. Reference profile measurements along both 

aths at the test sections were obtained using Dipstick. The profilers performed five repeat 

each test section at speeds of 56 and 80 km/h (35 and 80 mi/h). The left wheelpath

 at the test sections and the profiler driver

p

12 sections for comparing the profiler and Dipstick IRI. The following a

fr

In most cases, better agreement between profiler IRI and Dipstick IRI was obtained

the left wheelpath than for the right wheelpath. It was noted that the IRI for the right

profiler IRI and Dipstick IRI usually occurred at the rough sections. 

Significant differences between profiler IRI and Dipstick IRI were noted for several 

cases, with the profiler IRI being much higher than the Dipstick IRI. Most of the

where these significant differences occurred were along the right wheelpath at extremely 

rough sections that had Dipstick IRI values in excess of 4 m/km (254 inches/mi). An 

evaluation of the profiler data for such cases indicated saturation spikes in the dat

was the caus

under the shroud covering the sensors and contaminate the profile data. 

Evaluation of the profiles also indicated instances where lost lock occurred. This was 

another factor contributing to differences between Dipstick and profiler IRI values. 

An evaluation of the Dipstick IRI values obtained for the 12 sections indicated the 

following IRI distribution: 

(76 and 152 inches/mi), three sections had IRI values between 2.4 and 4.7 m/k

(152 and 298 inches/mi), and two sections had IRI values exceeding 4.7 m/km 

(298 inches/mi). 

o Right Wheelpath: Five sections had IRI values between 1.2 and 2.4 m/km 

(76 and 152 in

111 
(152 and 297 inches/mi), and two sections had IRI values exceeding 4.7 m/km 

As seen from these IRI values, many sections had extremely high IRI values. The side-to-side 

rocking motion of the profiler on some of these extremely rough sections caused sunlight to seep 

d contaminate the profile data. The majority of the 

TPP test sections will not have such high roughness values. Thus, contamination of profile data 

xtremely rough sections, sunlight 

eping under the shroud could contaminate the data by causing saturation spikes to appear in the 

son 

s were used in this study—four AC sections and four jointed PCC sections. 

hen selecting the test sections, two levels of roughness were considered—IRI less than 

 m/k l 

was to els. 

Dipstic

testing t wheelpath was 

arked on all of the test sections, and the profiler drivers were asked to align the profiler along 

s pat

profile 

The lef

80-km/

respect

the six ined 

for the 

(298 inches/mi). 

under the shroud covering the sensors an

L

by ambient light probably is not a major issue at the majority of the sections if the shroud 

covering the sensors is in good condition. However, at e

se

data. 

1991 COMPARISON: ANN ARBOR, MI 

The four LTPP DNC 690 profilers participated in this comparison. Details about this compari

were obtained from reference 11. 

Eight test section

W

2.0 m (127 inches/mi) and IRI between 2.0 and 4.7 m/km (127 and 298 inches/mi). The goa

establish two sections for each pavement type that fell into each of these roughness lev

k measurements were obtained along both wheelpaths on all of the test sections. Profile 

was performed at speeds of 64 and 80 km/h (40 and 50 mi/h). The lef

m

thi h when collecting data. Each profiler operator was instructed to obtain six error-free 

runs on each test section at each test speed. 

t- and right-wheelpath IRI values computed from the profiler data collected at the 

h (50-mi/h) test speed and from the Dipstick data are presented in tables 17 and 18, 

ively. The IRI values presented in these tables are the average IRI values computed from 

repeat runs. The Dipstick IRI values presented in these tables are the average IRI obta

two sets of measurements that were available for each wheelpath. 

Table 17. IRI values along the left wheelpath (1991). 

Device Left-Wheelpath IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 5 6 7 8 

North Atlantic Profiler 1.42 3.45 2.40 0.91 2.41 2.65 1.94 0.84 

North Central Profiler 1.14 3.33 2.35 0.99 2.67 2.62 1.91 0.91 

Southern Profiler 1.29 3.63 2.48 0.93 3.14 2.84 2.05 0.98 

Western Profiler 1.18 3.31 2.27 0.87 2.74 2.62 1.86 0.84 

Dipstick 1.20 3.44 2.10 0.84 2.65 2.43 1.81 0.98 

1 m/km = 5.28 ft/mi 

112 
Table 18. IRI values along the right wheelpath (1991). 

Device Right-Wheelpath IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 5 6 7 8 

North Atlantic Profiler 1.29 4.64 2.63 1.03 3.09 2.62 1.77 0.96 

North Central Profiler 1.34 4.29 2.54 1.01 2.89 2.65 1.74 0.96 

Southern Profiler 1.26 4.37 2.59 0.99 2.97 2.59 1.72 0.91 

Western Profiler 1.26 4.43 2.54 0.98 2.89 2.57 1.75 0.95 

Dipstick 1.26 3.91 2.48 0.93 2.65 2.41 1.75 1.10 

nted in tables 19 and 20, respectively. 

1 m/km = 5.28 ft/mi 

The differences between the average profiler IRI and the Dipstick IRI at the test sections for the 

left and right wheelpaths are prese

Table 19. Differences between profiler IRI and Dipstick IRI: Left wheelpath (1991). 

Device Profiler IRI – Dipstick IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 5 6 7 8 

North Atlantic 

Profiler 0.22 0.02 0.30 0.08 -0.24 0.22 0.13 -0.14 

North Central 

Profiler -0.06 -0.11 0.25 0.16 0.02 0.19 0.09 -0.06 

Southern Profiler 0.09 0.19 0.38 0.09 0.49 0.41 0.24 0.00 

Western Profiler -0.02 -0.13 0.17 0.03 0.09 0.19 0.05 -0.14 

Table 20. Differences between profiler IRI and Dipstick IRI: Right wheelpath (1991). 

1 m/km = 5.28 ft/mi 

Device Profiler IRI – Dipstick IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 4 5 3 6 7 8 

North Atlant Profile ic r 0.03 0.73 0.16 0.09 0.44 0.21 0.02 -0.14 

North Central Profiler 0.08 0 0 0 0 0 -0.02 .38 .06 .08 .24 .24 -0.14

Southern Profiler 0.00 0 0 0 0 0 .46 .11 .06 .32 .17 -0.03 -0.19

Western Profiler 0.00 0.52 0.06 0.05 0.24 0.16 0.00 -0.16

1 m/km = 5.28 ft/mi 

113 
The standard deviatio r the left and right 

wheelpaths are presented in tables 21 and 22, respectively. 

Table . Standard deviations of IRI lpath (1991). 

Device Standard Deviations of IRI (m/km) 

ns of the IRI values for the 80-km/h (50-mi/h) testing fo

 21 for 80-km/h (50-mi/h) runs: Left whee

 Surfa p o m ce Ty e and Secti n Nu ber 

 Asphalt Concrete 

 1 2 3 4 5 6 7 8 

North Atlantic Profiler 0.08 0.07 0.08 0.02 0.05 0.03 0.01 0.03 

North Central Profiler 0.03 0.03 0.03 0.02 0.04 0.07 0.04 0.01 

Southern Profiler 0.00 0.02 0.02 0.01 0.01 0.05 0.03 0.01 

Western Profiler 0.01 0.02 0.00 0.02 0.02 0.05 0.04 0.03 

1 m/km = 5.28 ft/mi 

Table 22. Standard deviations of IRI for 80-km/h (50-mi/h) runs: Right wheelpath (1991). 

Device Standard Deviations of IRI (m/km) 

 Surface Type and Section Number

 Asphalt Concrete

 1 2 3 4 5 6 7 8 

North Atlantic Profiler 0.03 .11 4 0.01 0.04 0.04 0.04 0.02 0 0.0

North Central Profiler 0.06 0.06 0.04 0.01 0.05 0.02 0.02 0.02 

Southern Profiler 03 8 0. 0.0 0.03 0.01 0.02 0.03 0.02 0.03 

Western Profiler 0.0 5 4 0.1 0.06 0.01 0.05 0.03 0.02 0.01 

The mai is s e

• An ANOVA performed on the IRI values indicated that the left-wheelpath IRI from the 

ANOVA wed that the left collected by the 

N rth Ce stern, and North erall mean IRI 

values for the profilers along the l g all of the 

profile runs at all of the test sections at the 80-km/h (50-mi/h) test speed were 1.98, 1.97, 

2.00, and 2.10 m/km (126, 5, 1 and 0 in es/m for t Nor entral, Western, 

N r c . c at the 

le i n a v han the 

d e e l

In th, th d h v o d rofile 

d the profiler combinations of North Central, Western, and North 

Atlantic; North Central, Western, and Southern; and North Central, North Atlantic, and 

Southern, were similar. Although the right sensor of the North Central profiler did not 

follow the same path as the other three profilers (because of the difference in sensor 

1 m/km = 5.28 ft/mi 

n findings from th tudy ar : 

Southern profiler was different from the IRI values from the other three profilers. An

 also sho

ntral, We

-wheelpath IRI computed from the data

o Atlantic profilers were similar. The ov

eft wheelpath computed by considerin

 12 27, 14 ch i) he th C

orth Atlantic, and Southern p ofilers, respe tively These values learly show th

ft sensor of the Souther

 oth

n prof ler is collecti g data that h ve higher IRI alues t

ata collected by the r thre profi ers. 

• the right wheelpa e ANOVA indicate that t e IRI alues btaine from p

ata collected by 

114 
spacing), all three cases in which the profilers were not significant in the ANOVA 

involved this profiler. Although the North Atlantic, Western, and Southern profilers have 

similar sensor spacing, the ANOVA indicated that at least one of the profilers was 

wheelpath d by consideri t sections at the 

80-km/h (5 test speed wer , 137, 143, and 

138 inches/mi) for the North Cent ern profilers, 

respectively. Although the ANOVA indicated that the IRI va four profilers 

were not similar, an examina n o e IR ilers the test 

sec i p l f a

com

• The results of the ANOVA for the left-wheelpa I e t g 

wa or the t lp h d s g nt factor 

for two of the profiler combinations out of a total of five combinations that were 

analyzed. 

