**DRAFT**

**GPS Data Standard for GIS**

US Forest Service

July 15, 2002

The GPS Data Standard for GIS consists of the following sections.

Section 1: Introduction

Section 2: GPS Accuracy Testing Guidelines

Section 3: Table of Tested GPS Accuracy

Section 4: GPS Accuracy Reporting and Metadata

References

Appendices 1-5

**Section 1: Introduction**

The GPS Data Standard provides direction on matching GPS equipment and
field procedures to the various GIS accuracy requirements of the agency.
The Standard is intended to provide agency direction for testing and
reporting GPS accuracy in a consistent and statistically meaningful way.
The Standard is based on a review of past agency and federal accuracy
standards with the intent of providing direction that is consistent with
current federal standards.

The GPS Data Standard does not define threshold accuracy values nor
define the minimum accuracy required for a given data theme or
application. The data steward or application manager is responsible for
deciding the accuracy values that are acceptable on a theme-by-theme
basis.

The GPS Data Standard provides GPS Accuracy Testing Guidelines (Section
2) for consistent and repeatable testing and accuracy reporting. The
test procedures will be used by the Forest Service to test and report
the accuracy of GPS equipment and/or procedures at designated GPS test
networks. The results will be published in a Table of Tested GPS
Accuracies (Section 3) that lists tested accuracies under various canopy
conditions*.*

GPS users can use this Table to select the GPS equipment and field
procedures that meet the accuracy requirements set by the data steward.
This Table also allows the reporting of "expected accuracy" of GPS data
for projects that use similar equipment, field procedures, and have
similar canopy conditions, thus eliminating the need for costly testing
of each data set. When data is logged using untested GPS equipment,
procedures or canopy conditions, users should test their own *data sets*
using the Testing Guidelines.

The GPS Data Standard is applicable to GPS data logged with: 1**)
Resource / Mapping GPS receivers**, defined as a C/A code receiver that
allows user configurable critical settings (DOP, SNR, elevation mask,
logging rate) and uses pseudorange data suitable for differential
corrections (real-time or post-processed), 2) **Recreation / Navigation
GPS receivers:** defined as C/A code or P/Y receivers that are generally
not user configurable for critical settings. Hybird configurations of
these two types of receivers do exist and are covered under this
Standard. To support agency activities in forested conditions, the
Standard will emphasize the reporting of the accuracy of these types of
receivers under forest canopy.

**GPS Data Standard Development Procedures**

The U.S. Forest Service GPS Steering Committee requested the development
of this Standard at their meeting on April 10, 2002, in San Diego,
California. The GPS Steering Group was requested to develop these
standards by line and staff officers at the national, regional and
forest level due to frequent questions of the accuracy, quality and
suitability of various sources of GPS data used in GIS applications for
land management decisions.

Relationship of GPS Data Standard to Existing Federal Data Standards

This GPS Data Standard is designed to comply with the Federal Geographic
Data Committee (FGDC), Geospatial Positioning Accuracy Standards Part 3,
NATIONAL STANDARD FOR SPATIAL DATA ACCURACY, FGDC-STD-007.3-1998.
(NSSDA).(Appendix 2)

The FGDC specifies that the NSSDA be used to evaluate and report the
positional accuracy of geospatial data produced, revised, or
disseminated by or for the Federal Government. "Executive Order 12906
Coordinating Geographic Data Acquisition and Access: the National
Spatial Data Infrastructure" designates the FGDC as responsible for
setting these standards. (Appendix 2)

The NSSDA was designed to replace the National Map Accuracy Standard
(NMAS). The Bureau of the Budget developed NMAS in 1947. The
applicability of NMAS is limited to graphic maps, as accuracy is defined
by map scale. The NSSDA was developed to report accuracy of digital
geospatial data that is not constrained by scale

Characteristics of The National Standard for Spatial Data Accuracy are:

-   Does not specify threshold accuracy. GIS managers set accuracy
    requirements for their applications.

-   Tests and reports positional accuracy so users can directly compare
    accuracy of data for their applications.

-   Compares data to test points with positions established at a higher
    accuracy standard.

-   Reports accuracy based on Root Mean Square Error (RMSE) reported at
    95% confidence at ground scale.

-   Minimum number of test points is 20.

-   Horizontal Accuracy =1.7308\*RMSEr.

-   Vertical Accuracy = 1.96\*RMSEz.

Characteristics of The National Map Accuracy Standard are:

-   Defines a threshold accuracy (pass / fail).

