U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2015–1137

Accuracy Testing of Steel and 

Electric Groundwater-Level 

Measuring Tapes: Test Method and 

In-Service Tape Accuracy
1

2

3

4

Cover illustration: 

1. Figure 3, Vernier scale during displacement measurement of a steel test tape (A) 

 with the steel reference tape (B). 

2. Figure 2C, Calibration Device 2 tensioner. 

3. Figure 4, Vernier scale during displacement measurement of an electric test tape (A)

 with a steel reference tape (B). 

4. Steel tape on reel. 
Accuracy Testing of Steel and Electric 

Groundwater-Level Measuring Tapes: 

Test Method and In-Service Tape Accuracy

By Janice M. Fulford and Christopher S. Clayton

Prepared in cooperation with the 

Open-File Report 2015–1137

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Fulford, J.M., and Clayton, C.S., 2015, Accuracy testing of steel and electric groundwater-level 

measuring tapes—Test method and in-service tape accuracy: U.S. Geological Survey Open-File Report 2015–1137, 

31 p., http://dx.doi.org/10.3133/ofr20151137.

ISSN 2331-1258 (online)
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Overview of Tape Calibration and Tape Corrections ...............................................................................2

Review of Tape Calibration Methods .........................................................................................................3

Well-Measurement Comparison Calibration Method.....................................................................3

Side-by-Side Calibration Method.......................................................................................................4

Tape Calibration Requirements ...................................................................................................................5

Recommended Calibration Method............................................................................................................5

Reference-Tape Requirements...........................................................................................................6

Displacement-Measurement Requirements....................................................................................6

Calibration-Interval and Test-Location Requirements....................................................................6

Tape Tensioning ....................................................................................................................................6

Steel tapes ....................................................................................................................................6

Electric Tapes ...............................................................................................................................6

Basic Calibration Steps ......................................................................................................................7

Calibration Device..........................................................................................................................................8

Displacement Measurement using the Vernier Scale..................................................................10

Estimated Calibration Improvement Using Calibration Method Indoors with 

Calibration Device.................................................................................................................12

Repeatability of Calibration Method with Calibration Device .............................................................12

Accuracy Testing of In-Service Groundwater-Level Tapes..................................................................14

In-Service Steel Tapes ......................................................................................................................14

In-Service Electric Tapes .................................................................................................................14

Comparison of In-Service with New Electric Tapes .....................................................................19

Comparison of In-Service Steel and Electric-Tape Results.........................................................27

Summary........................................................................................................................................................28

Acknowledgments.......................................................................................................................................28

References Cited..........................................................................................................................................29

Appendix 1. Calibration Worksheet .........................................................................................................30

Figures

 1. Photos showing calibration Device 2 anchor...................................................................................9

 2. Photos showing calibration Device 2 tensioner ..............................................................................9

 3. Photo showing close-up view of Vernier scale during displacement measurement 

of a steel test tape with a steel reference tape.....................................................................11

 4. Photo showing close-up view of Vernier scale during displacement measurement 

of an electric test tape with a steel reference tape .............................................................11

 5. Graph showing the cumulative measured displacement between 15 USGS 

in-service steel test tapes and the steel reference tape with an expected 

accuracy of 0.01 foot per 100-foot length ...............................................................................16
iv

 6. Graph showing the cumulative measured displacement for 100-foot graduation 

intervals between in-service steel test tapes less than 300 feet in length and 

the steel reference tape ............................................................................................................16

 7. Graph showing the cumulative measured displacement for 100-foot graduation intervals 

between in-service 300-foot steel test tapes and the steel reference tape.....................16

 8. Graph showing the cumulative measured displacement for 100-foot graduation 

intervals between in-service steel test tapes greater than 300 feet in length 

and the steel reference tape ....................................................................................................16

 9. Charts showing new and 14 USGS Water Science Center in-service electric test 

tapes, the cumulative measured displacement between electric test tapes 

and the steel reference tape with an expected accuracy of 0.01 foot 

per 100-ft length ..........................................................................................................................20

10. Charts showing the cumulative measured displacement between Solinst model 101 

electric test tapes and the steel reference tape with an expected accuracy of 

0.01 foot per 100-ft length for new and for USGS Water Science Center 

in-service electric test tapes....................................................................................................21

11. Charts showing the cumulative measured displacement between Slope Indicator 

electric test tapes and the steel reference tape with an expected accuracy 

of 0.01 foot per 100-ft length for new and for USGS Water Science Center 

in-service electric test tapes....................................................................................................22

12. Charts showing the cumulative measured displacement between Keck electric 

test tapes and the steel reference tape with an expected accuracy of 

0.01 foot per 100-ft length for new and for USGS Water Science Center 

in-service electric test tapes....................................................................................................23

13. Charts showing the cumulative measured displacement between Heron electric 

test tapes and the steel reference tape with an expected accuracy of 

0.01 foot per 100-ft length for new and for USGS Water Science Center 

in-service electric test tapes....................................................................................................24

14. Chart showing the cumulative measured displacement between in-service 

Solinst model 102 electric test tapes and the steel reference tape with an 

expected accuracy of 0.01 foot per 100-ft length..................................................................25

15. Chart showing the cumulative measured displacement between in-service 

RST model 3001 electric test tapes and the steel reference tape with 

an expected accuracy of 0.01 foot per 100-ft length ............................................................25

16. Chart showing for new and in-service electric test tapes with steel tape cores, the 

cumulative measured displacement between electric test tapes and the steel 

reference tape with an expected accuracy of 0.01 foot per 100-ft length........................26

17. Chart showing for new and in-service electric test tapes without steel tape cores, 

the cumulative measured displacement between electric test tapes and the 

steel reference tape with an expected accuracy of 0.01 foot per 100-ft length..............26

18. Chart showing for steel and in-service electric test tapes, the cumulative measured 

displacement between in-service test tapes and the steel reference tape with 

an expected accuracy of 0.01 foot per 100-ft length ............................................................27

Tables

 1. Example of tape marking correction table for a fictional electric tape .......................................3

 2. Electric groundwater-level measuring-tape probe and tape weights for various models 

and calibration tensions by tape interval used to simulate in-use tensions ......................7
v

 3. Summary statistics of measured displacement between test tape and reference 

tape over 100-foot intervals for all individual interval measurements...............................13

 4. Summary statistics of 10 replicated steel tape calibrations using four repeated 

measurements to determine the average displacement with reference tape 

in feet over 100-foot intervals ...................................................................................................13

 5. Summary statistics of 20 replicated steel tape calibrations using two repeated 

measurements to determine the average displacement with reference tape 

in feet over 100-ft intervals........................................................................................................13

 6. Effect of sample size on interval-displacement measurement accuracy using a 

measurement standard deviation of 0.0011 foot....................................................................13

 7. Information for in-service steel tapes manufactured by Lufkin and calibrated by 

proposed procedure and device..............................................................................................15

 8. Summary statistics of measured displacement between in-service steel tape and 

reference tape over 100-foot intervals for all individual interval measurements............15

 9. Summary statistics for 15 in-service steel tapes, using four repeated measurements 

to determine the displacement with the reference tape over a 100-ft interval................17

10. Based on the USGS Office of Groundwater survey, comparison of quantity and 

makes of in-service electric groundwater-level tapes with the study numbers 

and makes of in-service electric tapes...................................................................................17

11. Information for the sample of in-service electric tapes used to check accuracy 

of electric groundwater-level tapes used by the USGS.......................................................18

12. Summary statistics of in-service electric-tape measurements, individual 

interval-displacement measurements, and probe-offset errors ........................................18

13. Summary statistics of in-service electric groundwater-level tape displacement 

measurements using four measurements for each 100-foot interval................................19

14. Summary statistics of in-service electric groundwater-level tape displacement 

measurements over 100-ft tape intervals by model..............................................................19

15. New and in-service electric-tape calibration data. Displacement is the measured 

length difference between the electric tape and the steel reference tape .....................27

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

inch (in.) 0.08333 foot (ft)

foot (ft) 0.3048 meter (m)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius 

(°C) as follows: °C=(°F-32)/1.8

Specific conductance is given in microsiemens per centimeter at 25 degrees 

Celsius (μS/cm at 25 °C).
vi
Accuracy Testing of Steel and Electric 

Groundwater-Level Measuring Tapes: 

Test Method and In-Service Tape Accuracy

by Janice M. Fulford and Christopher S. Clayton

Abstract 

The accuracy of groundwater-level tapes was investigated 

by developing a tape calibration method and device and 

testing the accuracy of a sample of groundwater-level tapes 

with the calibration method and device. The sample of tapes 

included in-service U.S. Geological Survey (USGS) Water 

Science Center steel and electric groundwater-level tapes. 

The tape calibration method developed during the study 

is based on a National Institute for Standards and Technology 

(NIST) protocol, and compares the tape under calibration 

(test tape) with a NIST-traceable reference tape. The calibration method can be used to determine tape accuracy and tape 

corrections. The tape calibration device consists of an anchor 

and tensioner. The device allows individual tensioning of the 

test and reference tapes. The Vernier scale on the tensioner 

allows measurement of the difference (or displacement) 

between the test and reference tapes graduations with a resolution of 0.0005 foot (ft). The calibration method used with the 

calibration device has a repeatability (standard deviation) of 

0.0011 ft. 

The calibration device and proposed method were used 

to calibrate a sample of in-service USGS steel and electric 

groundwater tapes. The sample of in-service groundwater steel 

tapes were in relatively good condition. All steel tapes, except 

one, were accurate to ±0.01 ft per 100 ft over their entire 

length. One steel tape, which had obvious damage in the first 

hundred feet, was marginally outside the accuracy of ±0.01 ft 

per 100 ft by 0.001 ft. The sample of in-service groundwaterlevel electric tapes were in a range of conditions—from like 

new, with cosmetic damage, to nonfunctional. The in-service 

electric tapes did not meet the USGS accuracy recommendation of ±0.01 ft. In-service electric tapes, except for the 

nonfunctional tape, were accurate to about ±0.03 ft per 100 ft. 

A comparison of new with in-service electric tapes found that 

steel-core electric tapes maintained their length and accuracy 

better than electric tapes without a steel core. The in-service 

steel tapes could be used as is and achieve USGS accuracy 

recommendations for groundwater-level measurements. The 

in-service electric tapes require tape corrections to achieve 

USGS accuracy recommendations for groundwater-level 

measurements.

Introduction 

The U.S. Geological Survey (USGS) uses steel and 

electric measurement tapes to measure groundwater levels 

and to verify the accuracy of pressure transducers installed in 

groundwater wells. USGS groundwater-level measurements 

are used by water managers and scientists to make decisions 

about water allocations, to calibrate groundwater-flow models, 

and to assess groundwater flow and supply. The quality of 

groundwater-level measurements is directly related to the 

accuracy of the tapes used for those measurements. Because 

of the importance of groundwater-level measurements, USGS 

Water Science Centers (WSCs) are required by the USGS 

Office of Groundwater (OGW) to have a quality-assurance 

program (Brunett and others, 1997) that includes the calibration of all steel and electric tapes used for groundwater-level 

measurements. The details and frequency of the procedure 

used to calibrate WSC tapes is required to be part of the WSC 

quality assurance plan and is reviewed triennially during 

technical reviews of the WSC data program. 