• All profile d excellent rep elpaths, except 

at a ew se t had spikes i the repeatability 

of profilers was not affected b ), the level of 

rou ness, or the two speeds selec ting. Although th profiler was 

producing higher IRI values when compared to the other three profilers along the left 

wh R e ai y p r ble to 

tho e le

1992 COM S, IA

he four L rofilers participated in this comparison. The information presented in 

ent written about this comparison.(12)

ght te

(127 in

establis

Dipstic

measur

were ob

level m

one wh

measur

equest d profile runs at a section for each test speed. 

measur or 

determ

obtaine

different from the rest. The overall mean IRI values for the profilers along the right 

 compute

0-mi/h) 

ng all of the profile runs at all of the tes

e 2.18, 2.16, 2.26, and 2.15 m/km (138

ral, Western, North Atlantic, and South

lues for the

tio f th I values obtained by the prof at

tions did not indicate clear ev dence that a articu ar pro iler h d a bias when

pared to the other profilers. 

th IR show d tha the speed of testin

s not significant. F righ whee ath, t e spee of te ting was a si nifica

rs showe

ctions tha

eatability in IRI values along both whe

 f n

the

 the profile data. It was observed that

y the surface type (asphalt vs. concrete

gh ted for tes e Southern

eelpath, the repeatability of I I valu s obt ned b this rofile was compara

se obtained for the other thre profi rs. 

PARISON: AME

T TPP DNC 690 p

this section was obtained from a docum

Ei st sections were used in this study—four AC sections and four PCC sections. When 

selecting the test sections, two levels of roughness were considered (IRI less than 2.0 m/km 

ches/mi) and IRI between 2.0 and 4.7 m/km (127 and 298 inches/mi)). The goal was to 

h two sections from each pavement type that fell into each of these roughness levels. 

k measurements were obtained along both wheelpaths at all of the sections, with replicate

ements obtained at seven sections. At six sections, two sets of Dipstick measurements 

tained, with three sets of Dipstick measurements being obtained at one section. Rod-andeasurements were also obtained at these test sections; however, at some sections, only 

eelpath was surveyed. Two types of levels were used to perform the rod-and-level 

ements. The profilers collected data at speeds of 64 and 80 km/h (40 and 50 mi/h). It was 

r ed that each profiler operator obtain six goo

During data analysis, it was discovered that one of the levels that was used to obtain 

ements did not meet the resolution requirements for rod-and-level measurements f

ining IRI that are outlined in ASTM Standard E1364-95 (2000).(31) Measurements 

d from this level produced high IRI values. 

115 
The lef

(50 mi/ ely. 

The IR d 

from th

obtaine

availab

level th

(m/km) 

t- and right-wheelpath IRI values computed from the profiler data collected at 80 km/h 

h), Dipstick data, and rod-and-level data are presented in tables 23 and 24, respectiv

I values presented in these tables for the profilers are the average IRI values compute

e six repeat runs. The Dipstick IRI values presented in these tables are the average IRI 

d from the replicate measurements for cases where more than one set of data were 

le. For the rod and level, only the IRI values obtained from the data collected with the 

at met the resolution requirements outlined in ASTM Standard E1364 are shown. 

Table 23. IRI values along the left wheelpath (1992). 

Device IRI 

 Surface Type and Section Number 

 Asphalt Concrete 

 3 5 6 7 1 2 4 8

No Profile rth Atlantic r 1.42 1.18 0.66 1.53 1.59 2.21 4.23 1.42 

North Central Profiler 1.39 1.25 0.65 1.75 1.62 2.13 4.59 1.28 

Southern Profiler 1.39 1.23 0.60 1.55 1.74 2.37 4.37 1.47 

Western Profiler 1.39 1.23 0.60 1.53 1.69 2.33 4.34 1.44 

Dipstick 1.47 1.31 0.66 1.85 1.81 2.32 4.12 1.39 

Rod and Level N/A 1.45 N/A N/A N/A 2.35 4.15 N/A 

N/A = Measurements not obtained. 

Table 24. IRI values along the right wheelpath (1992). 

Device IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 3 5 6 7 1 2 4 8 

North Atlantic Profiler 1.39 0.93 0.79 3.50 2.02 1.67 5.68 1.09 

North Central Profiler 1.36 0.85 0.74 3.64 1.97 1.59 5.74 1.09 

Southern Profiler 1.32 0.88 0.74 3.08 1.97 1.59 5.55 1.09 

Western Profiler 1.45 0.93 0.76 3.61 2.02 1.61 5.68 1.12 

Dipstick 1.37 0.93 0.76 3.52 2.13 1.69 5.63 1.10 

Rod and Level N/A N/A 0.95 N/A N/A 1.66 5.57 1.20

N/A = Measurements not obtained. 

1 m/km = 5.28 ft/mi 

The differences between the average profiler IRI and the Dipstick IRI at the test secti

left and right wheelpaths are presented in tables 25 and 26, respectively. 

ons for the 

1 m/km = 5.28 ft/mi 

116 
Table 25. Differences between profiler IRI and Dipstick IRI: Left wheelpath (1992). 

Device Profiler IRI – Dipstick IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 3 5 6 7 1 2 4 8

North Atlantic Profiler -0.05 -0.13 0.00 -0.32 -0.22 -0.11 0.11 0.03 

North Central Profiler -0.08 -0.06 -0.02 -0.09 -0.19 -0.19 0.47 -0.11 

Southern Profiler -0.08 -0.08 -0.06 -0.30 -0.08 0.05 0.25 0.08 

Western Profiler -0.08 -0.08 -0.06 -0.32 -0.13 0.02 0.22 0.05 

Table 26. ifferences between pro ipstick IRI: Right wheelpath (1992). 

ro IR ps R m

D filer IRI and D

Device P filer I – Di tick I I (m/k ) 

 Sur y d n e face T pe an Sectio Numb r 

 Asphalt Concrete 

 3 5 6 7 1 2 4 8 

North Atlantic Profiler 0.02 0.00 0.03 -0.02 -0.11 -0.02 0.05 -0.02

North Central Profiler -0.02 -0.08 -0.02 0.13 -0.16 -0.09 0.11 -0.02

Southern Profiler -0.05 -0.05 -0.02 -0.44 -0.16 -0.09 -0.08 -0.02 

Western Profiler 0.08 0.00 0.00 0.09 -0.11 -0.08 0.05 0.02 

The standard deviatio 80 km/h (50 mi/h) 

for the left and right wheelpaths are presented in tables 27 and 28, respectively.

able 27. Standard deviations of IR

Device Standard Deviations of IRI (m/km) 

ns of the IRI values obtained from testing performed at 

T I for 80-km/h (50-mi/h) runs: Left wheelpath (1992). 

 Sur y d n e face T pe an Sectio Numb r 

 Asphalt Concrete 

 3 5 6 7 1 2 4 8 

North Atlantic Profiler 0.05 0.02 0.02 0.05 0.05 0.03 0.03 0.02 

North Central Profiler 0.02 0.02 0.02 0.04 0.04 0.02 0.11 0.03 

Southern Profiler 0.01 0.01 0.01 0.04 0.02 0.02 0.02 0.01 

Western Profiler 0.02 0.01 0.01 0.04 0.04 0.02 0.03 0.03 

1 /km = 5

1 m/km = 5.28 ft/mi 

 m .28 ft/mi 

1 m/km = 5.28 ft/mi 

117 
Table 28. Standard deviations of IRI for 80-km/h (50-mi/h) runs: Right wheelpath (1992). 

Device Standard Deviations of IRI (m/km) 

 Asphalt Concrete 

Section Number 

3 5 6 7 1 2 4 8

North Atlantic Profiler 0.04 0.02 0.02 0.07 0.03 0 0 0. .03 .04 02 

North Central Profiler 0.02 0.01 0.01 0.22 0.02 0 0 0 .02 .05 .02 

Southern Profiler 0.02 0.01 0.01 0.06 0.02 0. 0 0. 02 .04 02 

Western Profiler 0.02 0.02 0.01 0.09 0.02 0. 0 0. 02 .06 02 

The f

 

• All t repe ht wheelpaths, 

except for the right wheelpath of se h high-severity 

longitudinal cracking along the right wheelpath. Very high standard deviations in IRI 

were observed at this section. It was observed that the repeatability of the profilers was 

two ing or the e C CC

• ree a d n I the Dipstick data 

r data f ou e rs b e a a f 

 performed separately on the left- and right-wheelpath IRI values obtained 

• An ANOVA performed separately on the left- and right-wheelpath IRI values obtained 

• One o els that was used to meet the 

resolution required for profile meas Standard 

E1364-95 (2000).(31) IRI compute ofiles measured by this level showed poor 

agreement with both profiler IRI and Dipstick IRI, with the rod-and-level IRI being 

• e m em re b le at e tion 

owed g re t w e IRI com f e ck

r cases w th a te 1 i

e was poor agreement in the IRI for cases where the IRI was less than 

1.6 m/km (101 inches/mi). Errors in leveling because of instrument errors (repeatability 

errors) and the deviation of the rod from the vertical will introduce random variations 

(noise) into the profiles. An analysis indicated that random variations in a profile have a 

much greater effect on the IRI for smooth pavement than for rough pavement. It was 

concluded that the effects of random variations in profiles caused by the previously 

1 m/km = 5.28 ft/mi 

ollowing are the main findings of this study: 

profilers showed excellen atability in IRI for both the left and rig

ction 7, which is an AC pavement wit

not affected by the speeds of test s ac urf ty (A pe P and ). 

Generally, good ag ment w s foun betwee the IR computed from

and the profile or all f r of th profile along oth wh elpaths t the m jority o

the sections. 

• An ANOVA

from Dipstick and the four profilers showed that the device type was not significant. 

from the profilers indicated that the speed of testing was not significant. 

f the lev obtain elevation measurements did not 

urements that are specified in ASTM 

d from pr

higher. 

IRI obtained from th easur ents corded y the vel th met th ASTM resolu

requirements sh ood ag emen ith th puted rom th Dipsti and 

profiler data fo here e IRI w s grea r than .6 m/km (101 inches/m ). 

However, ther

118 
described factors caused the rod-and-level IRI to have poor agreement with both the 

Dipstick IRI and profiler IRI for wheelpaths that had an /km 

(101 i). 

• Results f m a study at one section indicated oncrete pavement could 

be affected considerably by slab curling. At that section, the morning and afternoon IRI 

e r fo ef lp er a 1 

/mi e , th es g s r

 an m (1 1 ch , ti

1998 : URBA L

he four LTPP T-6600 profilers participated in this comparison. This profiler comparison was 

rs since profile data for the LTPP program 

as first collected in late 1996. The information presented in this section was obtained from a 

cume

Four te e 

other tw

surface ection with 

n IRI greater than 2.2 m/km (139 inches/mi). Dipstick measurements were obtained along both 

Each pr

he left- and right-wheelpath IRI values computed from the profiler data and the Dipstick data 

profiler

left and

are 

present

The fol

The precision of a profiler along each wheelpath was evaluated by computing the 

. All of 

• g 

IRI of less than 1.6 m

inches/m

ro that the IRI for a c

values obtained by th same p ofiler r the l t whee ath w e 2.40 nd 2.1 m/km

(152 and 134 inches

 1.85

), resp ctively while e corr pondin value for the ight 

wheelpath were d 1.59 /km 17 and 01 in es/mi) respec vely. 