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-   Accuracy is scale dependent:

    -   12.2 Meters (40 Ft) for 1:24000 scale: Puerto Rico, Hawaii, and
        Continental US

    -   32.2 Meters (105.6 Ft) for 1:63,360 scale: Alaska

-   Reports accuracy at 90% confidence level at the map scale.

-   Horizontal Accuracy is 1.5175 \* RMSEr.

-   Vertical Accuracy = 1.645 \* RSMEz.

Relationship of GPS Data Standard to Existing Forest Service Data
Standards

There are currently two Forest Service documents that define agency
standards for GPS accuracy. They are the "GIS Data Dictionary, March 30,
2001" and "Guidelines for Digital Base Map Updates EM 7140-24". NMAS is
referenced as the accuracy standard in both these documents.

NMAS accuracy relative to NSSDA:

> NMAS uses a 90% confidence level value and the GPS Data Standard uses
> the NSSDA 95% confidence level; therefore, the conversions are listed
> below. (Appendix 3 Formulas)
>
> The two Forest Service documents above use the following accuracy
> standards:
>
> 1:24000 scale: Puerto Rico, Hawaii, and Continental US

-   NMAS at 90% confidence level= 12.2 Meters (40 Feet)

> Expressed as NSSDA:

-   NMAS at 95% confidence level = 13.9 Meters (45.6 Feet)

> 1: 63,360 scale: Alaska

-   NMAS at 90% confidence level = 32.2 Meters (105.6 Feet)

> Expressed as NSSDA:

-   NMAS at 95% confidence level = 36.7 Meters (120.4 Feet)

Source Codes used in existing Forest Service Standards:

Both of the above documents specify, under Source Code, three
classifications of GPS Accuracy:

02 GPS 2-5 Meter Receiver, 3-D.

03 GPS 2-5 Meter Receiver, 2D.

04 GPS Survey Grade and Sub-meter

The Data Dictionary document lists Source Code 02 and 03 as "GPS 2-5
meter" while the EM 7140-24 lists the same Source Code codes as "GPS 25
meter" (no dash between 2 and 5). It is assumed that the EM 7140-24
intended to list these as "2--5 meter" as this was a commonly quoted
accuracy figure provided by Trimble Navigation for the early Pathfinder
Professional GPS receivers. The "2-5 meter" figure was stated as a
Circular Error Probable (CEP), which is a 50% confidence level.

To convert to NSSDA accuracy (95% confidence level) from 5-meter CEP
(50% confidence level) data the following relationship is used:
(Appendix 3 Formulas).

RMSEr = CEP/0.83

NSSDA accuracy = 1.7308\*RMSEr

NSSDA accuracy = 10.4 meters

The Source Codes 04 Survey grade receiver classification uses carrier
phase. It is recommended that the existing BLM / Forest Service
Standards and Guidelines for GPS surveys be used for carrier phase
measurements. These standards and guidelines specify procedures for
network design, data logging, processing and analysis based on
least-squares network analysis.

The Source Code 04 Sub-meter classification also refers to carrier phase
measurements made using C/A code receivers. The specific processing
instructions contained in this document are inconsistent as MCORR400 is
not a carrier phase processing tool.

The descriptions of the Source Codes in the above documents are obsolete
as well as being internally inconsistent. Revision of these Source Codes
may be required to update and reconcile these inconsistencies.

**Section 2: GPS Testing Procedures**

Accuracy Test Guidelines

GPS Accuracy is tested comparing GPS measurements with independently
established coordinates of higher accuracy. NSSDA specifies that a
minimum of 20 check points shall be tested and that check points tested
should be distributed to reflect the geographic area of interest and the
distribution of error in the dataset.

To adapt the NSSDA for GPS testing, the number of known check points can
be less than 20; however, the number of measurement sets on the check
points should be much greater than 20.

Test data will be more repeatable and statistically valid with larger
sample sizes.

To avoid systematic errors due to satellite constellation bias, data
should be logged at times that are well distributed throughout the day.
To avoid systematic errors due to specific canopy conditions, data
should be logged at all check points in the network. This is especially
true for sites with heavy canopy.

The Forest Service maintains a set of GPS test networks at locations
across the country. These sites provide the unique characteristic of
having known independently established positions under forest canopy.
GPS accuracy in open conditions is relatively well documented and
understood. However the degree of GPS accuracy degradation under forest
canopy is not well documented. Since much of the GPS use in the Forest
Service will take place in forested locations, systematic study and
reporting of this is required. The various test sites were selected to
create a sample of the forest types and canopy conditions found on
national forest lands. Additional test sites may be added to this system
in the future.