Techniques and Methods 1–A1, “Groundwater Technical 

Procedures of the U.S. Geological Survey” (Cunningham 

and Schalk, 2011a, 2011b), describes the use of steel and 

electric tapes for USGS groundwater-level measurements. 

Its purpose is to “provide a citable document for technical 

field procedures” (p. 2) and guidance for USGS qualityassurance procedures. The report states that “graduated steel 

tape is commonly accurate to 0.01 foot” (Cunningham and 
2 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Schalk, 2011a, p. 5) and that “modern graduated electric 

tape is commonly accurate to ±0.01 foot” (Cunningham and 

Schalk, 2011b, p. 33). 

Techniques and Methods 1-A1 also gives general 

information on conducting a groundwater-level tape calibration to verify tape accuracy. The report recommends annual 

or more frequent calibration of steel and electric tapes when 

a tape is abnormally stressed or damaged in some way, and 

a steel tape for the calibration reference. However, the report 

does not mention whether the calibration reference tape 

needs traceability to a National Institute of Standards and 

Technology (NIST) reference. No specific calibration procedure for steel tapes and only vague calibration guidance for 

electric tapes are given in the Techniques and Methods report. 

Because no detailed tape calibration guidance is given, each 

WSC determines the detailed procedure used to calibrate 

their groundwater-level tapes. The lack of a uniform detailed 

calibration method may result in some groundwater measurements having a lower quality. Additionally, little information 

is readily available to verify the measurement performance of 

USGS in-service groundwater tapes. 

The USGS Hydrologic Instrumentation Facility (HIF) 

was tasked by the OGW to evaluate groundwater tape 

calibration methods and the accuracy of USGS in-service 

groundwater-level tapes. The HIF routinely evaluates the 

performance of hydrologic equipment and instruments used 

by the USGS Water Mission Area to measure, record, and 

transmit hydrologic data. The performance of the equipment is 

primarily evaluated by the HIF for compliance to an accuracy 

requirement. Evaluation of instrument accuracy requires the 

HIF to develop or use established methods to check whether 

an instrument meets an accuracy specification. In some cases, 

these methods can be used to calibrate the equipment. 

Purpose and Scope

The purpose of this study was to: (1) investigate tape 

calibration methods, and develop an accurate tape calibration 

method and device and (2) test the accuracy of a sample of 

USGS in-service steel and electric groundwater-level tapes 

using the calibration method and device. The report describes 

tape calibration and briefly reviews tape-calibration methods 

recommended by metrology laboratories and used by USGS 

WSCs. The selection criteria for the tape-calibration method 

are presented and the development of the tape-calibration 

method and device are described. Test data collected for a steel 

tape using the calibration method and device with a NISTtraceable, calibrated steel reference tape are used to determine 

the repeatability of the developed tape-calibration method. 

Accuracy of USGS in-service steel and electric 

groundwater-level tapes was investigated by test calibrating a 

sample of tapes from various USGS WSCs. Fifteen in-service 

steel tapes from five WSCs and 14 in-service electric tapes 

from seven WSCs were tested for accuracy using the tapecalibration method and device. In-service tape test results are 

compared with the expected accuracy of ±0.01 foot (ft) per 

100 ft. The accuracy tests for a sample of in-service steel and 

electric groundwater-level tapes using the tape calibration 

method are discussed and presented in tables and charts in 

the sections on the accuracy testing of in-service groundwater 

tapes.

Overview of Tape Calibration and Tape 

Corrections

A tape calibration compares the measurement by a test 

tape with a length reference of a known accuracy. A tape calibration can be used to verify that a tape is within its required 

accuracy or to produce a table of tape corrections or a tape 

correction equation. The comparison measurements are made 

at fixed intervals along the tape length and measure the length 

differences (or displacements) between the test tape and the 

length reference for each tape interval. For tapes of 100-ft 

nominal length or greater, the chosen interval can be from 1 to 

100 ft. 

Tape corrections can be used to correct measurements 

made by a less accurate tape to meet accuracy requirements. 

The tape measurement, for the ith interval, is corrected by:

Lcorrected,i =Luncorrected,i +∆i , (1)

where 

Luncorrected,i is the uncorrected tape measurement in feet, 

Δi is the measured length difference 

(displacement) or interval correction 

in feet, 

Lcorrected,i is the corrected tape measurement, and i

indicates the ith-measured tape interval. 

The displacement or interval correction, Δi

, is negative for a 

tape interval that is marked short, and is positive for a tape 

interval that is marked long. 

A tape that is marked short will measure a longer distance 

to the water level than is actual. For example, during a calibration, the test tape measures 0.01 ft shorter than the reference 

tape. Its measured correction is –0.01 ft. In equation 1, the 

minus correction will subtract the additional tape length, 

based on the tape markings needed to reach the water level. 

Similarly, a tape that is marked long will measure a shorter 

distance to the water level than is actual. For example, during 

a calibration, the test tape measures 1.00 ft longer than the 

reference tape. Its measured correction is +1.00 ft. If this tape 

makes a measurement in a well with a 100-ft actual water 

level, the tape will read 99 ft. In equation 1, the positive correction will add the additional tape length, based on the tape 

markings, needed to reach the water level. 

Tape corrections can be applied as a correction per foot of 

tape. A correction per foot of tape for the ith tape interval, c(i), 

can be computed from: 
Review of Tape Calibration Methods 3

Table 1. Example of tape marking correction table for a fictional electric tape. 

[ft, feet]

i, interval

Length 

interval 

in ft

 Actual 

marked 

length

∆i, interval 

correction 

in ft

ci, correction 

over interval 

in ft/ft

Cumulative 

correction 

in ft

1 0-100 100 -0.0130 -0.000133 -0.0130

2 100-200 100 -0.0022 -0.000022 -0.0152

3 200-300 100 -0.0110 -0.000110 -0.0263

ci= ∆i . (2) 

 

 

 

Luncorrected,i

This equation assumes that the error is linearly distributed 

over the ith measured interval and allows for unequal tape 

intervals. Table 1 is an example of a tape correction table. 

A corrected tape measurement, Mcorrected, is computed from the 

tape reading, Muncorrected, in the ith tape interval by: n 

Mcorrected=Muncorrected+{cn+1×[Muncorrected – nI]+∑i=1 ∆i }, (3)

where 

I is the calibration interval length 

(for example, 100 ft), 

n is the number of complete intervals equal 

to (Muncorrected÷I) truncated to a whole 

number, and 

cn+1 is the correction per foot of tape for the partial 

interval of the measurement. 

For example, the uncorrected measurement of 225.20 ft using 

the electric tape with the correction information in table 1 is 

corrected as follows:

Mcorrected=225.20+{–0.00011×[225.20–(2×100)]+0.0152} 

=225.20+(–0.02)=225.18 ft . (4)

This correction is only for the tape markings and does not 

include the probe correction. Alternatively, a simpler to use, 

but potentially less precise, correction method is to use an 

equation obtained by fitting with linear regression the cumulative tape-marking corrections at each interval. Using an equation derived from linear regression, a corrected tape measurement, Mcorrected, is computed from the tape reading, Muncorrected, 

by:

Mcorrected=Muncorrected×(1+Sregression ) , (5)

where Sregression is the slope of the straight line regression equation fixed to the cumulative tape-marking corrections. For the 

fictional electric tape in table 1, the regression slope, Sregression, 

is –0.00005 ft. The uncorrected measurement of 225.20 ft is 

corrected as follows:

Mcorrected=225.20[1+(–0.00005)]=225.19 ft . (6)

Review of Tape Calibration Methods 

 National and large professional metrology laboratories 

calibrate tapes similar to those used for groundwater-level 

measurement by laying out the tape to be calibrated on a 

flat, level surface and comparing it to a length reference 

whose accuracy (or uncertainty) has been evaluated. These 

laboratories use displacement-measuring interferometers 

to precisely measure the difference between the tape under 

calibration and the length reference. Displacement-measuring 

interferometers can have a resolution as small as 6 x 10-9 inch 

(in.). Because of their high expense, calibration methods using 

displacement-measuring interferometers were not considered. 

Tape-calibration methods reviewed were limited to those using 

simpler displacement-measurement methods to measure the 

length differences between the test and reference tapes. Tape 

calibration methods using simpler displacement-measurement 

methods can be classified into two basic calibration methods: 

(1) the well measurement comparison and (2) the side-by-side 

tape comparison. 

Well-Measurement Comparison Calibration 

Method

The well-measurement-comparison calibration method 

is described for electric tapes by Garber and Koopman (1968, 

p. 9) and is one of the electric-tape calibration methods 

listed by Cunningham and Schalk (2010, p. 35). The wellmeasurement-comparison calibration method is also described 

in internal USGS quality-assurance documents and used by 

some USGS offices.

The calibration method compares groundwater-level 

measurements made with the test tape (the tape to calibrate) 

to measurements made with a steel reference tape in one or 

more wells of various depths to water. A steel tape water-level 

measurement requires chalking the end section of the tape. 

The steel tape groundwater-level measurement is determined 

by subtracting the length of the wetted chalk mark (the cut 

line) on the steel reference tape from the graduation value held 

at the top of the well (the hold) during the measurement (Cunningham and Schalk, 2011a, p. 6). The measurement difference between the test and reference tape (the displacement) is 

the tape correction for that tape length. Ideally, measurements 
4 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

are made with the steel reference tape before and after the test 

tape measurement to account for any water-level changes in 

the well. 

Calibration measurements at several wells can be 

combined to generate a tape correction curve that covers the 

range of measured water levels. Using only one calibration 

well reduces calibration time but assumes that the tape error 

is distributed in a linear manner over the tape length. This can 

cause significant measurement inaccuracies if the tape error is 

not linear. 

A variation of the well-measurement-comparison 

calibration method utilizes a down-hole camera to compare the 

tapes in the well. This method permits a direct comparison of 

the tape readings. No tape chalking and reading of the cut line 

is needed. Major disadvantages of this variation are the extra 

expense required for the down-hole camera and the larger 

well-casing diameter required to permit the lowering and 

aligning of the reference and test tapes and a camera inside the 

well. 