 COMPARISON NA, I 

T

the first-ever comparison of the four T-6600 profile

w

do nt written about this comparison.(13)

st sections were used in this study. Two of the sections were asphalt-surfaced, while th

o were PCC sections. When selecting the test sections, the goal was to select for each 

 type one section with an IRI of less than 1.6 m/km (101 inches/mi) and one s

a

wheelpaths at all of the sections. Profile testing was conducted at a speed of 80 km/h (50 mi/h). 

ofiler conducted five error-free profile runs on a test section. 

T

are presented in tables 29 and 30, respectively. The IRI values presented in these tables for the 

s are the average IRI values computed from the IRI for the five repeat runs. 

The differences between the average profiler IRI and the Dipstick IRI at the test sections for the

 right wheelpaths are presented in tables 31 and 32, respectively. 

The standard deviations of the IRI values for the profilers for the left and right wheelpaths 

ed in tables 33 and 34, respectively. 

lowing are the main findings of this study: 

•

standard deviation of the IRI for that wheelpath using the IRI values obtained from the 

five repeat runs. A profiler was considered to have failed the precision criterion if the

standard deviation of the IRI for a wheelpath exceeded 0.04 m/km (2.5 inches/mi)

the profilers met the precision criterion along both wheelpaths at all of the sites. 

The IRI bias of a profiler along each wheelpath at a section was evaluated by computin

the difference between the profiler IRI (average IRI from five runs) along that wheelpath 

and the IRI obtained by Dipstick. A profiler was considered to have satisfied the bias 

criterion if this difference in IRI was within ±0.16 m/km (±10 inches/mi). The North

119 
Atlantic, Southern, and Western profilers passed the bias criterion for both wheelpath

all of the test sections. The North Central profiler passed the bias criterion for all of

cases, except for t

s at 

 the 

he right wheelpath of the rough AC section (section 2). The variability 

in the IRI values obtained by the North Central profiler for the repeat runs is considered 

to be the cause of the profiler failing the bias criterion at this section. 

Table 29. IRI values along the left wheelpath (1998). 

Device IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 

North Atlantic Profiler 1.02 2.49 1.30 2.77 

North Central Profiler 1.04 2.55 1.28 2.90 

Southern Profiler 1.02 2.56 1.27 2.83 

Western Profiler 1.02 2.59 1.28 2.90 

Dipstick 0.95 2.52 1.17 2.87 

Table 30. IRI values along the right wheelpath (1998). 

Device IRI (m/km) 

1 m/km = 5.28 ft/mi 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 

North Atlantic Profiler 1.08 2.38 1.26 3.09 

North Central Profiler 1.05 2.62 1.24 3.17 

Southern Profiler 1.04 2.39 1.24 3.05 

Western Profiler 1.04 2.50 1.22 3.20 

Dipstick 0.96 2.46 1.13 3.17 

1 m/km = 5.28 ft/mi 

Table 31. Differences between the profiler IRI and Dipstick IRI: Left wheelpath (1998). 

Device Profiler IRI – Dipstick IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 

North Atlantic Profiler 0.07 -0.03 0.13 -0.10 

North Central Profiler 0.09 0.03 0.11 0.03 

Southern Profiler 0.07 0.04 0.10 -0.04 

Western Profiler 0.07 0.07 0.11 0.03 

1 m/km = 5.28 ft/mi 

120 
Table 32. Differences between the profiler IRI and Dipstick IRI: Right wheelpath (1998). 

Device Profiler IRI – Dipstick IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 

North Atlantic Profiler 0.12 -0.08 0.13 -0.08 

North Central Profiler 0.09 0.16 0.11 0.00 

Southern Profiler 0.08 -0.07 0.11 -0.12 

Western Profiler 0.0 4 0. 8 0.0 09 0.03

evia f IRI: Left wheelpath (1998). 

Standard Deviations of IRI (m/km) 

Table 33. Standard d tions o

Device 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 

North Central Profiler 0.02 0.02 0.02 0.02 

North Atlantic Profiler 0.01 0.03 0.01 0.01 

Southern 0.01 0.03 0.01 0.01 Profiler

Western Profiler 0.01 0.02 0.03 0.01 

Table 34. Standard deviations of IRI: Right wheelpath (1998). 

Device Standard Deviations of IRI (m/km) 

 Surface Type and Section Number 

 Asphalt Concrete 

 1 2 3 4 

North Central Profiler 0.01 0.02 0.02 0.02 

North Atlantic Profiler 0.01 0.04 0.02 0.00 

Southern Profiler 0.01 0.02 0.01 0.01 

Western Profiler 0.01 0.02 0.01 0.03 

• The evaluation of the bias and precision values wed n

would indicate that it was diffe nt fr e oth fil

• An ANO t sep for ft- ght- path IRI values 

for the f ted that th ere no differences in IRI values obtained for 

the four e left and hee . 

1 m/km = 5.28 ft/mi 

m/km = 5.28 ft/mi 

m/km = 5.28 ft/mi 

1 

1 

sho

r p

o d

rs. 

istinct trend for a profiler that 

re o h m t e ro e

VA that was carried ou arately the le and ri wheel

our profilers indica ere w

profilers in both th right w lpaths

121 
•

for the North Atla rth Centr ing wavebands: 

(1) 1 to 3 m (3 t (2) less 66 ft), and 

(4) 20 to 0 m (66 to 100 ft). The S results, except 

for the r gh PCC site. At this site, the profiler s repeatability for 

wavebands between 10 and 20 m (33 and 66 ft), and 20 and 30 m (66 and 100 ft), with 

the repe th r w d. T y be caused by the 

conditio een rofi and hic er completion of 

the com ion een ofile nd filer vehicle in 

the Southern profiler were inspected and tightened. Subsequent te ndicated that this 

profiler was able to collect repeatab file n rou ctio

2000 COMPARISON: COLLEGE STATION

The four LTPP T-6600 profilers participated in th

this section was obtained ocumen

Five pavement sections were used for profile testing: (1) a smooth AC site, (2) a medium-rough 

AC site, (3) a chip-seal secti , (4) a smooth PCC site, and (5) a rough PCC site. The smooth AC 

and the medium within the Riverside Cam xas A&M 

University and ectio certi Tex part of Transportation 

(TxDOT) profil ute measuremen

profiler were av o section ev wal rofi ta for these two 

sections were no y TxDOT use sites sed tifying profilers. 

Measurements m ence device (e.g., Dipstick or a walking profiler) were not 

performed at the other three test sections. Profile testing was conducted at a speed of 80 km/h (50 

mi/h). Each profi ction. 

The IRI values computed e profiler the left and 

right wheelpaths on the tests sections are p ely. The IRI 

values presented in these tables for the profiler verage IR mputed from the 

IRI for the five repeat runs. 

The differences file nd alking Profiler t the test sections 

are presented in

The standard de I values f prof or th and wheelpaths are 

presented in tables 38 and 39, respectively. 

A visual review of the profile data plots indicated excellent repeatability of the profiles

ntic, No

o 100 ft),

al, and Western profilers for the follow

0 than 10 m (33 ft), (3) 10 to 20 m (33 to

 3 outhern profiler also exhibited similar

ou Southern howed poor

atability being poorer for e latte aveban his ma

n of the connection betw

nnect

 the p ler bar the ve le. Aft

parison test, the co s betw the pr r bar a the pro

sting i

le pro data o gh se ns. 

, TX 

is comparison. The information presented in 

 from a d t written about this comparison.(13)

on

-rough AC sections were located 

are used as calibration s

pus of Te

ns to fy the as De ment 

ers. The IRI values comp d from ts made with an ARRB walking 

ailable for these tw s. How er, the king p ler da

t made available b beca these are u for cer

ade with a refer

ler was required to obtain five error-free profile runs on a test se

 from th data and the walking profiler data for 

resented in tables 35 and 36, respectiv

s are the a I values co

 between the average pro

 table 37. 

r IRI a the W IRI a

viations for the IR or the ilers f e left right 

122 
Table 35. IRI values along the left wheelpath (2000). 

Device IRI (m/km) 

 Test Section 

 Smooth Medium Chip Smooth Rough 

 AC AC Seal PCC PCC 

North Atlantic Profiler 1.13 1.93 3.00 1.54 2.94 

North Central Profiler 0.98 1.81 2.72 1.52 2.68 

Southern Profiler 1.01 1.85 3.31 1.48 2.78 

Western Profiler 1.04 1.94 3.33 1.56 2.90 

Walking Profiler 1.01 1.85 N/A N/A N/A 

N/A = Measurements not performed. 

1 m/km = 5.28 ft/mi 

Table 36. IRI values along the right wheelpath (2000). 

Device IRI (m/km) 

 Test Section 

 Smooth Medium Chip Smooth Rough 

 AC AC Seal PCC PCC 

North Atlantic Profiler 0.72 1.77 2.02 1.70 2.95 

North Central Profiler 0.79 1.75 2.77 1.68 2.92 

Southern Profiler 0.73 1.81 1.98 1.63 2.96 

Western Profiler 0.66 1.72 2.11 1.63 2.88 

Walking Profiler 0.67 1.75 N/A N/A N/A 

N/A = Measurements not performed. 

Table 37. Differences between the profiler IRI and the walking profiler IRI (2000). 

(m/km) 

1 m/km = 5.28 ft/mi 

Section Wheelpath Average Profiler IRI – Reference IRI 

 Profiler 

 North North Southern Western 

 Atlantic Central 

Smooth Asphalt Left 0.12 -0.03 0 0.03 

Smooth Asphalt Left 0.08 -0.04 0 0.09 

Medium Asphalt Right 0.05 0.12 0.06 -0.01 

Medium Asphalt Right 0.02 0 0.06 -0.03 

1 m/km = 5.28 ft/mi 

123 
Table 38. Standard deviations of IRI: Left wheelpath (2000). 

Profiler Standard Deviations of IRI (m/km) 

 Test Section 

 Smooth Medium Chip Smooth Rough 

 A A a C C Se l PCC PCC 

North Atlantic 0.03 0.02 0.1 .0 0.0 0 0 1 1 

North Central 0.0 0.0 0.0 .01 0.0 5 3 6 0 3 

Southern 0.0 0.0 0.0 .02 0.03 1 2 7 0

Western 0.0 0.0 0.0 .01 0.01 1 2 8 0

 IRI: Right wheelpath (2000). 