The GPS test networks consist of a set of monumented check points, each
of which has independently established coordinates. The coordinates of
the check points at each site were determined by a combination of
carrier phase GPS (survey grade) and conventional theodolite/EDM survey
methods. The GPS measurements were tied to the National Spatial
Reference System via the state High Accuracy Reference Network; only
horizontal coordinates are published for these networks. Test Network
coordinates are generally accurate to less than 5 cm (95% confidence
level).

Tests of GPS accuracy in open conditions with no forest canopy should be
made using monumented and published geodetic positions from the National
Spatial Reference System. These can also serve as "control group" sites
for testing in canopy.

The Forest Service GPS test network site locations and descriptions are
listed in APPENDIX 4.

Steps for Testing of GPS accuracy:

> 1\. Test Site Selection
>
> 2\. Receiver Configuration
>
> 3\. Observation Length
>
> 4\. Data Logging
>
> 5\. Data Processing
>
> 6\. Compute Accuracy
>
> 7\. Documentation and Reporting

**1. Test Site Selection**

A test site should be selected based on the canopy type and density
where the GPS data will be logged.

**2. Receiver Configuration**

When making receiver tests all receiver configuration variables should
be noted in the test documentation.

Almanac Files: Receivers should be turned on and allowed to receive GPS
signals for approximately 20 minutes before the tests are run. This
procedure will insure that a current almanac is stored in the receiver
before any test data is recorded.

Positioning Mode: A minimum of 4 satellites should be observed. Only
3-dimensional (X, Y, and Z) positions should be logged 2-dimensional
data is not acceptable for test data.

Elevation Mask: The minimum satellite elevation mask should be 15
degrees above the horizon, if user configurable.

Antenna Type: The receiver antenna type, i.e. internal or external,
should be recorded. External antennas have been found to have an effect
on both the accuracy and efficiency of the GPS data logged under canopy.
It is recommended that receivers with manufacturer supplied external and
internal antennas be tested with both to assess any differences in
performance. The test should use an antenna height equivalent to those
used for GIS fieldwork and the height should be documented.

PDOP or EPE: The Position Dilution of Precision (PDOP) and/or the
manufacturers Estimated Position Error (EPE) values used in the test
should be logged if the receiver is configurable to do so. This data
will allow the analysis of the horizontal accuracy based on specific
PDOP or EPE settings.

SNR: The Signal-to-Noise (SNR) used in the test should be documented if
the SNR mask is configurable.

Data Logging Rates: Logging rates should be the same as those used to
log GIS data. Commonly used settings are 1 position / second for point
features and 1 position / 5 seconds for line (arc) and area (polygon)
features.

DGPS Signals: When using real-time DGPS, select the base with the
strongest signal regardless of whether that base is the closest to the
test site. If the receiver configuration allows reception of multiple
real time sources, it should be set for one frequency only. Systematic
errors in the position can be introduced if the receiver is configured
to use multiple broadcast signals due to unexpected changes in the
correction source.

**3. Observation Length**

Point Features: Receiver accuracy for *point* features can be determined
using several approaches:

One method is when using single point receivers. A single position test
is where a single GPS position is logged per point feature, then
compared with the independent data. Some Recreation / Navigation
receivers can only operate in a single point mode; they cannot be
configured to log multiple positions into one "waypoint."

Another method is a systematic analysis of the relationship between the
number of positions and accuracy. This method is used to determine the
optimum number of GPS *positions* that should be logged to create one
*point* feature. The accuracy of a point position is analyzed relative
to the number of positions recorded. Using this method allows the tester
to determine the minimum number of positions for each point feature that
will be used in the data-logging portion of the test.

Yet another method is to adopt a commonly used number of positions for
each point feature. For each point feature could consist of the
manufacturers recommended number of positions or for example under
canopy conditions be increased to a larger number of positions such as
30, 60, 120, or more.

Line Features: Line (arc) and area (polygon) features consist of a
series of linked single point positions; therefore, single point tests
can be used to estimate the accuracy of line and area features.

**4. Data Logging**

To avoid systematic errors due to specific canopy conditions, data
should be logged at all check points in the network. To avoid systematic
errors due to satellite constellation bias, data should be logged at
times that are well distributed throughout the day.

**5. Data Processing**

Data processing is considered to be all operations and computer
processing made to the raw GPS data logged in the field. The data
differential correction method used e.g. real-time or post processed
should also be noted. To allow duplication of test results, the data
processing methods and procedures used for data downloading,
differential correction, and export should be documented. This should
include all the software packages and version used. If data is
differentially corrected, the base and the distance from the test course
should be noted.