Accuracy of the well-comparison method is affected 

by the accuracy of the steel reference tape, the straightness 

of the well(s) casing, the range of water levels used for the 

calibration, and the changes in air temperature with location 

inside the well (usually very small). This method calibrates the 

test tape in field conditions and eliminates the need to apply 

tension to the tape during calibration. Differences in tensile 

load on the tape from tape construction and operation (steel 

versus electric) are automatically factored into the calibration measurement. For electric test tapes, the measurement 

difference between the test and reference tape includes any 

inaccuracy contributed by the operation and location of the 

water-level-indicating probe, as well as the length difference 

from the graduation-marking inaccuracies. The disadvantages 

of this method are as follows: (1) calibration time increases 

with the number of wells, (2) the reference tape is exposed to 

wear and tear, (3) test and reference tapes may not have identical alignment and distance down to the water surface, and 

(4) the calibration is not valid beyond the maximum depth of 

the measured wells. The method may require significant travel 

time to multiple wells. 

Side-by-Side Calibration Method

The side-by-side-calibration method is described by 

American Society of Mechanical Engineers (ASME, 2009) 

and by NIST (NIST, 1986). The side-by-side calibration 

method is also described in internal USGS quality-assurance 

documents and used by some USGS offices. The ASME Standard B89.1.7 is intended for steel measuring tapes. The NIST 

Standard Operating Procedure (SOP) No. 12 for tape calibration (NIST, 1986) describes the method in detail for metal 

tapes used for land surveying and construction applications. 

In general, the side-by-side-comparison calibration 

method compares the test tape to the reference tape while 

they are stretched out next to each other on a flat surface. It 

is similar to the method used by precision metrology laboratories to calibrate tapes. The method requires a location that 

has a flat, smooth, level surface that is slightly longer than (or 

a multiple of the calibration interval) needed to calibrate the 

tape. After the tapes are laid out side by side, they are aligned 

at the zero-interval marking or the graduation marking that is 

closest to zero. Usually, tension is applied to the tapes at the 

opposite end so that they lie flat. The displacement between 

the test-tape and the reference-tape interval graduation marks 

is measured at the opposite end. This displacement measurement is the measured difference between the test tape and 

the reference tape. Accuracy of this method is affected by the 

tensions applied to the test and reference tapes, tape temperature, accuracy of the reference tape, alignment of the test- and 

reference-tape marks at one end, resolution, and accuracy 

of the instrument used to measure the displacement and the 

closeness to parallel alignment of the tapes to each other.

 One version of the side-by-side method used by USGS 

offices has one technician holding the tapes aligned to the 

lower interval gradation while another technician at the 

opposite end of the tapes pulls out the slack and measures 

the displacement between the next higher interval gradation 

(for example, the 100-ft mark) of the test and reference tapes. 

This version restricts the calibration interval to no longer than 

100 ft, uses a steel reference tape, and measures only one 

displacement measurement for each calibrated tape interval. 

The method does not specify the amount of pull force to apply 

to the tapes, how to measure the displacement between the test 

tape and the reference tape, or require the measurement of the 

accuracy of the water-level probe.

The ASME document gives broad guidance on side-byside calibration methods. The guidance recommendations are 

to (1) apply a 10-pound (lb) tension to tape lengths less than or 

equal to 100 ft, (2) use a reference tape traceable to the NIST 

standards, and (3) use a comparator instrument, such as a measuring microscope, to measure the displacement between the 

test and reference tapes. The document also gives information 

on how to evaluate the accuracy of the calibration. 

The NIST side-by-side calibration method calls for three 

replicated measurements between the tapes, with the tension 

released and reapplied after each measurement. NIST specifies a 10-lb pull force for all steel tapes and millimeter (mm) 

graph paper for measuring the displacement between the tapes. 

The accuracy of the calibration is limited by the graph paper to 

between 1 and 2 mm or 0.003 ft and 0.006 ft. The NIST SOP 

(NIST, 1986) includes an explanation of using graph paper 

to measure the displacement between the test and reference 

tapes. Similar to the ASME document, the NIST documentation gives no recommendations on how to attach the scales to 

the tapes or how to anchor the other tape ends. 

There are advantages and disadvantages to the side-byside comparison. The side-by-side method can be very 

accurate and repeatable. Because it can be conducted either 

in a long office hallway or outdoors on a level area, it does 

not require access to a number of wells or special equipment. 

The advantages are excellent repeatability, small likelihood of 
Recommended Calibration Method 5

reference tape damage, fast calibration times, and the entire 

tape length can be calibrated. The disadvantages are (1) long 

tapes must be calibrated in intervals, which increases calibration time and potential error; (2) two technicians or specialized 

equipment are needed; (3) different pull forces may be needed 

for tapes of different construction, such as electric tapes; 

(4) pull forces may not equal the tape tension during a well 

measurement; and (5) the measurement inaccuracy contributed 

by the probe is not measured. 

Tape Calibration Requirements

The calibration method accuracy needed for 

groundwater-level measuring tapes should be better than the 

expected accuracy of the groundwater-level tapes (0.01 ft). 

Additionally, WSCs should be able to use the method to 

perform groundwater-tape calibrations at their offices and the 

method should not be cost prohibitive. 

Cost and effort of a calibration method is directly affected 

by the calibration accuracy requirements. USGS Techniques 

and Methods 1–A1, “Groundwater Technical Procedures of 

the U.S. Geological Survey” states that steel and electric tapes 

are commonly accurate to 0.01 ft (Cunningham and Schalk, 

2011a, 2011b; p. 8, 35, 37). Based on traditional metrology 

requirements (International Standards Organization, 1992; 

ISO 10012-1) for test accuracy ratio, the reference tape 

should have an accuracy that is one-third to one-quarter of 

the accuracy of the tape being calibrated. This requires the 

reference tape to have an accuracy of better than 0.0033 ft to 

meet an accuracy of 0.01 ft. However, the test accuracy ratio 

does not explicitly include calibration test errors that come 

from sources other than the reference tape. Ideally, to meet a 

measurement accuracy of 0.01 ft, the accuracy of the overall 

calibration method should be better than 0.0033 ft. 

The law of propagation of uncertainty, as presented in the 

International Standards Organization’s Guide to Expression 

of Uncertainty in Measurement (GUM, 1993), can be used 

to estimate the accuracy of the displacement measurement 

(length difference) and the reference tape needed to meet the 

3-to-1 test accuracy ratio. An estimate for the combined standard uncertainty, u∆, of the calibration is:

u∆=[ureference +udisplacement ]1⁄2 , (7)

where 

ureference is the standard uncertainty of the calibrated 

reference tape, and

udisplacement is is the standard uncertainty of the 

displacement measurement. 

Assuming that the resolution of the displacement 

measurement and the reference tape bound the uncertainty 

of those sources of calibration uncertainty, an estimate of 

the calibration uncertainty can be computed. Both of these 

2 2

2

2

assumptions may be optimistic, but give a ball park estimate 

of the calibration uncertainty using various measurement 

devices. Because the resolution of each uncertainty source 

is assumed to have a rectangular distribution and bound the 

uncertainty, the resolution is divided by √3 to give the standard deviation, which is equal to the standard uncertainty. If a 

steel reference tape, with a calibrated accuracy of 0.001 ft, and 

1-mm graph paper are used to calibrate a tape, the estimated 

combined standard uncertainty using equation 5 is:

u∆ = 0.005 ft 

2

+ 1mm × 0.0032808 ft/mm 2

 

1/2

= 0.02ft , (8) √3 √3 [( ) ( ) ]

and is equal to an acceptable test ratio of 1 to 5. This estimate 

ignores any uncertainty contributed by temperature differences 

between the two tapes, tensioning errors, uneven calibration 

surfaces, or test repeatability. 

Recommended Calibration Method

The recommended calibration method for groundwaterlevel measurement tapes is a side-by-side method. The sideby-side calibration method was selected because it allows for 

the entire tape length to be calibrated, is recognized by NIST 

and ASME as a method that can be used to calibrate tapes 

similar to those used for groundwater-level measurements, 

and does not require a large investment in equipment or access 

to wells to perform. The recommended calibration method 

is a modified version of NIST SOP No. 12 “Tape-to-Tape 

Method” (NIST, 1986) and is applicable to steel and electric 

groundwater-level tapes. This modified NIST method, along 

with a custom-designed and fabricated calibration device (not 

required but very helpful), forms the calibration system used 

by this study to test the accuracy of in-service tapes. 

The equipment needed for the side-by-side tape 

calibration includes: (1) a NIST-traceable steel tape calibrated 

to an appropriate accuracy; (2) equipment or person to anchor 

one end of the tapes (for example, surveying tape clamp handle, and stake(s), concrete screws, surveying pins, or weights); 

(3) two tape tension handles (for example, Lufkin 586 30-lb 

tape handle) or scales to apply accurate pull forces to the 

tapes; (4) graph paper or some other means to measure the 

displacement between the test and reference tapes, and (5) an 

appropriate flat test area that is at least 50 ft long (preferably 

longer). Alternatively, a calibration device designed for tape 

calibration can be substituted for items 2 through 4.

Additionally, at least one technician (possibly two, 

depending on equipment used) is needed. A worksheet for 

recording all necessary calibration information, including tape 

information, calibration information, and correction calculations for the calibration method, is in appendix 1. 
6 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Reference-Tape Requirements

The reference tape should be a steel tape at least 100 ft 

long. The calibrated length of the reference tape should be at 

least 100 ft long but may be less than the total length of the 

reference tape. The calibrated length of the reference tape 

should have a NIST-traceable calibration to at least ±0.001 ft 

or better. The reference tape should not be used for routine 

water-level measurements and should be handled and stored 

carefully to preserve the tape accuracy. 

Displacement-Measurement Requirements

The equipment or materials used to measure the 

displacement between the test and reference tape graduations 

should have a resolution of at least 0.0033 ft (0.04 in. or 

1 mm). Graph paper marked in centimeter and millimeter 

graduations can be used as described in the NBS SOP 12 

(NIST, 1986). Graph paper has the advantage of having lines 

that can be used to ensure that the test and reference tapes are 

aligned properly. Alternatively, calibrated digital calipers, or 

a precision steel rule marked in 1/32-in. (or 0.5-mm) increments can be used to measure the displacement. Measuring the 

displacement in length units other than feet is acceptable, but 

requires the units to be converted into feet. 

Calibration-Interval and Test-Location 

Requirements

The ideal test location is indoors in a hallway or 

warehouse. This will provide temperature stability and eliminate solar heating. Alternatively, an outdoors test location with 

a smooth, level surface, such as concrete or asphalt, is acceptable on a cloudy day. The clouds will minimize solar heating 

of the tapes. As long as there are no bumps or dips, a concrete 

or asphalt surface, such as a parking lot or sidewalk, will be 

acceptable. Conditions that create unacceptable calibration 

accuracy include outdoors on a sunny day or a gravel or grass 

surface. The sun will heat the tape in an uncontrollable and 

irregular manner and will cause substantial errors. A gravel 

or grass surface is irregular and the tapes will likely take 

slightly different paths, causing random errors. A temperature 

correction is not necessary if the reference and test tapes are 

allowed to equalize with the test location ambient temperature. 

However, temperature differences between the reference and 

test tapes can be a significant source of test error, especially 

if the test and reference tapes are made of different materials. 