Profiler Standard Deviations of IRI (m/km) 

Table 39. Standard deviations of

 Test Section 

 Smooth Medium Chip Smooth Rough 

 AC AC Seal PCC PCC 

North Atlantic 0.01 0.02 0.02 0.01 0.03 

North Central 0 0 0 .06 .02 0. 6 0.01 0.03 

Southern 0.02 0.02 0.0 .0 0.03 9 0 2 

Western 0.0 0.0 0.0 .01 0.01 0 1 1 0

The following are the main findings of this study: 

• At b ctions, the difference between e profiler IRI 

and the walking profiler IRI was within the LTPP-specified bias criterion of ±0.16 m/km 

(±10 l of the profilers fo both wheelpaths. With the exception of the left 

between er IR lk

(6 inches/mi). 

• At the chip-seal section, the orth Ce ofi n h 4 m/km 

(47 inches/m higher than th avera t h

wh r th heelp a ue th as 0.49 

(3 r th average r the other three profilers. A sting was 

co C iler operator in that sensor might not 

ha h section rofile er su diti ly the 

ac e r compu he profile. This can explain why the IRI from 

the ofiler was different f m the IRI values obtained by the other three 

profilers at this site. 

mi 

i 

1 m/km = 5.28 ft/

1 m/km = 5.28 ft/m

oth the smooth AC and medium AC se th

 inches/mi) for al r 

wheelpath on the smooth AC section for the North Atlantic profiler and the right 

wheelpath on the smooth AC section for the North Central profiler, the difference 

 the profil I and the wa ing profiler IRI was less than 0.1 m/km 

N ntral pr ler had a IRI value t

ree p

at was 0.7

i)

h

e 

 w

ge IRI for

ath, it had

he other t

n IRI val

rofilers in the right 

eelpath, w ile fo t w

1 inches/mi) lowe

e left

an the

a /km

fter te

m

 IRI fo

mpleted, the North entral prof dicated the covers

ve been taken off w en this was p d. Und ch con ons, on

celerometer data ar used fo ting t

 North Central pr ro

124 
• All profile C, medium AC, 

smooth PCC ium PC criterion for IRI 

that is used in the LTPP comp .04 m/km 

(2.5 inches/mi)) was satisfied by all four profilers along both wheelpaths, except by the 

North C tral profiler at t it t l profiler had 

IRI prec ion values of 0.05 0.06 m i) along the left and 

right wh tiv e IRI sion chip-seal section that had 

signific s va , with ion s f heel or the four 

profilers ranging from 0.01 0 m/k 6 to es

• An evaluation of the point-to-point profile repeatability values for the left, right, and 

center paths for the profilers did not show any trends that would suggest that a specific 

profiler was different when compared to the other profilers. 

• It was noted that at the smooth AC a Western 

profiler was noisy when comp ise was not 

influencing the IRI values computed from however, it may influence the 

ride ind s that weigh sh e r i

• The wheelpaths on the smooth AC and um cti ere m with white 

paint do d w aths h pac t w ghtl ter than the 

LTPP p acing. Therefore, when p g es, L s profilers 

profiled t was slig nside arke . S pike e noted on some 

of the p lected at t e, and ap h en c when the sensor 

traversed over the wheelpath markings. However, these spikes did not influence the 

compute

A visual examination of the profile data plots showed good agreement in profiles 

d by 

hen 

ntral 

e inconsistencies in the data collected with the center sensor. The North 

Central region indicated that they were aware of this problem with this sensor and were 

2003 C

The fou

Mn/RO

profiler

T-6600 , 

and this profiler was also used in the comparison. The information presented in this section was 

btained from a document written about this comparison.(15)

rs showed excellent repeatability of IRI values at the smooth A

, and med C sites. At these four sites, the precision

arison studies (a precision of less than 0

en the smooth AC site. A this s e, the Nor h Centra

is and /km (3.2 and 3.8 inches/m

eelpaths, respec ely. Th preci at the

ant cracking wa riable precis value or a w path f

 to 0.1 m (0. 6 inch /mi). 

nd medium AC sites, the left sensor of the 

ared to the other three profilers. This no

these profiles; 

ice orter wav lengths mo e heav ly. 

 medi

ad a s

AC se

ing tha

ons w

as sli

arked

ts. These marke heelp y grea

rofiler sensor sp rofilin the sit TPP’

 a path tha htly i the m d path mall s s wer

rofiles col his sit these pear to ave be aused

d IRI values. 

•

collected by all of the profilers along the left, center, and right wheelpaths, except for two 

cases: The two cases involved data collected by the North Central profiler along the left 

and right wheelpaths at the chip-seal section. At this section, the data collected by the 

North Central profiler along both wheelpaths were different from the data collecte

the other profilers. (Note: It is believed that the profiler had the sensor covers on w

the profile data were collected at this site.) At the two PCC sites, the North Ce

profiler had som

in the process of correcting it with the help of K.J. Law Engineers. 

OMPARISON: MN/ROAD, ALBERTVILLE, MN 

r LTPP ICC profilers were used in this comparison, which was conducted at the 

AD facility in Albertville, MN. This was the first comparison of four of LTPP’s ICC

s after they began collecting data for the LTPP program in August 2002. One of LTPP’s 

 profilers that was used to collect profile data for the LTPP program was still operational

o

125 
st sections were used for profile testing: (1) a smooth AC section, (2) a rough AC section

ooth PCC section, (4) a medium-rough PCC section, and (5) a chip-seal section.

ements were obtained along both wheelpaths at all of the test sections. Profile testing wa

ted at a speed of 80 km/h (50 mi/h). Each profiler was required to obtain five error-free 

runs on a test section. 

t- and right-wheelpath IRI values computed from the profiler data and the Dipstick da

ented in tables 40 and 41, respectively. The IRI values presente

Five te , 

(3) a sm Dipstick 

measur s 

conduc

profile 

The lef ta 

are pres d in these tables for the 

profilers are the average IRI values computed from the IRI for the five repeat runs. 

Table 40. IRI values along the left wheelpath (2003). 

Device IRI (m/km) 

 Section 1 Section 2 Section 3 Section 4 Section 5

 Smooth AC Rough AC Smooth PCC Medium PCC Chip Seal

North Atlantic: ICC 1.27 2.76 0.92 1.45 2.25 

North Central: ICC 1.26 2.75 0.93 1.57 2.15 

Southern: ICC 1.29 2.78 0.93 1.45 2.15 

Western: ICC 1.28 2.75 0.91 1.43 2.20 

K.J. Law Engineers 1.31 2.75 0.94 1.47 2.25 

Dipstick 1.17 2.80 0.88 1.32 2.24 

Table 41. IRI values along the right wheelpath (2003). 

Device IRI (m/km) 

 Section 1 Section 2 Section 3 Section 4 Section 5

 Smooth AC Rough AC Smooth PCC Medium PCC Chip Seal

North Atlantic: ICC 1.68 2.81 0.98 1.70 2.54 

North Central: ICC 1.73 3.01 1.02 1.72 2.54 

Southern: ICC 1.69 2.62 0.96 1.67 2.54 

Western: ICC 1.66 2.54 0.97 1.71 2.50 

K.J. Law Engineers 1.64 2.46 0.96 1.70 2.44 

Dipstick 1.81 2.79 0.99 1.64 2.63 

1 m/k

1 m/km = 5.28 ft/mi 

m = 5.28 ft/mi 

The differences between the average profiler IRI and the Dipstick IRI on the test sections for the 

left and right wheelpaths are presented in tables 42 and 43, respectively. 

126 
Table 42. Differences between the profiler IRI and Dipstick IRI: Left wheelpath (2003). 

Profiler Average Profiler IRI - Dipstick IRI (m/km) 

 Section 1 Section 2 Section 3 Section 4 Section 5

 Smooth AC Rough AC Smooth PCC Medium PCC Chip Seal

North Atlantic: ICC 0.10 -0.03 0.04 0.13 0.01 

North Central: ICC 0.09 -0.04 0.05 0.25 -0.09 

Southern: ICC 0.12 -0.02 0.05 0.13 -0.09 

Western: ICC 0.11 -0.05 0.03 0.11 -0.03 

K.J. Law Engineers 0.14 -0.05 0.06 0.15 0.01 

an I: R pa

Profiler i i

Table 43. Differences between the profiler IRI d Dipstick IR ight wheel th (2003). 

Average Prof ler IRI - Dipst ck IRI (m/km) 

 Section 1 Section 2 Section 3 Section 4 Section 5

 Smooth AC Rough AC Smooth PCC Medium PCC Chip Seal

North Atlantic: ICC -0.13 0.02 -0.01 0.05 -0.09 

North Central: ICC -0.08 0.22 0.03 0.08 -0.09 

Southern: ICC -0.12 -0.18 -0.03 0.03 -0.08 

Western: ICC -0.15 -0.25 -0.02 0.07 -0.13 

K.J. Law Engineers -0.18 -0.33 -0.03 0.05 -0.19 

The standard deviations of the IRI from the profilers for the left and right wheelpaths are 

presented in tables 44 and 45, respectively. 

Table 44. Standard deviations of l

Standard Deviations of IRI (m/km) 

1 m/km = 5.28 ft/mi 

1 m/km = 5.28 ft/mi 

IRI: Left whee path (2003). 

Profiler 

 Section 1 Section 2 Section 3 Section 4 Section 5

 Smooth AC Rough AC Smooth PCC Medium PCC Chip Seal

North Atlantic: ICC 0.02 0.03 0.01 0.03 0.02

North Central: ICC 0.01 0.04 0.01 0.04 0.02

Southern: ICC 0.02 0.02 0.01 0.03 0.03

Western: ICC 0.01 0.04 0.01 0.01 0.04 

K.J. Law Engineers 0.02 0.03 0.01 0.01 0.06 

1 m/km = 5.28 ft/mi 

127 
P

Table 45. Standard deviations of IRI: Right wheelpath (2003). 

rofiler Standard Deviations of IRI (m/km) 

 Section 1 Section 2 Section 3 Section 4 Section 5

 Smooth AC Rough AC Smooth PCC Medium PCC Chip Seal

North Atlantic: ICC 0.07 0.11 0.02 0.02 0.02

North Central: ICC 0.01 0.13 0.01 0.01 0.03

Southern: ICC 0.07 0.08 0.02 0.03 0.04

Western: ICC 0.08 0.09 0.01 0.01 0.03

K.J. Law Engineers 0.04 0.08 0.02 0.02 0.03

The

• Overall, all of the profilers ap se data did 

not indicate that a particular profiler was behaving differently than the other profilers as 

far as IRI repeata e

n stu that th standard deviations from iple run

e less .5 inches owever, times this criterion 

distr re pre ong the wheelpath, because even a slight shift in 

iled can signif pact on IRI. The IRI precision criterion was 

 of the case pt for llowing: profilers, except for the North 

along ht wh h of the s AC secti

 along the right wheelpath of the rough AC section (section 2); (3) left 

wheelpath of the rough AC section (section 2) and medium-rough PCC section 

ction 

s profiler. Distresses were present along the right 

wheelpath on the smooth AC section (section 1) and along both wheelpaths on the rough 

AC section rs was the most 

se of the failure of the p t these two sites. 

• Good agreement i o

LT u o ie

pstick nd the p r IRI should be within ±0 km 

ion wa for all of xcep e follow ) 

elpath of th oth AC n (sectio d the chip-seal section (section 5) 

.J. Law Eng profiler eft wheelpath of the medi ugh PCC on 

the No ntral pr , and (3) heelpath o rough AC ion 

 by all profilers, except for the North Atlantic profiler. Extensive distresses 

were present on the rough AC section (section 2), and the failure of the profilers to meet 

the specified criterion at this site may be caused by the differences in the way downward 

features in the pavement are measured by Dipstick and the profilers. 

• An evaluation of the profile data collected by the North Central, North Atlantic, and 

Western ICC profilers indicated that the profile data collected by these profilers generally 

1 m/km = 5.28 ft/mi 

 main findings of this study are: 

pear to be obtaining repeatable IRI values. The

bility is concerned. The pr cision criterion for IRI that is used in the 

LTPP compariso dies is e IRI mult s on a 

section should b

et if 

 than 0.04 m/km (2 /mi). H some

cannot be m

the path prof

esses a

have a

sent al

icant im

met for all s, exce the fo (1) all 

Central profiler the rig eelpat mooth on (section 1); (2) all of 

the profilers

(section 4) for the North Central profiler; and (4) left wheelpath of the chip-seal se

(section 5) for the K.J. Law Engineer

 (section 2), and variability on the paths profiled by the profile

likely cau rofilers to meet the specified criterion a

 between prof

PP criterion 

ler IRI and D

sed for the c

ipstick IRI was

mparison stud

btained for the m

s is that the diffe

ajority of 

the cases. The rence 

between the Di IRI a rofile .16 m/

(±10 inches/mi). This criter s met the cases, e t for th ing: (1

right whe e smo sectio n 1) an

by the K ineers , (2) l um-ro secti

(section 4) by rth Ce ofiler right w f the sect

(section 2)

128 
have similar repeatability. The K.J. Law ngineers and Southern ICC profilers showed 

much hig lpaths when 

 to the other three ICC p btained by the 

rofilers te file

capturing similar r f e p 

 t l le

other ation wa ormed to tigate the r bility of

lected e Southern profiler. This investigation indicated that t

profiler wa ining re le profil that was co able to th 

 by the othe ICC profilers on the Mn/ROAD test sections. The poor profile 

taine e Southern profiler on the Mn/ROAD test sections m e 

d by problems with the operational procedures that were followed by the 

profiler operator (e.g., insufficient lead-in, not maintaining a constant speed, etc.). 

Collection of profile data at speeds of 35, 50, 65, 80, 95, and 110 km/h (22, 38, 41, 50, 

•

utes the 

between RoadRuf and ProQual. 

E

her variability in the profile data for repeat runs along both whee

compared

four ICC p

rofilers. A comparison of the profiles o

 on the five st sections indicated that all four of the pro rs are 

 profile featu

he profiles co

es. A profile

lected by the

eature that app

other ICC profi

ared in any ICC 

rs. After the pro

rofiler was

also esent on filer 

comparison, an

 pr

 evalu s perf inves epeata the 

profile data col by th he 

Southern s obta peatab e data mpar e data

obtained r three

repeatability ob d by th ay hav

been cause

•

59, and 69 mi/h) was performed by the K.J. Law Engineers and Southern ICC profilers 

on one section. The analysis of the data indicated that the IRI value did not appear to be 

influenced by the speed of testing. 

There were differences in IRI values computed for the Dipstick data using ProQual and 

RoadRuf. RoadRuf is a software program developed by UMTRI. However, a comparison 

of IRI values obtained by ProQual and RoadRuf for profiler data showed that the IRI 

values were similar. For the Dipstick data collected along 10 wheelpaths (from 5 

sections), the differences in the IRI values from ProQual and RoadRuf ranged from 

0.004 to 0.079 m/km (0.25 to 5 inches/mi), with the IRI values from ProQual being

higher than those obtained by RoadRuf for all of the cases. When ProQual comp

IRI from the Dipstick data, it first applies a filter that has an upper-wavelength cutoff of 

100 m (328 ft), then it uses the filtered data to compute the IRI value. When RoadRuf 

computes the IRI from the Dipstick data, the software uses the Dipstick elevation profile 

to compute the IRI without any prefiltering of the data. The filtering of the Dipstick data 

performed by ProQual may be the cause of the differences in the Dipstick IRI values 

129 

APPENDIX B: PROFILER VERIFICATION STUDIES 

1996 VERIFICATION TEST 

In 1996, FHWA purchased four K.J. Law Engineers T-6600 profilers to replace the K.J. Law 

Engineers DNC 690 profilers that were used in the LTPP program. Each region compared the 

DNC 690 and T-6600 profilers before using the T-6600 profiler to collect profile data for the 

LTPP program. 

Each region used four test sections for the verification test; each test section was 152.4 m (500 ft) 

long. Two sections were surfaced with AC, while the other two were jointed PCC pavements. 

When selecting the test sections, the aim was to select two sections for each pavement type so 

that one section will have an IRI value of less than 1.6 m/km (101 inches/mi), and the other 

section will have an IRI value of greater than 2.2 m/km (139 inches/mi). Dipstick measurements 

were performed on both wheelpaths at all four test sections. Each profiler performed three 

measurement sequences at each test section, with one measurement sequence being performed 

on 1 day. A measurement sequence consisted of obtaining a set of profile runs (a minimum of 

five good profile runs) on a test section at test speeds of 64 and 80 km/h (40 and 50 mi/h). 

Therefore, at each section, each profiler collected six data sets (two speeds x three sequences). 

During testing, each profiler collected 24 data sets (4 test sections x 2 test speeds x 3 

measurement sequences). The wheelpaths at the test sections were not marked, and the profiler 

drivers judged the location of the wheelpath when profiling the sections. 

The guidelines for testing recommended that concurrent measurements be made at the test 

sections with the two profilers with a minimum time lag between measurements. The guidelines 

also recommended that an effort should be made to obtain the three profiler measurement 

sequences for a test section on 3 consecutive days. Each RSC was asked to compute the IRI 

value of the profiler data and Dipstick data using ProQual. The bias in the IRI value of a profiler 

for a wheelpath on a test section was determined by computing the difference between the 

average profiler IRI at a specific speed for a data set (computed by averaging the IRI values 

obtained from replicate runs) and the Dipstick IRI. The profiler was deemed to have satisfied the 

IRI bias criterion if this difference in IRI was within ±0.16 m/km (±10 inches/mi). For each data 

set, the standard deviation of the IRI for each wheelpath was computed by using the IRI values 

obtained for the replicate profile runs. The precision of the profiler was evaluated by using this 

standard deviation of the IRI. The profiler was deemed to have satisfied the precision criterion if 

the standard deviation of the IRI was less than 0.04 m/km (2.5 inches/mi). 

Each RSC prepared a report documenting the results of this comparison test. (See references 24, 

25, 26, and 27.) A summary of the findings contained in these reports is presented separately for 

the four regions. 

North Atlantic Region 

For all profiler testing, the same driver operated each profiler. A review of the report indicated 

that, in many cases, more than five repeat runs were conducted for each data set. Thereafter, five 

131 
runs were selected for computing the average IRI and the standard deviation of the IRI. It 

appears that t re 

reviewed to s es 

btained for the 80-km/h (50-mi/h) profiler runs and IRI obtained from the Dipstick 

at were extracted from the report are presented in table 46. The 

ifferences between the average profiler IRI and the Dipstick IRI for the profile runs obtained at 

Table 46. Profiler IRI for 80-km/h (50-mi/h) runs and Dipstick IRI: North Atlantic region. 

he five runs were selected after the IRI values from all available runs we

elect values that were similar to each other. The average profiler IRI valu

o

measurements at the four sites th

d

80 km/h (50 mi/h) are presented in table 47. It should be noted that testing resulted in six data 

sets at each section, and these tables only show the results obtained for 80-km/h (50-mi/h) 

testing, which comprise three data sets from a test section. 

Site Sequence Test Average IRI (m/km) 

 Date Left Wheelpath Right Wheelpath 

 T-6600 DNC 690 Dipstick T-6600 DNC 690 Dipstick

Smooth AC 1 10/07/96 0.78 0.86 0.83 0.89 0.87 0.92 

 2 10/09/96 0.74 0.86 0.88 0.86 

 3 10/10/96 0.75 0.87 0.87 0.86 

Rough AC 1 10/07/96 2.44 2.26 2.30 1.92 1.89 1.91 

 2 10/09/96 2.11 2.27 1.67 1.92 

 3 10/10/96 2.28 2.28 1.72 1.87 

Smooth PCC 1 10/03/96 1.19 1.25 1.27 1.37 1.30 1.39 

 2 10/04/96 1.18 1.22 1.34 1.31 

 3 10/04/96 1.20 1.21 1.40 1.33 

Rough PCC 1 10/07/96 1.92 1.81 1.93 2.11 2.12 2.22 

 2 10/09/96 1.88 1.89 2.23 2.15 

 3 10/10/96 1.88 1.88 2.18 2.18 

Note: Only one data set was obtained for Dipstick at each site. 

Table 47. Differences between the profiler IRI and the Dipstick IRI: North Atlantic regio

Site Sequence Test Profiler IRI – Dipstick IRI (m/km) 

1 m/km = 5.28 ft/mi 

n. 

 Date Left Wheelpath Right Wheelpath 

 T-6600 DNC 690 T-6600 DNC 690 

Smooth AC 1 10/07/96 -0.05 0.03 -0.03 -0.05 

 2 10/09/96 -0.09 0.03 -0.05 -0.06 

 3 10/10/96 -0.08 0.04 -0.05 -0.06 

Rough AC 1 10/07/96 0.14 -0.04 0.01 -0.02 

 2 10/09/96 -0.20 -0.04 -0.24 0.00 

 3 10/10/96 -0.02 -0.02 -0.20 -0.05 

Smooth PCC 1 10/03/96 -0.08 -0.02 -0.03 -0.09 

 2 10/04/96 -0.09 -0.05 -0.05 -0.08 

 3 10/04/96 -0.07 -0.06 0.01 -0.06 

Rough PCC 1 10/07/96 -0.01 -0.13 -0.11 -0.10 

 2 10/09/96 -0.05 -0.05 0.01 -0.08 

 3 10/10/96 -0.05 -0.05 -0.04 -0.04 

1 m/km = 5.28 ft/mi 

132 
The main findings from the study are: 

• Both profilers passed the IRI bias criterion for all data sets, except for the followin

cases: (1) at the rough AC site, the T-6600 profiler failed the criterion for two

both the left and right wheelpaths, and (2) the DNC 690 profiler failed the criterion at the 

rough PCC site along the right wheelpath for one data set. 