A difference in temperature by 2 oC between steel tapes gives 

a calibration measurement error of 0.0023 ft (Moffit and 

Bouchard, 1975). 

The recommended calibration interval is 100 ft. Shorter 

calibration intervals introduce additional time and technician 

error with each tape reset for the measurement of the next 

interval section. However, good calibrations are possible with 

a shorter interval, such as a 50-ft calibration interval in a 60-ft 

hallway. The measurement uncertainties from the shorter 

calibration interval and the increased number of measurements 

in the controlled environmental conditions in an office hallway 

are small compared to the uncertainties from solar heating 

and unknown surface irregularities of a grass field or gravel 

driveway. 

Tape Tensioning

Ideally, a calibrated scale pulled mechanically should 

be used to tension the tapes during calibration. It will assure 

calibration accuracy and repeatability. Pulling the tapes by 

hand without any force measurement device does not allow 

an accurate pull force to be consistently applied. A portable 

scale, such as a fishing scale, pulled manually is acceptable as 

long as the technician pulling the scales can maintain the pull 

forces during the measurement and repeat the same pull for 

each measurement. The scales or tape-tension handles should 

have an accuracy of ±0.5 lb or better, resolution of ±0.5 lb, and 

a range of 0 to 20 lbs. For steel tapes, a weight-scale accuracy 

and resolution of 0.5 lb gives a possible calibration error of 

0.000672 ft (Moffit and Bouchard, 1975).

Steel tapes

A standard pull force of 10 lbs is prescribed during 

calibration for all steel tapes over a 25- to 100-ft calibration 

interval. This is the tension force used by NIST and ASME 

for steel tape calibrations. If no weight is attached, a steel 

tape would require at least 1,200 ft to be lowered down a well 

to achieve about 20 lbs of tensile force at the very top of the 

tape. If weight is attached to the tape, usually a few ounces 

or a couple of pounds, the tensile force at the top of the tape 

is increased by the amount of the attached weight. Steel tapes 

that are lying flat and are not bent or crimped can handle tensile forces exceeding 50 lbs without permanent damage.

Electric Tapes

A standard tension does not apply to the calibration of 

electric tapes. Because the probe on most electric tapes also 

acts as the weight, the tape down tension is constant and 

known. Precision scales can apply a simulated weight of 

the electric-tape probe and tape as a function of length. This 

provides the most realistic calibration as the tension applied is 

approximately the same as the actual in-use tension for each 

tape section. Tape tension increases linearly with length. The 

in-use down-hole tension for an electric tape is the calculated 

average tension over the calibration interval. The calibration 

tension for the 100- to 200-ft calibration interval is equal to 

the weight of the probe plus the weight of the 0- to 100-ft 

tape interval plus one half of the weight of the 100- to 200-ft 

tape calibration interval. Table 2 lists tensions to apply during 

electric-tape calibrations for tape calibration intervals from 

100- to 500-ft for several electric-tape models. As long as the 
Recommended Calibration Method 7

tape elongation is linear (assumed in all tape calibrations), 

an average tension applied horizontally will cause the same 

stretch as the tape suspended vertically down a well.

Basic Calibration Steps 

The basic steps of the proposed calibration procedure are:

1. Acclimate the reference and test tapes to the temperature 

of the test area. Allow the tapes at least 1 hour or longer 

for larger initial differences in temperature between the 

tapes and the ambient temperature in the test area before 

beginning calibration. 

2. Record test date, tape identification numbers, test 

personnel names, and test location on the calibration 

record sheet. Also, record the average surface temperature and ambient air temperature. The test conditions 

should also be recorded, such as weather (if outdoors) 

and test surface conditions. Record the calibrationinterval information, the calibration interval used, and 

the number of intervals in the tape. Calibrating a 1,000-ft 

tape with a 100-ft interval would need 10 100-ft interval 

measurements. Record the type of instrument and its 

accuracy used to measure the displacement between test 

and reference tapes graduation markings and the units of 

the measurement on the calibration sheet.

3. Clear the test area of any dirt and debris. The test area 

should be a reasonably level, hard surface, such as a 

smooth asphalt road, a concrete floor, or a carpeted hallway. Any bumps or irregularities should affect both tapes 

equally, or measurement errors could occur.

4. Roll out the test and reference tapes parallel to each 

other with the graduations face up and increasing in 

the same direction. Inspect the tapes for any damage, 

such as kinks or curling that would make either of them 

Table 2. Electric groundwater-level measuring-tape probe and tape weights for various models and calibration tensions by tape 

interval used to simulate in-use tensions. 

[lbs, pounds; ft, feet; DGSI, Durham Geo Slope Indicator]

Model Probe 

weight (lbs)

Tape weight 

per ft (lbs)

Calibration tension by interval

0 to 100 ft 100 to 200 ft 200 to 300 ft 300 to 400 ft 400 to 500 ft

Solinst Model 

101

0.322 0.0107 0.86 1.92 2.99 4.06 5.13

Solinst Model 

102

0.142 0.0105 0.67 1.72 2.77 3.82 4.87

DGSI 0.123 0.0121 0.73 1.94 3.15 4.35 5.56

RST Model 3001 0.169 0.0127 0.80 2.07 3.34 4.62 5.89

Heron Dipper-T 0.352 0.0134 1.02 2.37 3.71 5.06 6.40

InSitu 200 0.352 0.0134 1.02 2.37 3.71 5.06 6.40

Geotech Keck 

Water Level 

0.339 0.0137 1.02 2.39 3.76 5.13 6.50

Waterline 0.434 0.0085 0.86 1.71 2.57 3.42 4.27

unusable and note the location of any damage. If the 

reference tape is damaged, do not proceed with the 

calibration. Record information on the test and reference 

tape condition. 

5. Attach one end of each of the tapes to a mechanical 

system fixed to the ground or use a technician to hold the 

tapes in position. Tapes should be clamped with either 

specialty surveying tape clamps or with clamps having 

rubber-padded jaws.

6. Attach each of the other tape ends to scales (one for each 

tape) either attached to a mechanical pull system or use a 

technician to hold and pull the scales. 

7. Ensure that the anchor and puller are square with the 

tapes and each other, the tapes are parallel, and the tape 

markings aligned precisely on the anchor side. Misalignment of either the tapes or the tape markings at the 

anchor end will adversely affect the calibration accuracy.

8. Apply an initial tension of 15 or 20 lb and “snap” the 

tapes until they are straight and parallel with each other. 

The tapes should be plucked with a vertical motion (just 

like plucking a chalk line) a few times to get the tapes 

to settle into parallel, straight lines. The “snap” does not 

need to be violent. 

9. Reduce the tension in the reference tape to 10 lb 

(standard tension) to give the calibrated length of the 

reference tape.

10. Reduce the tension in the test tape to 10 lb (steel tape) 

or the desired in-use tension (electric tape). The in-use 

tension is typically used for electric tapes and is the 

average tension on that tape interval during a wellwater-level measurement, one-half the weight of the tape 

interval being calibrated.
8 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

11. Check the alignment of the tapes at the anchor end and 

make adjustments as needed. After any tape position 

correction, check the tension forces and adjust as needed 

to the required tension. Record the actual tension forces 

for both the reference tape and the tape under calibration 

on the calibration sheet.

12. Measure the displacement between the test and reference 

tapes interval graduations using digital calipers, 

precision steel ruler, or graph paper (NIST, 1986) to 

determine the displacement (or difference) in the tape 

lengths. Record the tape displacement measurement on 

the calibration record sheet.

13. Switch the tapes’ positions. For example, if the reference 

tape is on the right side and the test tape is on the left, 

switch them so that the reference tape is on the left side 

and the test tape is on the right.

14. Repeat steps 5 through 13 until there are four 

measurements made over the segment (two measurements in each of two positions). If two measurements 

over the same interval differ by more than twice the 

accuracy of the measurement, repeat the interval measurement. Average the four good measurements for the 

interval to determine the accuracy of the test tape over 

that interval. 

15. Repeat steps 5 through 14 for each interval of tape until 

the entire tape calibration is completed.

After the calibration is completed, roll up the test and 

reference tapes. Store the reference tape in a protective case 

and a dry, secure location. The test tape can be put into service 

if its calibration is within its expected accuracy or if the 

resulting correction table gives corrections that are within its 

expected accuracy. Groundwater tapes are usually expected 

to have an accuracy of better than 0.01 ft or 0.01 percent 

over any tape interval, whichever is largest. If the test tape is 

kinked or curled, the tape is permanently deformed and should 

be removed from service. However, it may be possible to 

remove the deformed section to allow the shortened tape to be 

recalibrated and returned to service. If the test tape conditions 

are unacceptable, label it as unusable and plan to dispose of 

the test tape. 

Calibration Device

A calibration device was designed and constructed to 

reduce the time and effort needed to align and tension the 

tapes during calibration using the side-by-side method. The 

device was also constructed to be easy to move and store. 

The calibration device has two parts, an anchor and a 

tensioner. Both the anchor and the tensioner were constructed 

to give a fixed, stable platform on which to attach the tapes. 

The anchor is used to hold the tape ends that are aligned to a 

common interval graduation. The tensioner is used to apply 

tensions on each tape and includes two scales to measure the 

applied tensions. The weight scales (CCI Scale Company, Inc. 

model HS–30) installed on the tensioner have a resolution of 

±0.02 lb. Weights are placed on the anchor and tensioner to 

hold them in place when the tape is tensioned.

Two calibration devices were constructed during the 

study. The first design, CD1, was used for initial method testing with in-house steel tapes. Two people were used to operate 

the first device. The first anchor design used a single screwdown rubber-coated clamp to hold both tapes and a machinist’s square to align the graduation marks. The first tensioner 

design attached the tape to the scales with rubber-surface cam 

assemblies. The cam assemblies rode on linear rails and were 

attached with ropes to the scales. A rope-and-pulley system 

was used to apply the tension to the tapes. Tape tension was 

fixed by positioning the ropes in spring-loaded cleats. The 

displacement between the test and reference tapes graduation 

marks was measured with a dial caliper. The initial design was 

functional but had several problems. The design made applying two different tape tensions difficult because of the single 

tape clamp on the anchor. The stretchiness of the ropes made 

tension adjustment time consuming because the ropes would 

relax after the initial tension was applied, requiring additional 

adjustment. Additionally, the dial caliper required patience, as 

well as skill to align with the tape graduations, and the thickness of the anchor and tensioner added a small amount of tape 

sag near the anchor and tensioner. 

The first design was modified and a new calibration 

device, CD2, was built to improve the initial design and calibration efficiency. The CD2 anchor (fig. 1A) has two toggle 

clamps for individual tape adjustment, and milled lines to aid 

in tape graduation alignment. The CD2 tensioner (fig. 2) uses 

a chain and acme screw system instead of the original ropeand-pulley system. The chain and acme screw system (fig. 2C) 

allows for coarse (chain) and fine (acme screw) tension adjustment, and the tapes to be individually and rapidly tensioned. 