• Both profilers passed the precision criterion for all data sets, except for the followin

cases: (1) at the rough AC site, the DNC 690 profile

g 

 data sets in 

g 

r failed the criterion for two data sets 

in the right wheelpath; the T-6600 profiler failed the criterion along the left wheelpath for 

, the ro ailed the criterion for t wheelpath; the 

600 ed iterion along the right wheelpath f

th pro rs, i d f e r l

 eld tr r bot oth a ugh 

pavements, ell an sur 

ost cas , the s o d by ofile e les thos ined

Dipstick. 

I valu vari ore for the r ectio en c ed to moo

sections. The major e o nce lved th -6600 ler. Overall, the IRI 

s obtai fro 6 file ared to be more table thos

ed fro he T fil is m ve be res the T pro

being operat by a en iver killed at consis tracking 

egion 

For all profiler testing, 

a camera system lp th cle 90 and the 

T-6600 profilers h different nsor gs s), 

respectively). 

The report i icated that the t fiv b obt at a fo data set were 

used to com te the average an deviation of the IRI for each wheelpath. The 

average profiler IRI values obtained -k 50- rofi s a obtained 

from the Dipstick measurements at the four site er cte the

presented in ble 48. The dif enc n rag iler d the Dipstick IRI for 

the profiler runs obtained at 80 km/ ) sen tab t should be noted that 

testing resu d in six data set t ea , se only show the results obtained for 

the 80-km/h (50-mi/h) testing hic e om ec

four data sets and along the right wheelpath for three data sets, and (2) at the rough PCC

site

-6

 DNC 690 p

prof r fail

filer f two data sets in the righ

T ile the cr or three data sets. 

• For bo

were insensitive to the two testing speeds.

file t was foun e that th IRI values or th both 

u o

 l eft and ig e ht whe paths 

This h e f h smo nd ro

as w as for AC d PCC faces.

• For m es IRI value btaine the pr rs er w s than e obta b y

• The IR es ed much m ough s ns h w ompar the s th

ity of thes ccurre s invo e T profi

value

obtain

ned

m t

m the DNC

-6600 pro

90 pro

er. Th

r appe

ight ha

 repea

ult of 

 than 

-6600

e 

en the filer 

ed n inexperi ced dr who was not as s tently

the wheelpa s th th 69 iler. th a e driver of e DNC 0 prof

North Central R

the same drivers operated each profiler. The drivers of both profilers used 

 to he them align e vehi along the right wheelpath. The DNC 6

ad se spacin (1,676 and 1,422 mm (66 and 56 inche

nd firs e accepta le runs ained section r each

pu IRI d standard

 for the 80 m/h ( mi/h) p ler run nd IRI

s that w e extra d from report are 

 ta fer es betwee the ave e prof IRI an

h (50 mi/h are pre ted in le 49. I

lte s a ch section and the tables 

, w h compris three data sets fr a test s tion. 

133 
Table 48. Profiler IRI for 80-km/h (50-mi/h) runs and Dipstick IRI: North Central region. 

Site Sequence Test Average IRI (m/km) 

 Date Left Wheelpath Right Wheelpath 

 T-6600 DNC 690 Dipstick T-6600 DNC 690 Dipstick

Smooth AC 1 10/07/96 1.07 1.00 0.98 1.12 1.05 1.08 

 2 10/09/96 1.05 1.01 1.05 1.07 

 3 10/10/96 1.02 1.00 1.08 1.05 

Rough AC 1 10/07/96 3.94 3.95 3.82 4.73 4.98 4.86 

 2 10/09/96 3.96 3.97 4.78 4.92 

 3 10/10/96 3.94 3.95 4.79 4.90 

Smooth PCC 1 10/03/96 1.11 1.11 1.05 1.07 1.09 1.15 

 2 10/04/96 1.11 1.10 1.08 1.07 

 3 10/04/96 1.11 1.11 1.08 1.09 

Rough PCC 1 10/07/96 2.64 2.75 2.55 3.00 3.06 3.01 

 2 10/09/96 2.67 2.70 2.95 2.99 

 3 10/10/96 2.63 2.74 3.02 3.02 

Note: Only one data set was obtained for Dipstick at each site. 

Table 49. Differences between the profiler IRI and the Dipstick IRI: North Central region. 

Site Sequence Test Profiler IRI – Dipstick IRI (m/km) 

 Date Left Wheelpath Right Wheelpath 

 T-6600 DNC 690 T-6600 DNC 690 

Smooth AC 1 10/07/96 0.09 0.03 0.04 -0.03 

 2 10/09/96 0.07 0.04 -0.04 -0.01 

 3 10/10/96 0.05 0.02 0.00 -0.03 

Rough AC 1 10/07/96 0.12 0.13 -0.12 0.12 

 2 10/09/96 0.15 0.15 -0.07 0.07 

 3 10/10/96 0.12 0.14 -0.07 0.04 

Smooth PCC 1 10/03/96 0.06 0.05 -0.08 -0.06 

 2 10/04/96 0.06 0.04 -0.07 -0.08 

 3 10/04/96 0.06 0.06 -0.08 -0.06 

Rough PCC 1 10/07/96 0.08 0.20 -0.01 0.05 

 2 10/09/96 0.11 0.15 -0.06 -0.02 

 3 10/10/96 0.08 0.19 0.01 0.01 

The findings from the study are: 

• Both profilers satisfied the IRI bias criterion along both wheelpaths on the smooth AC 

and smooth PCC sections. On the rough AC section, the DNC 690 profiler 

1 m/km = 5.28 ft/mi 

passed the 

bias criterion for both wheelpaths for all of the cases, while the T-6600 profiler failed the 

bias criterion along the left wheelpath for all data sets. On the rough PCC section, the 

DNC 690 profiler failed the bias criterion for four data sets along the left wheelpath, 

1 = 5.28 ft/mi m/km

134 
while the T-6600 profiler passed the bias criterion for all of the cases along both 

wheelp

• oth profile satisfi IR

d smo h PC

ion cr rion le elpa thre

wheelpath f one h DN profiler failed the criterion along the right 

two oug sect he D 90 a T-66

ilers fai the cr n for ata s g th whe and

data set alo the r p

ion 

 review of the rep sho e ge I d stand d deviation of the IRI along each 

r each ta se vaila ns f dat he a e 

ofiler IRI values aine 0 (50 profile runs and IRI obtained from the 

ipstick measurem ts at te wer cted f the are ted in table 

pstick IRI for the profile runs 

/h (50 mi/h) are presented in table 51. Testing resulted in six data sets at each 

section; these tables only show the results obtained for 80-km/h (50-mi/h) testing, which 

com

Table 50. Profiler IRI for 80-km -m gion. 

Site Sequence Test 

aths. 

B rs ed he t I precis

s. e

ion criterion

 rou C 

 along bot

sec th

h wheelp

e T 0 

a s ths on the

pro fai

mooth 

AC an led

precis

ot C section On th gh A tion, 

e d ta sets, and along the right 

-660 filer the 

ite along the ft whe th for a

or 

or 

data set, w

 data sets. O

ile the

n the r

C 690

wheelpath f h PCC

prof

io t

et alon

n, NC 6 nd the 0

 one 

0 

led precision iterio one d e left elpath

ng ight wheel ath. 

Southern Reg

A ort wed that th avera RI an ar

wheelpath fo da t were computed using all a ble ru or that a set. T verag

pr obt d for the 8 -km/h -mi/h) r 

D en the four si s that e extra rom report presen

50. The differences between the average profiler IRI and the Di

obtained at 80 km

prise three data sets from a test section. 

/h (50 i/h) runs and Dipstick IRI: Southern re

Average IRI (m/km) 

 Date f Le t el Whe path Righ eel t Wh path 

 T-6600 DNC 690 Dipstick DNC 690 Dipstick T-6600

Smooth AC 1 11/0 96 6/ 0.72 0.73 0.70 0.74 0.73 0.75 

 2 11/08/96 0.71 0.74 0.74 0.73 

 3 11/08/96 0.70 0.73 0.73 0.73 

Rough AC 1 11/0 96 4/ 1.88 1.66 1.84 1.87 2.12 2.18 

 2 11/04/96 1.91 1.63 1.90 2.12 

 3 11/04/96 1.96 1.61 1.92 2.12 

Smooth PCC 1 10/3 96 0/ 1.76 1.79 1.67 1.78 1.69 1.77 

 2 10/31/96 1.74 1.77 1.76 1.69 

 3 11/01/96 1.83 1.81 1.75 1.69 

Rough PCC 1 10/3 96 0/ 2.05 2.15 2.28 2.48 2.09 2.35 

 2 10/31/96 2.02 2.16 2.50 2.09 

 3 11/01/96 2.12 2.18 2.52 2.09 

Note: Only one data set was obtained for Dipstick at each site. 