The tensioner has a pair of rollers that holds the tapes close 

to the ground as they come off the device to minimize tape 

sag from the anchor and tensioner (figs. 2B and 2C). An 

integrated, custom, moving Vernier scale was installed in the 

tensioner to measure the displacement between the test and 

reference tapes (figs. 2B and 2C). The Vernier scale allows 

displacement measurements with a resolution of 0.0005 ft. A 

single technician can efficiently operate the device, but placing 

a technician at each end eliminates the need to walk back 

and forth between the anchor and the tensioner and reduces 

calibration time. 
Calibration Device 9

A B

A

B

C

Figure 1. Calibration Device 2 anchor. A, Overhead view of anchor showing 

toggle clamps and tape alignment lines. B, Tape end view of anchor with 

weights piled onto plate to hold anchor to floor during tape tensioning.

Figure 2. Calibration Device 2 tensioner. A, Overhead view of tensioner showing plate with weight to hold tensioner to floor 

during tape tensioning. B, View of tape grippers, roller, and Vernier scale. C, Close-up overhead view showing chains for coarse 

tension adjustment and acme screws for fine-tension adjustment and two weigh scales used to measure applied tension.
10 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Displacement Measurement using the 

Vernier Scale

A Vernier scale is composed of two scales, one fixed and 

one moving. The moving Vernier scale slides past the fixed 

one, and indicates a higher precision measurement in-between 

the marks of the fixed scale. In the case of the CD2, the 

fixed scale is the tape graduations of the reference tape. At 

the anchor end, the reference tape 101-ft graduation mark is 

aligned with the interval graduation mark (for example, the 

100-ft mark) of the test tape. The displacement measurement is 

made at the puller end using the custom moving Vernier scale 

(figs. 3 and 4) as follows: 

1. Align one of the moving Vernier scale grooves to the 

interval graduation mark on the test tape. If the interval 

graduation mark of the test tape is closer to the anchor 

end than the 1-ft mark of the reference tape, the test tape 

is short and the displacement is negative. If the interval 

graduation mark of the reference tape is closer to the 

anchor end than the interval graduation mark of the test 

tape, the test tape is long and the displacement is positive. 

2. Make the coarse measurement by counting the 0.01-ft 

tape graduation marks on the reference tape, j, between 

the two tapes’ interval marks (100-ft, 200-ft, and so on). 

The count, j, is multiplied by 0.01 ft to give the coarse 

measurement.

3. Make the fine measurement by counting the number of 

moving scale grooves, k, from the scale groove aligned 

to the test tape interval graduation mark in step 1 to the 

scale groove that aligns with a 0.01-ft mark on the reference tape. If the test tape interval is short, count the scale 

grooves to the right of the aligned groove. If the test tape 

interval is long, count the scale grooves to the left of the 

aligned groove. The Vernier moving-scale groove count 

is multiplied by 0.0005 ft to give the fine measurement.

4. Compute the displacement by adding the fine measurement to the coarse measurement (k×0.01+j×0.0005).

5. Determine the sign of the displacement. If the test tape 

interval graduation mark is closer to the anchor than the 

reference tape mark, the difference is negative. If the test 

tape interval graduation mark is farther from the anchor 

than the reference tape mark, the displacement is positive. 

Determining the displacement measurement after aligning 

the Vernier groove to the test-tape interval mark is not difficult. 

For example, figure 3 shows the displacement measurement for 

the 100- to 200-ft interval of a steel test tape. In figure 3, the 

anchor is to the right of the photo and the test tape is the upper 

tape in the figure. The test tape 200-ft graduation mark has 

been aligned with the 101-ft graduation mark of the steel reference tape at the anchor. The reference tape in figure 3 is the 

tape showing the 1-ft graduation mark. The Vernier groove has 

been aligned to the test tape 100-ft interval graduation mark. 

The test tape is short because the test tape interval graduation 

mark is closer to the anchor than the reference tape 1-ft graduation mark. To determine the displacement measurement using 

figure 3:

1. Count the number of 0.01-ft graduation markings that 

are between the reference tape’s 1-ft interval graduation 

mark and the test tape’s 100-ft interval graduation mark. 

The interval graduation markings for the tapes are within 

0.01 ft of each other, indicating that the displacement is 

less than 0.01 ft and the coarse measurement is zero. 

2. Because the test tape interval is short, count the number 

of Vernier grooves to the right of the test tape’s 100-ft 

mark until the Vernier grooves align with a 0.01-ft 

mark on the reference tape. It takes five Vernier groove 

marks to align the Vernier grooves with a 0.01-ft mark 

on the reference tape, so the fine measurement is 

5×0.0005=0.0025 ft.

3. Sum the coarse and fine measurements, 

0+0.0025=0.0025 ft.

4. Determine the sign of the correction. The test tape 

is shorter than the reference tape and the correction 

becomes -0.0025 ft for this interval section of the test 

tape. 

Figure 4 shows the displacement measurement for the 

100- to 200-ft interval of a test electric tape. As in figure 3, 

the anchor is to the right of the photo, but the reference tape is 

at the top of the figure. The test tape 200-ft graduation mark 

is aligned with the reference tape 101-ft graduation marks at 

the anchor. As in the previous example, the test tape is short 

because the test tape 100-ft interval graduation mark is closer 

to the anchor than the reference tape 1-ft graduation mark. The 

Vernier groove has been aligned to the test electric-tape 100-ft 

interval graduation mark. To determine the displacement using 

figure 4:

1. Count the number of 0.01-ft graduation markings that 

are between the reference tape’s 1-ft graduation interval 

mark and the test tape’s 100-ft interval mark. The test and 

reference tapes’ interval graduation marks are separated 

by two 0.01-ft increments. The coarse measurement is 

2×0.01 = 0.02 ft.

2. Because the test tape is short, count the number of 

Vernier grooves to the right of the electric-tape 100-ft 

mark until the Vernier grooves align with a 0.01-ft mark 

on the reference tape. A count of one aligns the Vernier 

groove with a 0.01-ft mark on the reference tape. The fine 

measurement is 1×0.0005=0.0005.

3. Sum the coarse and fine measurements: 

(2×0.01)+(1×0.0005)=0.0205 ft. 

4. Determine the sign of the correction. Because the test 

tape is shorter than the reference, the tape correction is 

-0.0205 ft for the tape in figure 4. 
Calibration Device 11

Figure 3. Close-up view of Vernier scale during displacement measurement of a steel test tape (top tape) with a steel 

reference tape (bottom tape). Red arrow, A, points to the alignment of the moving Vernier scale groove to the test-tape intervalgraduation mark. No 0.01-foot (ft) marks are between the 100-ft and 1-ft graduation marks, giving a count of “0” for the coarse 

measurement. Red arrow, B, points to the Vernier scale groove that aligns with the reference tape 0.01-ft graduation mark. The 

double-head arrow line indicates the five grooves to count for the fine measurement.

Figure 4. Close-up view of Vernier scale during displacement measurement of an electric test tape (bottom tape) with a 

steel reference tape (top tape). Red arrow, A, points to alignment of the moving Vernier scale groove to the test-tape intervalgraduation mark. Two 0.01-foot (ft) marks, denoted by the larger orange arrow, are between the 1-ft and 100-ft graduation 

marks, giving a count of 2 for the coarse measurement (0.02 ft). Red arrow, B, points to the Vernier scale groove that aligns 

with the reference tape 0.01-ft graduation mark. The smaller orange arrow indicates the “1” groove to count for the fine 

measurement of 0.0005 ft.
12 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Estimated Calibration Improvement 

Using Calibration Method Indoors with 

Calibration Device

For a calibration conducted outdoors without the CD2, 

the expanded uncertainty of a calibration (International Organization for Standardization, 1993) estimated by: 

U∆=2×[ureference+udisplacement+utension+utemperature]

1⁄2 , (9)

assuming a displacement measurement accuracy of 1/32 in. 

(0.0033 ft), a reference tape accuracy of 0.001 ft, an error of 

0.0003 ft due to tensioning accuracy of ±0.5 lb, and an error of 

0.0023 due to a 2 degree Celsius (oC) difference between the 

test and reference tapes, is 0.005 ft. The expanded uncertainty 

has a 95-percent confidence interval in comparison to the standard uncertainty, which has a 66-percent confidence interval. 

The individual calibration errors and accuracies are assumed 

to have a rectangular distribution and are divided by √3 to 

give the standard deviation, u, which is used in the uncertainty 

calculation. For a calibration conducted indoors with the CD2, 

the expanded uncertainty of a calibration uncertainty (International Organization for Standardization, 1993) estimated 

by assuming a NIST-traceable reference tape calibrated to 

+/-0.0001 ft, a displacement measurement accuracy of 0.0005 

ft, an error of 0.0001 ft due to tensioning accuracy of ±0.2 lb, 

and no error from temperature differences, is 0.001 ft.

Repeatability of Calibration Method 

with Calibration Device 

The repeatability (or precision) of the calibration method 

using the calibration device was determined by comparing a 

calibrated reference tape to the test tape, a new 300-ft Lufkin, 

Super Hi-way Nubian steel tape, and computing the standard 

deviation from a number of repeated tests. The calibrated reference tape used for the calibrations was also a 300-ft Lufkin, 

Super Hi-way Nubian steel tape. The calibrated steel reference tape is NIST-traceable and was calibrated by Lockheed 

Martin–Stennis Integrated Metrology Center, a metrology 

laboratory that is ACLASS accredited to ISO 17025. The reference tape total error over the 100-ft test section is 0.00092 ft. 

All test calibrations were conducted indoors on a level, 

smooth concrete floor in the HIF Hydraulic Laboratory. The 

calibration interval used for the testing was 100 ft. Prior to 

testing, the tapes were acclimated to the ambient air temperature in the Hydraulic Laboratory. During the testing, the maximum and minimum air temperatures observed in the Hydraulic 

2 2 2 2

Laboratory were 29.6 and 27.7 oC, respectively. Because the 

tapes were equalized to the ambient air temperature over several hours and both tapes are of the same material, the dimensional differences between the tapes due to temperature was 

negligible and no temperature corrections were necessary. 

Measurements were taken over the entire 300-ft length 

of test tape and compared to the 100-ft calibrated interval of 

the reference tape. Each calibration required 12 displacement 

measurements; four repeated measurements per 100-ft tape 

interval over the 300-ft test-tape length. A total of 10 calibrations were conducted using a total of 120 interval measurements. Table 3 lists the summary statistics for all of the 

individual interval measurements made during the repeated 

calibrations. Based on the standard deviation computed from 

the individual interval displacement measurements, any single 

interval measurement made with the calibration device has 

a standard uncertainty (or standard deviation) of 0.0011 ft. 

Table 4 lists the summary statistics for the 10 repeated tape 

calibrations. Each interval measurement for a calibration is the 

average of four measurements and the statistics in table 4 are 

for those interval measurements. The statistics are similar to 

those listed in table 3. 