1 m/km = 5.28 ft/mi 

135 
T n. 

 Sequence Test Profiler IRI – Dipstick IRI (m/km) 

able 51. Differences between the profiler IRI and the Dipstick IRI: Southern regio

Site

 Date Left Wheelpath Right Wheelpath 

 T-6600 DNC 690 T-6600 DNC 690 

Smooth AC 1 11/06/96 -0.01 0.00 -0.05 -0.01 

 2 11/08/96 -0.02 0.01 -0.08 -0.01 

 3 11/08/96 -0.03 0.00 -0.08 -0.02 

Rough AC 1 11/04/96 -0.24 -0.45 -0.33 -0.31 

 2 11/04/96 -0.21 -0.49 -0.38 -0.28 

 3 11/04/96 -0.15 -0.51 -0.37 -0.26 

Smooth PCC 1 10/30/96 0.07 0.10 -0.10 0.01 

 2 10/31/96 0.05 0.08 -0.07 -0.01 

 3 11/01/96 0.14 0.11 -0.08 -0.02 

Rough PCC 1 10/30/96 -0.04 0.06 -0.08 0.13 

 2 10/31/96 -0.07 0.07 0.02 0.15 

 3 11/01/96 0.03 0.10 0.04 0.17 

The findings from the main study are: 

 

1 m/km = 5.28 ft/mi 

• Both profilers passed the IRI bias criterion at the smooth AC site. At the rough AC site, 

0 

filer te ong the left wheelpat e smooth PCC 

, the pr passed t

rofiler pas the b er

riterio long

rofiler ssed r for ta set ept ee d ts alo 

right wheel h; th p pas bias c erion l dat excep r 

Both profile pass c riter the sm th AC At t gh AC te, 

C 69 rof ion c n for ata sets, while the T-6600 

r fail the r alo left w ath l dat and a

the right wh lpath a . At ooth PCC site NC rofiler met 

the precisio riter d ts; th 600 pro a he c n for all data 

xcept r thr s the heelp t th h PC , bot

profilers me e p it for a sets, e ept a e le elpath 

where the T-6600 profiler failed the criterion for four ta se

• ses where the bias or the precis criteria were not met, testing was performed 

at 56 km/h (35 mi/h). The drivers appeared to have difficulty running the same wheelpath 

consistently at the slower test speed of 56 km/h (35 mi/h). 

both profilers failed the IRI bias criterion for all of the cases, except where the T-660

pro

ite

 met the cri

 DN 690 

rion al h for one data set. At th

s C ofiler r all data set

ias rit

he bias criterion fo

l a

s, while the T-6600

p sed c ion for a e f

h th

l data sets

 fo e d

long the r

ata A

ight whe

t t ug

lpath, but 

h P ite,

ailed the 

bias c th C

690 p

n a the left w eelpa r thre sets.

s, xc

he ro CC s e DN

 pa ng he

pat

 the bias c

e T-6600 

iterion

rofiler

 all da

sed the

e

rit

for thr

 for al

ata se

a sets,

 t

t fo

two data sets along the right wheelpath. 

• rs ed the pre ision c ion at oo site. he rou si

the DN

profile

0 p iler met the precis riterio a d

heelp

ll 

ed precision c iterion ng the for al a sets long 

ee for two d ta sets the sm , the D 690 p

n c ion for all ata se e T-6 filer p ssed t riterio

sets, e fo ee data set along left w ath. A e roug C site h 

t th recision cr erion ll data xc long th ft whe

 da ts. 

In most ca ion 

136 
Weste

For all profiler t , eac w at eport 

indicated that nine profile runs sually o ting the 

IRI bias for a particular wheelpath fo a ted by 

averaging t that w e o r ile r th da and then 

subtracting the Dipstick IRI from this value. Wh puting the standard deviation of the IRI 

at a site for wheelp an ic eed rofi ob for the three 

test sequen sidered

The average profiler IRI values obta e /h 

obtained fr k me rem he sites er ted

are presented in table 52. The fere ee aver rofi ob for the 

80-km/h (50-mi/h) runs and the Dipstick IRI for est s ce sen table 53. The 

values pres 53 w co m alue n 52 eport on this 

comparison did not give the values shown in tab sinc iffe bet profiler IRI 

and Dipstick IRI were computed diff s d ed p usly

Table 52. Profiler IRI for 80-km/h (50-mi/h) runs and Dipstick IRI: Western region. 

Average IRI (m/km) 

rn Region 

es g tin h pro iler f as oper

 w re c

ed by the same driver. A review of the r

 u e nducted

ee 

for each da

ver ro

ta set. W

fi I 

hen compu

r a test sp wa pu

btained fo

d, the

all prof

age p

runs fo

ler IR

e three

s com

he IRI values er ta sets

en com

 a particular

ces were con

ath d a specif test sp , all p le runs tained 

. 

ined for th 80-km (50-mi/h) profiler runs and IRI 

om the Dipstic asu ents for t four that w e extrac from the report 

dif nces betw n the age p ler IRI tained 

each t equen are pre ted in 

ented in table ere mputed fro the v s show in table . The r

le 53, e the d rences ween 

erently, a escrib revio . 

Site Sequence Test 

 Date Left Wheelpath Right Wheelpath 

 T-6600 DNC 690 Dipstick T-6600 DNC 690 Dipstick

Smooth AC 1 10/10/1996 0.78 0.79 0.86 0.94 0.92 0.97 

 2 10/10/1996 0.78 1.15 0.93 0.91 

 3 10/10/1996 0.75 0.80 0.91 0.93 

Rough AC 1 10/15/1996 2.63 3.42 1.96 2.58 2.55 2.10 

 2 10/16/1996 2.53 3.50 2.52 2.63 

 3 10/17/1996 2.60 3.13 2.67 2.56 

Smooth PCC1 1 10/11/1996 1.10 1.18 1.16 1.04 1.08 1.04 

 2 10/11/1996 0.98 1.03 0.88 0.94 

 3 10/16/1996 0.97 0.95 0.87 0.90 

Rough PCC 1 10/10/1996 2.24 2.33 1.99 2.46 2.41 2.12 

 2 10/11/1996 2.30 2.32 2.37 2.40 

 3 10/16/1996 2.58 2.55 2.60 2.70 1

 On smooth PCC data collected with T-6600 on 10/10/96. 

Note: Only one data set was obtained for Dipstick at each site. 

he main findings from this study are: 

•

ort indicated that the high 

bias values were probably caused by the way Dipstick measurements were performed. 

The report also indicated that the documentation for the Dipstick measurement 

procedures being used at that time included the following paragraph: “The footpads 

1 m/km = 5.28 ft/mi 

T

Very high IRI bias values that were outside of the acceptable range were observed for 

both profilers at the rough AC and rough PCC sites. The rep

137 
should be placed to avoid minor localized cracks, holes, open joints, the edge of open 

joints or wide cracks, and loose stones or debris.” The report indicated that this procedure 

ey 

After reviewing the report from this research project, the precision criterion failure at 

ces 

k IRI (m/km) 

was followed during Dipstick measurements. However, when profilers collect data, th

do not avoid these features, so there were high bias values for the profilers at the rough 

AC and rough PCC sites. 

• High IRI precision values that did not meet the specified criterion were obtained in many

cases. 

•

many of the sections is attributed to the procedure that was used for computing the 

precision. The other regions computed the precision for each data set. In the analysis 

performed by the Western region, all of the profiler runs obtained for all three sequen

were used to compute the precision. 

Table 53. Differences between the profiler IRI and the Dipstick IRI: Western region. 

Site Sequence Test Profiler IRI – Dipstic

 Date Left Wheelpath Right Wheelpath 

 T-6600 DNC 690 T-6600 DNC 690 

Smooth AC 1 10/10/96 -0.08 -0.07 -0.04 -0.05 

 2 10/10/96 -0.08 0.29 -0.04 -0.06 

 3 10/10/96 -0.11 -0.06 -0.06 -0.04 

Rough AC 1 10/15/96 0.66 1.46 0.48 0.45 

 2 10/16/96 0.56 1.54 0.42 0.53 

 3 10/17/96 0.63 1.16 0.57 0.46 

Smooth C PC 1 10/11/96 -0.06 0.01 0.00 0.03 

 2 10 6 -0.18 -0.13 -0.16 -0.11 /11/9

 3 10 6 -0.20 -0.21 -0.18 -0.14 /16/9

Rough C PC 1 10/10/96 0.25 0.35 0.34 0.29 

 2 10/11/96 0.31 0.33 0.25 0.27 

 3 10/16/96 0.59 0.57 0.48 0.58 

1 m/km = 5.28

Overall Comment th

age IRI and the standard 

I were different. The following procedures are used by each region in 

omputing the average IRI and the standard deviation of the IRI: 

North Atlantic Region: Five runs from each data set were used in the computations; 

es 

• in the 

 ft/mi 

 on e Results 

The procedures used by the four regions for computing the aver

deviation of the IR

c

•

however, these runs appeared to have been selected based on the review of the IRI valu

obtained from all of the replicate runs. 

North Central Region: The first five error-free runs for each data set were used

computations. 

138 
•

• site, for a particular test speed, all runs that were collected at 

the site for all three test sequences were used in the computations. 

Because of the different procedures used by the four regions in computing IRI bias and IRI 

recision, the results from the regions cannot be compared. 

Overall a 

collecte

profiler

002 VERIFICATION STUDY 

In 2002, FHWA purchased four ICC profilers t 00 

profilers. Each R omp tw fil ta 

collection. A mini um of five test site u f and each 

region was t at l t on e e ooth AC 

section with an IRI less than 1.6 m/k c

than 1.6 m/ (101 inches/mi 3) s ith I g han /km (139 

inches/mi), CC sec n w g tha /km inc i), and (5) a 

chip-seal section. Dipstick me rem e n aine e te s. The wheelpaths at 

the test site ere not marked d t o rs ju the n o heelpath 

when profi g the test section

The purpos f this profiler co ari re t val th les obtained 

by the two profilers. Each region collected data test ns f ng 

collection p cept t th lec t the sect as performed in the 

afternoon. ch region was asked to I for rror-free profile runs on each 

section for each profiler. At m y o th pro olle eas

me day. However, at several sites, there was a time difference of up to 1.5 months between the 

easurements collected by the two profilers. Each region submitted the results of the comparison 

es contractor, who prepared a report documenting the results 

btained from all four regions. Table 54 shows the IRI values that were obtained from the 

ion of 

aths. 

e ma

• RI for left- and right-wheelpath IRI) obtained by 

the two profilers showed good agreement. The difference in the mean IRI between the 

f 

Southern Region: All available replicate runs for each data set were used in the 

computations. 

Western Region: At each 

p

, it appears that the T-6600 profilers are performing satisfactorily and, generally, dat

d by the T-6600 profilers appear to be similar to the data collected by the DNC 690 

s from an IRI viewpoint. 

2

o replace the K.J. Law Engineers T-66

SC c ared these o pro ers before using the ICC profilers for da

m s ere w sed in e o

t

ach region 

following r

or this c

eq

mparison, 

 asked to selec eas e site me nt m

m (101 in

ing the

hes/mi), (2) smooth PCC section with an IRI less 

uirem s: (1) s

km ), ( rough AC ection w an IR reater t 2.2 m

 (4) rough P tio ith an IRI reater n 2.2 m (139 hes/m

asu ents wer ot obt d at th st site

s w , an he profiler perato dged locatio f the w

lin s. 

e o mp son was to compa he IRI ues and e profi

at the sectio ollowi normal LTPP data 

rocedures, ex tha e data col tion a PCC ions w

Ea submit IR values five e

an f the sites, e two filers c cted m urements on the 

sa

m

to the LTPP technical support servic

(28) o

testing. This table shows the region, site number, surface type, dates when the sites were 

profiled, average left- and right-wheelpath IRI from each profiler, and the standard deviat

the IRI obtained from each profiler along the left and right wheelp

Th in findings of this study are: 

Overall, the mean IRI values (average I

two profilers at the test sites was within ±0.05 m/km (±3.1 inches/mi) for the majority o

the sites. 