Reducing the time and effort needed to determine the 

measurement was investigated by using half the number of 

replicated measurements for each calibration of the steel test 

tape. Table 5 lists the summary statistics for 20 repeated steel 

test tape calibration measurements using only two replicated 

measurements per interval. The difference in the statistics 

in table 5 compared to those in table 4 are small and within 

the resolution of the moving Vernier scale used for the 

measurements. 

Table 6 lists the error contributed by the number of 

repeated measurements using the standard deviation from table 

3 of 0.0011 ft. Each repeated measurement of the displacement 

between the test and reference tape is a sample of the actual 

displacement. The error or uncertainty, usample size, contributed 

by the number of repeated measurements or sample size (Eberhardt, 1990) can be estimated with a 95-percent confidence 

interval from usample size=

1.96 S

, where s is the standard deviation 

of the population, and n is the number of samples or repeated 

measurements. In order to achieve a sampling error of the 

same size as the calibration device resolution of 0.0005 ft, 

20 repeated measurements would be needed. However, two 

repeated measurements have only a slightly larger error than 

four repeated measurements, 0.0016 ft versus 0.0011 ft. Both 

measurements are within the 0.0033-ft goal and closely match 

the estimated expanded combined uncertainty estimated in the 

previous section. Because the calculation uses data for a steel 

test tape, the results may not be applicable to an electric tape.

√n
Repeatability of Calibration Method with Calibration Device 13

Table 3. Summary statistics of measured displacement between test tape and reference tape over 100-foot intervals for all 

individual interval measurements. Positive displacements are for tapes that measure long, and negative displacements are for 

tapes that measure short. 

[ft, feet]

Number of interval 

measurements

Average 

in ft

Standard Deviation 

in ft

Maximum 

in ft

Minimum 

in ft

120 -0.0007 0.0011 0.0020 -0.0025

Table 4. Summary statistics of 10 replicated steel tape calibrations using four repeated measurements to determine the average 

displacement with reference tape in feet over 100-foot intervals. Positive displacements are for tapes that measure long, and 

negative displacements are for tapes that measure short. 

[ft, feet]

Tape interval 

in ft

Number of 

calibrations

Average 

in ft

Standard 

deviation in ft

Maximum 

in ft

Minimum 

in ft

Standard error 

in ft

0–100 10 0.0002 0.0009 0.0014 -0.0009 0.0003

100–200 10 -0.0011 0.0008 0.0009 -0.0020 0.0003

200–300 10 -0.0013 0.0002 -0.0009 -0.0015 0.0001

All intervals 30 -0.0007 0.0010 0.0014 -0.0020 0.0003

Table 5. Summary statistics of 20 replicated steel tape calibrations using two repeated measurements to determine the 

average displacement with reference tape in feet (ft) over 100-ft intervals. Positive displacements are for tapes that measure 

long, and negative displacements are for tapes that measure short. 

[ft, feet]

Tape interval 

in ft

Number of 

calibrations

Average 

in ft

Standard 

deviation in ft

Maximum 

in ft

Minimum 

in ft

Standard error 

in ft

0–100 20 0.0003 0.0010 0.0018 -0.0010 0.0002

100–200 20 -0.0023 0.0009 0.0013 -0.0025 0.0002

200–300 20 -0.0025 0.0004 -0.0005 -0.0018 0.0001

All intervals 60 -0.0007 0.0010 0.0018 -0.0025 0.0001

Table 6. Effect of sample size on interval-displacement 

measurement accuracy using a measurement standard 

deviation of 0.0011 foot. 

[ft, feet]

Number of 

repeated 

measurements

Error 

in ft

2 0.0016

4 0.0011

20 0.0005
14 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Accuracy Testing of In-Service 

Groundwater-Level Tapes

Accuracy of USGS in-service steel and electric 

groundwater-level tapes was tested by calibrating a sample 

of tapes from various USGS WSCs. A total of 29 in-service 

groundwater-level tapes were tested for compliance with the 

expected accuracy of ±0.01 ft per 100 ft. The expected accuracy is actually plotted in all figures as ±0.0144 ft to account 

for the rounding errors and the limitations imposed by recording water-level measurements to two significant figures. The 

previously described calibration method was conducted on the 

tapes indoors at the HIF Hydraulic Laboratory using the CD2 

calibration device. Test results were also used to assess the 

repeatability of the test method for steel and electric tapes. The 

results for new and in-service electric tapes and for steel and 

electric in-service tapes were compared. 

In-Service Steel Tapes 

Fifteen in-service steel tapes, three each from WSCs 

in Arizona, Arkansas, New Jersey, Tennessee, and Pacific 

Islands, were calibrated. All tested in-service steel tapes are 

made by Lufkin. Selected information for the in-service steel 

tapes is listed in table 7. Overall, the sample of in-service 

steel tapes was legible and in good condition. Tape age did not 

seem to affect condition. Some of the in-service tapes were 

missing the first few feet of tape. 

The total number of individual interval measurements 

was 264. Summary statistics for the individual interval 

measurements are listed in table 8. Table 9 lists summary 

statistics for the calibration tests for the 15 tapes. Average 

measured interval displacements were similar, regardless 

of a measured interval, and ranged from –0.0024 ft (300- to 

400-ft interval) to –0.0088 ft (500- to 1,000-ft interval) and 

averaged –0.0048 ft. Missing the first few feet of tape did not 

appear to greatly affect the accuracy of the tape. The standard 

deviation for all in-service steel tape measurements, a measure 

of the length variability in the 100-ft intervals, was 0.0048 ft 

(table 9). A box plot of the calibration data for the in-service 

steel tapes, plotted with the expected accuracy of ±0.01 ft, is 

shown in figure 5. Figures 6, 7, and 8, respectively, show the 

cumulative displacement between the sample tapes and the 

reference tape for short (<300-ft), 300-ft, and long (>300-ft) 

tapes. All in-service tapes were within the expected accuracy 

of ±0.01 ft. 

The standard deviations computed from the individual 

interval measurements for each in-service tape were pooled by 

averaging those standard deviations. The pooling removed the 

effect of different length sections from the standard deviation 

so that the random uncertainty for the calibration method 

using CD2 could be estimated. The pooled standard deviation 

for the in-service steel tapes was 0.0005 ft for 51 100-ft 

segments.

Data for a NIST calibration of the 1000-ft in-service steel 

tape became available after the study work was completed. A 

simple comparison of the NIST calibration with the study calibration for this tape found a testing bias of –0.007 ft per 100-ft 

interval. This bias may be limited to this tape calibration. The 

testing bias can be estimated from the data by assuming that 

the group of the in-service steel is unbiased and any nonzero 

median is due to a testing bias. Estimating the testing bias 

from the median of the data for the 100-, 200-, 300-, 400-, and 

500-ft-tape intervals, gives an average bias of –0.004 ft per 

100-ft interval. Both bias estimation methods indicate that the 

calibration method is adequate for calibrating tapes to ±0.01 ft, 

but could be improved.

In-Service Electric Tapes 

Fourteen in-service electric tapes, two each from the 

Arkansas, Nebraska, Nevada, North Carolina, Ohio, Pennsylvania, and South Carolina WSCs, were calibrated. Table 10 

compares the number of electric-tape makes and models used 

by WSCs based on OGW survey results (Rod Sheets, U.S. 

Geological Survey, Office of Groundwater, written commun., 

2011) with the study sample of in-service electric-tape makes. 

The sample of electric tapes from the WSCs approximately 

matches the proportions of makes and models used by WSCs. 

Four models, Heron, GeoTech (Keck), RST, and In-Situ, are 

overrepresented in the sample. The Waterline model, used 

by 20 percent of the WSCs, was not included in the sample. 

Table 11 lists selected information for the submitted in-service 

electric tapes. The sample of in-service electric tapes had signs 

of wear. The plastic jacket of most of the electric tapes showed 

signs of abrasion and wear that made the markings hard to 

read over the first few feet. One electric tape was submitted 

with a nonfunctional water-level indicator. 

The electric-tape calibration was a two-step process. The 

distance between the 1-ft mark and the probe activation level 

(probe offset) was calibrated using the method described by 

Jelinski and others (2015). The electric-tape length between 

the 1-ft mark and the end of the electric tape was calibrated 

using the calibration method and the CD2 device (CD2) 

described in this report. The combined measured probe offset 

and measured tape correction gave the tape correction. 

The probe-offset calibration used a device that consisted 

of a frame with three sliders. One slider held the tape at the 

1-ft mark and the other sliders held hook gauges. A cup filled 

with water, having a specific conductance of 1,000 microsiemens per centimeter (μS/cm) at 25 degrees Celsius, was 

placed below the tape probe and hook gauge. The tape probe 

was lowered into the water until it indicated contact with the 

water surface. A hook gauge was used to measure that distance 

from the 1-ft mark to a water surface. A detailed description of 

the probe-offset calibration device and procedure is given in 

Jelinski and others (2015). 

Table 12 lists summary statistics for all the individual 

interval-displacement measurements and for the probe-offset 
Accuracy Testing of In-Service Groundwater-Level Tapes 15

Table 7. Information for in-service steel tapes manufactured by Lufkin and calibrated by proposed procedure and device. 

[WSC, U.S. Geological Survey Water Science Center; ID, identifier; ft, feet]

WSC ID Model1 Condition2 Age1 Usage3 Original length 

in ft

Current length1

in ft

Arizona

Arizona

Arizona

Arkansas 

Arkansas 

Arkansas

New Jersey

New Jersey

New Jersey

Tennessee

Tennessee

Tennessee

Pacific Island

Pacific Island 

Pacific Island 

001

011

012

F-05

E-04

E-05

NJ-001

NJ-002

NJ-003

Logger #1

Logger #2

TN WSC 

500

PIWSC 

Reference

ST07-04

OAST-10

030402

Unknown

Unknown

Unknown

2967058

Unknown

24103

180165

Unknown

240910

180165

Unknown

Unknown

030402

18065

Good

Excellent

Good

Good

Fair, some rust

Good, 

minor rust

Good, many 

small kinks

Good, 

moderate rust

Good, 

minor rust

Well used, 

moderate rust

Good, 

some rust

Well used, 

moderate rust

Good

Good

Good

10 years

8 years

3 years

>10 years

>10 years

>10 years

“Older”

“Newer”

“Newer”

>30 years

>30 years

>30 years

10 years

5 years

Unknown

Calibration only

50 wells ~17 

trips/year

50 wells ~17 

trips/year

50/year

50–100/year

50–100/year

Regular use, 

toxics

Heavy use, 

GW network

10–15/year

10–20/year

10–20/year

Light use, 

few/year

Light use/

reference tape

Never, 

calibration 

problem

Moderate use

500

200

300

500

300

300

100

200

300

200

200

500

1000

500

100

500

200

300

493

268

271

99

200

300

90

200

471

1000

500

100

1

Read off tape. Length is approximate, to first whole foot mark.

2

Condition estimate based on appearance. Most issues occurred within first 50 ft of tape. 

3

Information provided by individual U.S. Geological Survey Water Science Center.

Table 8. Summary statistics of measured displacement between in-service steel tape and reference tape over 100-foot intervals 

for all individual interval measurements. Positive displacements are for tapes that measure long, and negative displacements are 

for tapes that measure short. 

[ft, feet]

Number of interval 

measurements

Average 

in ft

Standard Deviation 

in ft

Maximum 

in ft

Minimum 

in ft

264 -0.0049 0.0048 0.01082 -0.0154
16 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Figure 5. The cumulative measured displacement between 

15 U.S. Geological Survey Water Science Center in-service steel 

test tapes and the steel reference tape plotted with an expected 

accuracy of 0.01 foot (ft) per 100-ft length. The test tapes were 

tested at a reference tension of 10 pounds and over each 100-ft 

graduated interval.

Figure 6. The cumulative measured displacement for 100-foot (ft)

graduation intervals between in-service steel test tapes less 

than 300 ft in length and the steel reference tape. The tapes were 

tested at a reference tension of 10 pounds and are plotted with 

the expected accuracy of 0.01 ft per 100-ft length.

Figure 7. The cumulative measured displacement for 100-foot (ft)

graduation intervals between in-service 300-ft steel test tapes and 

the steel reference tape. The tapes were tested at a reference 

tension of 10 pounds and are plotted with the expected accuracy 

of 0.01 ft per 100-ft length.

Figure 8. The cumulative measured displacement for 100-foot (ft)

graduation intervals between in-service steel test tapes greater 

than 300 ft in length and the steel reference tape. The tapes were 

tested at a reference tension of 10 pounds and are plotted with 

the expected accuracy of 0.01 ft per 100-ft length.
Accuracy Testing of In-Service Groundwater-Level Tapes 17

Table 9. Summary statistics for 15 in-service steel tapes, using four repeated measurements to determine the displacement with 

the reference tape over a 100-ft interval. Positive displacements are for tapes that measure long, and negative displacements are for 

tapes that measure short. 

[ft, feet]

Tape Interval 

in ft

Number 

of tapes

Interval 

count

Average 

in ft

Standard 

deviation in ft

Maximum 

in ft

Minimum 

in ft

Median 

in ft

 0–100 14 14 -0.0052 0.0059 0.0108 -0.0154 -0.0045

100–200 13 13 -0.0043 0.0037 0.0041 -0.0109 -0.0039

200–300 9 9 -0.0040 0.0047 -0.0051 -0.0109 -0.0034

300–400 5 5 -0.0024 0.0055 0.0071 -0.0094 -0.0039

400–500 5 5 -0.0050 0.0025 -0.0029 -0.0099 -0.0034

500–1,000 1 5 -0.0088 0.0011 -0.0074 -0.0109 -0.0089

All intervals 15 51 -0.0048 0.0048 0.0108 -0.0154 -0.0044

measurements. Eleven of the 14 probes were tested. One tape, 

the Solinst Model 102, failed calibration of the probe offset 

because of an electrical fault in the probe. The average probe 

offset was 0.0026 ft and ranged from 0.0147 to -0.0102 ft. 

Probe offsets are usually small and not a large source of 

electric-tape error. For a field calibration, the probe-offset calibration may not be necessary unless the probe has been pulled 

from its original position on the tape. Probe errors are generally much less than the tape errors, especially on longer tapes, 

and should not change during the life of the tape, as long as 

the probe is not damaged and the electrodes are kept clean. 

However, probes should be checked for function and offsets 

should be measured prior to first use of the tape.

The reference tape and CD2 were used to calibrate 

the in-service electric tapes. The total number of individual 

interval-displacement measurements was 168 (table 12). The 

standard deviation for all in-service electric-tape measurements was 0.0433 ft and is a measure of the length variability 

in the 100-ft intervals for the 14 electric tapes. Differences 

in materials and construction used by the various electrictape models, as well as variation in the tape lengths over the 

intervals, increased the standard deviation when compared to 

the steel tapes. Table 13 lists statistics for the calibrations of 

the 14 in-service electric tapes for each measured interval. The 

average measured interval displacement for the electric tapes 

ranged from 0.0190 (500- to 1,000-ft intervals) to -0.0077 ft 

(200- to 300-ft intervals) and averaged -0.0274 ft. Table 14 

lists statistics for the measured interval displacement of the 

14 in-service electric tapes for each model over the 0- to100-ft 

interval and for the average of all the measured intervals. The 

combined average for all the intervals ranged from 0.0349 

ft for six Solinst 101 tapes to -0.0660 ft for one RST model 

3001 tape. The average interval displacement, or the average 

interval accuracy, measured for the in-service electric tapes 

exceeded ±0.01 ft. The range of average interval accuracy 

(average displacement) from -0.0660 to 0.0349 ft is similar 

to the accuracy range of -0.0453 to 0.0376 ft for new electric 

tapes measured by Jelinski and others (2015) and fits within 

the accuracy range of 0.02 to 0.1 ft for electric tapes mentioned by Holmes and others (2001).

The standard deviations computed from the individual 

interval measurements for each in-service electric tape were 

pooled by averaging those standard deviations. The standard 

deviations were pooled to estimate the measurement repeatability (or random uncertainty) for the method used to check 

the tapes. The pooled standard deviation for the in-service 

electric tapes was 0.0006 ft for 42 100-ft intervals. In general, 

tape stretch and friction between the concrete floor and the 

electric tapes made it more difficult to evenly apply tension 

to electric tapes than to steel tapes. Tapes with a flat steel 

strength member were easier to evenly tension than those with 

round coaxial cable members. However, electric-tape calibrations were about as repeatable as those for steel tapes and the 

models using coaxial cable (Durham Geo Water Slope Indicator) were as repeatable as models using a flat tape. The pooled 

standard deviation for Durham Geo Water Slope Indicator 

(DGSI) tapes was 0.0007 ft for 8 100-ft segments and for 

models using a flat tape, the standard deviation was 0.0006 ft 

for 34 100-ft segments. 

Table 10. Based on the U.S. Geological Survey Office of 

Groundwater survey, comparison of quantity and makes of 

in-service electric groundwater-level tapes with the study 

numbers and makes of in-service electric tapes. 

[%, percent; Durham Geo Slope Indicator, DGSI]

Make Total 

in service

Percentage of 

in-service 

tapes

Number 

in study

Percent 

in study

Solinst 286 47% 5 43%

DGSI 135 22% 4 29%

Heron 21 3.5% 1 7.1%

Waterline 103 17% 0 0%

RST 

Instruments

14 2.3% 1 7.1%

In-Situ 13 2.1% 1 7.1%

GeoTech, 

(Keck)

34 5.6% 1 7.1%
18 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Table 11. Information for the sample of in-service electric tapes used to check accuracy of electric groundwater-level tapes used 

by the U.S. Geological Survey (USGS). 

[WSC, USGS Water Science Center; ID, identifier; ft, feet; DGSI, Durham Geo Slope Indicator]

WSC ID Make Model Condition1 Age2 Usage2 Length in ft

Arkansas B–04 DGSI Water Level 

Indicator

Good 4 years 125–500/year 300

Arkansas K–05 Solinst Model 102 Non-functional3 2 years 400/year 300

Nebraska SN: 33890 Solinst Model 101 Very good Unknown Unknown 200

Nebraska Beep Solinst Model 101 Well used 10 years 300/year 300

Nevada ET–12 Solinst Model 101 Good Unknown Unknown 1,000

Nevada WL 3247 RST Model 3001 Good Unknown Unknown 500

North Carolina CFO–008 DGSI Water Level 

Indicator

Good >2 years None recently 100

North Carolina KECK 2045 Geotech Keck Water Level 

Meter

Fair 6 years 60/year 100

Ohio SN: 15279 DGSI Water Level 

Indicator

Good 6 years 150/year 150

Ohio SN: 27330 Solinst Model 101 Good 6 years 350/year 200

Pennsylvania EX–E01 DGSI Water Level 

Indicator

Good >8 years 5–25/year 150

Pennsylvania NC–E03 Solinst Model 101 Good Unknown Unknown 200

South Carolina SN: 19088 Heron dipper-T Good Unknown Unknown 300

South Carolina SN: 25975 In-Situ Model 200 Excellent Unknown Unknown 300

1

Condition estimate based on appearance. Most wear occurred within first 50 ft of tape. 

2

Information provided by individual WSC.

3

Tape probe did not detect water. Tape length calibrated from 1-ft mark.

Table 12. Summary statistics of in-service electric-tape measurements, individual interval-displacement measurements, and probeoffset errors. Positive displacements are for tapes that measure long, and negative displacements are for tapes that measure short. 

[ft, feet]

Measurement Number of 

units tested

Number of 

measurements

Average 

in ft

Standard 

deviation in ft

Maximum 

in ft

Minimum 

in ft

Interval markings 14 168 -0.0274 0.0433 0.1136 -0.1754

Probe offset 10 40 0.0026 0.0067 0.0147 -0.0102
Accuracy Testing of In-Service Groundwater-Level Tapes 19

Table 13. Summary statistics of in-service electric groundwater-level tape displacement measurements using four measurements 

for each 100-foot interval. Positive displacements are for tapes that measure long, and negative displacements are for tapes that 

measure short. 

[ft, feet]

Tape interval 

in ft

Number 

of tapes

Interval 

count

Average 

in ft

Standard 

deviation in ft

Maximum 

in ft

Minimum 

in ft

Median 

in ft

 0–100 14 14 -0.0534 0.0424 0.0067 -0.1790 -0.0527

100–200 12 12 -0.0276 0.0242 0.0141 -0.0709 -0.0194

200–300 7 7 -0.0077 0.0554 0.1136 -0.0674 -0.0109

300–400 2 2 -0.0292 0.0336 0.0026 -0.0614 -0.0292

400–500 2 2 -0.0275 0.0363 0.0066 -0.0619 -0.0272

500–1,000 1 5 0.0190 0.0032 0.0231 0.0136 0.0193

All 

measurements

14 42 -0.0274 0.0433 0.1136 -0.1754 -0.0242

Table 14. Summary statistics of in-service electric groundwater-level tape displacement measurements over 100-ft tape intervals by 

model. Positive displacements are for tapes that measure long, and negative displacements are for tapes that measure short. 

[ft, feet; unmeas., unmeasured]

In-service 

tape model

Number 

of tapes

0–100 interval 

average in ft

Number of 

measured segments

Average for all 

intervals in ft

Standard 

deviation in ft

Average probe 

offset in ft

Durham Geo Slope 

Indicator

4 -0.0732 8 -0.0252 0.0776 0.0032

Solinst 101 5 -0.0541 24 -0.0264 0.0349 0.0002

Solinst 102 1 -0.0131 3 -0.0088 0.0058 not work

Geotech Keck Water 

Level Meter

1 -0.0330 1 -0.0330 0.0105

Heron Dipper-T 1 -0.0646 3 -0.0602 0.0047 0.0060

In-Situ Model 200 1 -0.0324 3 -0.0299 0.0027 -0.0043

RST Model 3001 1 -0.0708 5 -0.0660 0.0045 unmeas.