139 
140 

e higher in magnitude than the differences observed when the mean IRI 

values were compared. When individual wheelpath IRI values at the test sites were 

comparison study that 

was conducted in 2000 in Texas indicated that the differences in the IRI that were 

our LTPP K.J. Law Engineers 

profilers. This indicates that the IRI values obtained from the data collected with the 

The comparison of the profile plots indicated that there are differences in long 

the K.J. Law Engineers and ICC profilers at 

many of the sites. Visual reviews of the profiles showed that similar pavement features 

• When individual wheelpath IRI values were compared between the two profilers, the 

differences wer

compared, for 70 percent of the cases, the differences in the IRI were within ±0.10 m/km

(±6 inches/mi). An evaluation of the results from the LTPP profiler 

observed between the K.J. Law Engineers and ICC profilers were comparable to the 

differences in the IRI that were observed among the f

(14)

ICC profilers show reasonable agreement with the IRI obtained from the data collected

by the K.J. Law Engineers profilers. 

•

wavelengths in the profile data recorded by 

were being recorded by both profilers. 
0.01 

0.02 0.04 0.03 0.08 

Table 54. IRI values obtained from the 2002 verification study. 

Region Site Surface Profile Date Average IRI Left Average IRI Right Std. Dev. IRI Left Std. Dev

Number Type K.J. Law ICC Wheelpath (m/km) Wheelpath (m/km) Wheelpath (m/km) Wheelpa

 Law ICC Law ICC Law ICC Law 

N. Atlantic 251002 AC 7/25/2002 7/25/2002 4.22 4.45 1.48 1.30 0. 0.22 28 0.06 N. Atlantic 361011 AC 7/24/2002 7/24/2002 0.91 0.89 0.87 0.89 0. 0.03 04 0.01 N. Atlantic 364018 PCC 7/25/2002 7/25/2002 2.64 2.87 2.22 2.18 0. 0.09 16 0.06 N. Atlantic 245807 PCC 6/05/2002 8/01/2002 1.56 1.41 1.58 1.53 0. 0.07 02 0.02 N. Atlantic 872811 AC 4/20/2001 7/31/2002 1.50 1.43 1.57 1.65 0. 0.21 02 0.07 N. Atlantic 360801 AC 7/23/2002 7/23/2002 1.11 1.08 1.09 1.06 0. 0.01 01 0.02 N. Atlantic 360802 AC 7/23/2002 7/23/2002 1.38 1.38 1.77 1.70 0. 0.01 03 0.01 N. Atlantic 360859 AC 7/23/2002 7/23/2002 1.06 1.04 1.15 1.13 0. 0.01 01 0.02 N. Central 17A001 AC 7/16/2002 7/16/2002 1.01 0.95 1.22 1.18 0. 0.05 02 0.06 N. Central 17A002 AC 7/16/2002 7/16/2002 2.68 2.78 2.83 2.69 0. 0.04 02 0.03 N. Central 17A003 PCC 7/17/2002 7/17/2002 1.06 1.08 1.15 1.16 0.02 0.01 

0.02 0.03 0.12 0.11 0.10 0.07 0.02 

0.02 

N. Central 17A004 PCC 7/17/2002 7/17/2002 4.01 4.07 4.14 4.16 0.02 0.02 N. Central 17A005 CS 7/16/2002 7/16/2002 3.20 2.95 3.65 3.76 0.08 0.08 Southern 481064 AC 7/24/2002 9/10/2002 1.89 1.94 1.75 1.75 0.05 0.04 Southern 481070 AC 7/24/2002 9/10/2002 1.58 1.63 1.73 1.82 0.05 0.08 Southern 48B350 CS 7/24/2002 9/10/2002 1.84 1.67 2.32 2.45 0.13 0.16 Southern 483003 PCC 7/24/2002 9/11/2002 2.11 2.10 2.31 2.26 0.06 0.02 Southern 485253 PCC 7/24/2002 9/11/2002 1.29 1.30 1.67 1.56 0.02 0.01 Western 320110 AC 6/10/2002 7/31/2002 0.57 0.57 0.74 0.74 0.01 0.01 Western 320209 PCC 6/10/2002 7/31/2002 1.03 1.21 0.96 1.11 0.03 0.03 Western 67454 AC 7/26/2002 7/26/2002 2.24 2.27 2.32 2.19 0.03 0.02 Western 69107 PCC 7/25/2002 7/25/2002 2.58 2.50 2.29 2.38 0.07 0.03 Western 169034 CS 6/04/2002 7/29/2002 1.81 1.74 2.02 1.98 0.05 0.03 Note: AC = Asphalt Concrete, PCC = Portland Cement Concrete, CS = Chip Seal. T141

1 m/km = 5.28 ft/mi 

REFERENCES 

1. Karamihas, S.M.; Gillespie, T.D.; Perera, R.W.; and Kohn, S.D., Guidelines for 

Longitudinal Pavement Profile Measurement, NCHRP Report 434, Transportation 

Research Board, Washington, DC, 1999. 

2. Sayers, M.W. and Karamihas, S.M., The Little Book of Profiling: Basic Information 

About Measuring and Interpreting Road Profiles, UMTRI, Ann Arbor, MI, 1998. 

3. Bertrand, C.; Harrison R.; and Hudson, W.R., Evaluation of a High-Resolution Profiling 

Instrument for Use in Road Roughness Calibration, Transportation Research Record 

1291, Transportation Research Board, Washington, DC, 1991. 

4. Manual for Profile Measurements: Operational Field Guidelines, Report No. SHRP-P378, SHRP, Washington, DC, 1994. 

5. Perera, R.W.; Kohn, S.D.; and Rada, G.R.J., Operational Procedures for K.J. Law T6600 Profilometer: Legacy Document, FHWA, Washington, DC, 2002. 

6. Perera, R.W.; Kohn, S.D.; and Rada, G.R.J., LTPP Manual for Profile Measurements and 

Processing, Version 4.1, FHWA, Washington, DC, 2004. 

7. Macpherson, D.; Olmedo, C.; and Merrill, C., ProQual 2004: User Guide, FHWA, 

Washington, DC, 2004. 

8. Macpherson, D., ProQual 2004: Overview, Federal Highway Administration, 

Washington, DC, 2004. 

9. Sayers, M.; Gillespie, T.D.; and Paterson, W.D.O., Guidelines for Conducting and 

Calibrating Road Roughness Measurements, World Bank, Washington, DC, 1986. 

10. Hadley, W.O., and Roper, H., Comparative Testing of Strategic Highway Research 

Program Profilers, Transportation Research Record 1311, Transportation Research 

Board, Washington, DC, 1991. 

11. Soil and Materials Engineers, Inc., Profilometer Comparison, SHRP North Central 

Region, Plymouth, MI, November 1992. 

12. Perera, R.W. and Kohn, S.D., Comparison of the SHRP Profilometers, Report No. 

SHRP-P-639, SHRP, Washington, DC, 1993 

(http://gulliver.trb.org/publications/shrp/SHRP-P-639.pdf). 

13. Soil and Materials Engineers, Inc., Comparison Testing of LTPP Profilers—July 1998, 

Plymouth, MI, 1998. 

143 
14. Soil and Materials Engineers, I ing of LTPP Profilers—December 

2000, Plymouth, MI, 2001. 

5. Soil and Materials Engineers, Inc., Comparison Testing of LTPP Profilers, Plymouth, 

16. ational Experiment to Harmonize Longitudinal 

and Transverse Profile Measurement and Reporting Procedures, Niveau, France, 2000. 

17. 

Report, Research Report No. 92–30, Soil and Materials Engineers, Inc., Plymouth, MI, 

18. Correlation, Final 

Report, Soil and Materials Engineers, Inc., Plymouth, MI, 1995. 

19. file Variability, Report No. FHWA-RD-00-113, 

FHWA, Washington, DC, June 2000. 

20. 60, 

Transportation Research Board, Washington, DC, 1990. 

21. ctive Comparison of Profiles, 

Report 2002–36, UMTRI, Ann Arbor, MI, 2002. 

22. A.G., Random Data: Analysis and Measurement Procedures, 

Wiley InterScience®, New York, NY, 1971. 

23. ransportation Research Institute, RoadRuf: Software for 

nalyzing Road Profiles, Tutorial, Ann Arbor, MI, October 1997. 

24. 

Profilometers and Dipstick®, Champaign, IL, 1996. 

25. s 

gion, Amherst, NY, 1996. 

1996. 

st: 

nc., Comparison Test

1

MI, 2004.

PIARC (World Road Association), Intern

Perera, R.W. and Kohn, S.D., Road Profiler Data Analysis and Correlation, Final 

1994. 

Perera, R.W. and Kohn, S.D., Road Profiler Data Analysis and 

Evans, L. and Eltahan, A., LTPP Pro

Sayers, M.W., Profiles of Roughness, Transportation Research Record 12

Karamihas, S., Development of Cross-Correlation for Obje

Bendant, J.S. and Piersol,

University of Michigan T

A

ERES Consultants, Inc., Comparison Testing of the North Central Region New/Old 

ITX Stanley Ltd., LTPP Profiling Equipment Comparison Test: New/Old Profilometer

and Dipstick® , North Atlantic Re

26. Brent Rauhut Engineering, Inc., Comparison Testing of New/Old Profilometers and 

Dipstick®: Southern Region, Austin, TX, 

27. Nichols Consulting Engineers, Chtd., New/Old Profiler—Dipstick® Comparison Te

Western Region, Reno, NV, 1996. 

28. Soil and Materials Engineers, Inc., 2002 Comparison of LTPP K.J. Law and ICC 

Profilers, Plymouth, MI, 2004. 

144 
 

29. American Society for Testing and Materials, “E1926–98 (2003), Standard Practice of 

Computing International Roughness Index of Roads From Longitudinal Profile 

Measurements, Volume 4.03, Road and Paving Materials,” Annual Book of ASTM 

0. Sayers, M.W., On the Calculation of International Roughness Index from Longitudinal 

hod 

als,” Annual Book of ASTM Standards, West Conshohocken, PA, 2004. 

Standards, West Conshohocken, PA, 2004. 

3

Road Profile, Transportation Research Record 1501, Transportation Research Board, 

Washington, DC, 1995. 

31. American Society for Testing and Materials, “E1364–95 (2000), Standard Test Met

for Measuring Road Roughness by Static Level Method, Volume 4.03, Road and Paving

Materi

145