Comparison of In-Service with 

New Electric Tapes

Calibration data from the in-service electric tapes are 

plotted in figure 9 with the calibration data for new electric 

tapes (Jelinski and others, 2015). The new electric-tape data 

shown in figure 9 include calibrations for each of the following: Solinst Model 101, DGSI, Geotech Keck Water Level 

Meter, Heron dipper-T, and In-Situ Model 200. Figures 10, 

11, 12, and 13 are plots of the cumulative displacements 

for the new and in-service electric groundwater-level tapes 

with the reference tape. Figure 14 is a plot for the in-service 

Solinst 102 and figure 15 is a plot of in-service RST model 

3001 for the cumulative difference between the electric tape 

and the reference tape. No comparison data for new tapes were 

available for these models.

Table 14 lists accuracy measurements for new and inservice electric tapes grouped by tape construction. The DGSI, 

Solinst models 101 and 102, and RST model 3001 tapes do 

not have a flat steel core. The other in-service tape models, 

Geotech Keck Water Level Meter, In-Situ Model 200, and 

Heron dipper-T, have a flat steel core. Figures 16 and 17 show 

the cumulative difference between new and in-service electric 

tapes with steel cores and those without them with the reference tape. The data show that the steel-cored tapes maintain 

their length and accuracy fairly well during use, especially 

when compared to tapes without a steel core. The tapes without steel cores may require a much more rigorous calibration 

plan to maintain accurate measurements. 
20 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Figure 9. For new (top chart) and for 14 U.S. Geological Survey Water Science Center 

in-service electric test tapes (E-TAPES) (bottom chart), the cumulative measured 

displacement between E-TAPES and the steel reference tape plotted with an expected 

accuracy of 0.01 foot (ft) per 100-ft length. The E-TAPES were tested with in-use tension.
Accuracy Testing of In-Service Groundwater-Level Tapes 21

Figure 10. The cumulative measured displacement between Solinst model 101 electric 

test tapes (E-TAPES) and the steel reference tape plotted with an expected accuracy of 0.01 

foot (ft) per 100-ft length for new (top chart) and for U.S. Geological Survey Water Science 

Center in-service E-TAPES (bottom chart). The E-TAPES were tested with in-use tension.
22 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Figure 11. The cumulative measured displacement between Slope Indicator electric test 

tapes (E-TAPES) and the steel reference tape plotted with an expected accuracy of 0.01 

foot (ft) per 100-ft length for new (top chart) and for U.S. Geological Survey Water Science 

Center in-service E-TAPES (bottom chart). The E-TAPES were tested with in-use tension.
Accuracy Testing of In-Service Groundwater-Level Tapes 23

Figure 12. The cumulative measured displacement between Keck electric test tapes 

(E-TAPES) and the steel reference tape plotted with an expected accuracy of 0.01 foot (ft)

per 100-ft length for new (top chart) and for U.S. Geological Survey Water Science Center 

in-service E-TAPES (bottom chart). The E-TAPES were tested with in-use tension.
24 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Figure 13. The cumulative measured displacement between Heron electric test tapes (E-TAPES) 

and the steel reference tape plotted with an expected accuracy of 0.01 foot (ft) per 100-ft length for 

new (top chart) and for Water Science Center in-service E-TAPES (bottom chart). The E-TAPES were 

tested with in-use tension.
Accuracy Testing of In-Service Groundwater-Level Tapes 25

Figure 14. The cumulative measured displacement between in-service 

Solinst model 102 electric test tapes (E-TAPES ) and the steel reference tape 

plotted with an expected accuracy of 0.01 foot (ft) per 100-ft length. The 

E-TAPES were tested with in-use tension.

Figure 15. The cumulative measured displacement between in-service RST 

model 3001 electric test tapes (E-TAPES) and the steel reference tape plotted

with an expected accuracy of 0.01 foot (ft) per 100-ft length. The E-TAPES were 

tested with in-use tension.
26 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Figure 16. For new and in-service electric test tapes (E-TAPES) with steel tape cores, the 

cumulative measured displacement between E-TAPES and the steel reference tape plotted 

with an expected accuracy of 0.01 foot (ft) per 100-ft length. The E-TAPES were tested with 

in-use tension. Vertical error bars are the maximum and minimum displacement measured.

Figure 17. For new and in-service electric test tapes (E-TAPES) without steel tape cores, 

the cumulative measured displacement between E-TAPES and the steel reference tape 

plotted with an expected accuracy of 0.01 foot (ft) per 100-ft length. The E-TAPES were 

tested with in-use tension. Vertical error bars are the maximum and minimum displacement 

measured.
Accuracy Testing of In-Service Groundwater-Level Tapes 27

Comparison of In-Service Steel and ElectricTape Results

The in-service electric tapes showed more signs of wear 

than the in-service steel tapes (table 15). Almost all electric 

tapes had signs of wear. One electric tape did not detect water 

because of a broken wire in either the tape length or probe. 

The steel tapes, by comparison, were in good, legible condition. Some steel tapes had minor rust, were missing sections, 

and had kinks in a few intervals. These wear and tear issues 

were minor and did not affect the accuracy or use of the steel 

tapes. 

Electric tapes were not as easy to calibrate as the steel 

tapes because the tension applied to the electric tapes is 

different than the tension applied to the reference steel tape 

and the friction from the electric-tape coatings makes the tapes 

harder to align and tension. Surface friction can cause the 

electric tape to stretch unevenly and contribute to measurement errors. 

Figure 18 compares the displacements measured during 

calibrations for the in-service steel and electric tapes and 

shows that the range of electric-tape measurement variation 

can be much larger than the steel tapes. The in-service steel 

tape intervals measured within 0.01 ft of the reference over the 

100-ft interval. Any of the in-service steel tapes, when used 

properly, will give an accurate water-level measurement without a tape correction. None of the in-service electric tapes in 

the study sample will give a water-level measurement within 

±0.01 ft per 100 ft without a tape correction. 

Table 15. New and in-service electric-tape calibration data. Displacement is the measured length difference between the electric 

tape and the steel reference tape. Positive displacements are for tapes that measure long, and negative displacements are for tapes 

that measure short. 

[ft, feet; w/o, without]

Tape models

New tapes In-service tapes

Number 

of tapes

Number of 

intervals

Displacement

Number of 

tapes

Number of 

intervals

Displacement

Average 

in ft

Standard 

deviation 

in ft

Average 

in ft

Standard 

deviation 

in ft

Durham Geo Slope 

Indicator

3 9 -0.0010 0.0241 4 8 -0.0252 0.0776

Solinst 101 3 9 -0.0040 0.0143 5 24 -0.0264 0.0349

Steel-core tapes 9 36 -0.0311 0.0061 3 7 -0.0433 0.0161

Tapes w/o steel core 6 27 -0.0022 0.0195 11 35 -0.0246 0.0475

Figure 18. For steel and in-service electric test 

tapes (E-TAPES), the cumulative measured 

displacement between in-service E-TAPES and 

the steel reference tape plotted with an expected 

accuracy of 0.01 foot (ft) per 100-ft length. The 

E-TAPES were tested with in-use tension. Vertical 

error bars are the maximum and minimum 

displacement measured.
28 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Summary

The accuracy of groundwater-level tapes was investigated 

by developing a calibration method and device, and testing 

the accuracy of a sample of in-service steel and electric 

groundwater-level measuring tapes. The recommended 

calibration method is a side-by-side tape comparison method 

based on a National Institute for Standards and Technology (NIST) standard operating procedure (NIST, 1986). The 

calibration method can be performed by U.S. Geological 

Survey Water Science Centers with readily available equipment. The procedure is applicable to all well tapes—steel and 

electric—and can accurately calibrate well tapes with a calibrated steel reference tape. A mechanical calibration device, 

composed of an anchor and tensioner, was also developed 

and can be used with the calibration procedure to enhance 

accuracy and repeatability. The estimated expanded measurement uncertainty for conducting the calibrations outdoors with 

the method and not using the CD2 device (CD2) is 0.005 ft. 

The estimated expanded measurement uncertainty for conducting calibrations indoors with the method and the CD2 is 

0.001 ft. Replicated calibration testing of a steel tape indoors 

with the method and the CD2 had a standard uncertainty of 

0.0011 ft for an interval measurement from a single measurement; 0.0011 ft for an interval measurement computed from 

four averaged, replicated measurements; and 0.0016 ft for an 

interval measurement computed from two averaged, replicated measurements. Two testing-bias estimation methods 

gave test-bias estimates from -0.007 ft to -0.004 ft per 100-ft 

interval and indicate that the calibration method is adequate 

for calibrating tapes to ±0.01 ft, but could be improved. 

The accuracy of a sample of 29 in-service steel and 

electric groundwater-level tapes was investigated using the 

calibration method and the CD2. The condition of the tapes 

was noted and the tapes were tested for compliance with the 

accuracy requirement of ±0.01 ft per 100 ft. The in-service 

steel tapes were all in good condition. In contrast, the electric 

tapes had signs of wear and were harder to read. All of the 

in-service steel tapes could be used as is, without tape corrections, to achieve an accuracy of ±0.01 ft. The in-service 

electric tapes were accurate to about ±0.03 ft per 100 ft. All of 

the in-service electric tapes require tape corrections to achieve 

an accuracy of ±0.01 ft per 100 ft. A comparison of new with 

in-service electric tapes found that steel-core electric tapes 

maintained their length and accuracy better than electric tapes 

without a steel core.

Acknowledgments

The USGS Office of Ground Water (OGW) funded the 

development of the calibration method and device and the 

study of in-service groundwater tapes described in this report. 

The authors thank William Cunningham (Chief, USGS OGW) 

and Rodney Sheets (USGS Water Science Field Team) for 

their help, patience, and support for this study. The authors 

also thank Jim Jelinski (retired) whose ideas and technical support significantly contributed to the study and report 

contents. The following U.S. Geological Survey Water Science Centers are thanked for volunteering their groundwaterlevel tapes for the in-service tape study: Arizona, Arkansas, 

Nebraska, Nevada, New Jersey, North Carolina, Ohio, Pacific 

Islands, Pennsylvania, South Carolina, and Tennessee.
References Cited 29

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Number 12.
30 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

Appendix 1. Calibration Worksheet
Appendix 1. Calibration Worksheet 31
32 Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy

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Fulford and Clayton—Accuracy Testing of Steel and Electric Groundwater-Level Measuring Tapes: Test Method and In-Service Tape Accuracy—Open-File Report 2015–1137

ISSN 2331-1258 (online)

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