**6. Compute Accuracy**

The NSSDA uses root-mean-square error (RMSE) to estimate positional
accuracy. RMSE is the square root of the average of the set of squared
differences between dataset coordinate values and coordinate values from
an independent source of higher accuracy for identical points. Accuracy
is reported in ground distances at the 95% confidence level.

The accuracy should be computed using the NSSDA error analysis
spreadsheet in Appendix 2.

Although not a measure of GPS accuracy, it is recommended that receiver
reports make at least a cursory examination of receiver efficiency.
Receiver Efficiency = number of positions logged / (total time data was
logged in seconds/ data logging rate). Some GPS receiver critical
settings can yield relatively accurate positions; however, due to low
efficiency, logging GIS data with those settings could take a very long
time.

**7. Documentation and Reporting**

Documenting detailed information about the methods, procedures, and
equipment used in GPS testing will make the test results useful and
valid for application by others. Minimum documentation should include
the receiver type, antenna type, observation length, receiver settings
(SNR, PDOP, logging rates, elevation mask, etc), number of positions per
point feature, differential -correction method and base station used (if
applicable), and any field other relevant field procedures. Specific
information regarding the receiver serial numbers, model numbers, and
software application versions should also be documented.

The accuracy of tests should be reported as NSSDA accuracy.

**Section 3:**

  ------------------------------- -------------- -------------- -- -------------- ---- -------------- ---- -------------- ---- --------------- ----
  **Table of Tested GPS                                                                                                                        
  Accuracy**                                                                                                                                   

                                                 **Open**          **Medium**          **Heavy**           **Heavy**           **Heavy**       

                                                 **Open**          **Canopy**          **Canopy**          **Canopy**          **Canopy**      

  **RECEIVER TYPE**               **COMMENTS**   **95%**           **95%**             **95%**             **95%**             **95%**         

                                                                   **Lubrecht**        **Powell**          **Clackmas**        **Hardwoods**   

                                                 **(meters)**      **(meters)**        **(meters)**        **(meters)**        **(meters)**    

  **Recreational Grade                                                                                                                         
  Receivers**                                                                                                                                  

  **Magelan XL12 -**single        1 Position     11                                                        33                                  
  reading 1)                      Reading                                                                                                      

  **Magelan Blazer 12** - single  1 Position     10                                                        50                                  
  reading 1)                      Reading                                                                                                      

  **Magellan GPS Companion** -    1 Position                       20-24                                                                       
  Single Reading                  Reading                                                                                                      

                                                                                                                                               

  **Garmin eTrex** - Single       1 Position     10                                                        63                                  
  Reading 1)                      Reading                                                                                                      

  **Garmin GPS III** - Single     1 Position     2.6                                                                                           
  Reading                         Reading                                                                                                      

  **Garmin GPS III** - Single     1 Position     4                                                         68                                  
  Reading 1)                      Reading                                                                                                      

  **Garmin GPS III** - Single     1 Position     2.6                                                                                           
  Reading RT-corrected            Reading                                                                                                      

  **Garmin GPS III** - Averaging  60 Position    3.5                                                       30                                  
  1)                              Ave                                                                                                          

  **Garmin GPS III** - with       60 Position                                          2-16                15-17                               
  Real-time Beacon                Ave                                                                                                          

                                                                                                                                               

  **Garmin GPS Map76** - Single   1 Position                                                                                                   
  Reading                         Reading                                                                                                      

  **Garmin GPS Map76** - Single   1 Position                                                                                                   
  Reading RT-corrected            Reading                                                                                                      

  **Garmin GPS Map76** - Single   1 Position                                                                                                   
  Reading With WAAS               Reading                                                                                                      

  **Garmin GPS Map76** -          60 Position                      26                  3-20                17-20                               
  Averaging                       Ave                                                                                                          

  **Garmin GPS Map76** with                                                            3-20                22                                  
  Real-time Beacon                                                                                                                             

  **Garmin GPS Map76** with WAAS                                                       3-20                22                                  

                                                                                                                                               

  **Trimble Pathfinder Pocket** - 1 Position                                                                                                   
  Single Reading                  Reading                                                                                                      

  **Trimble Pathfinder Pocket** - 60 Position    5                 9-13                                                                        
  Averaging                       Ave                                                                                                          

  **Trimble Pathfinder Pocket**                                                                                                                
  with Real time Beacon                                                                                                                        

                                                                                                                                               

  **Resource Grade Receivers**                                                                                                                 

  **Trimble Pro XR-**             1 Position Ave                                                                                               
  un-corrected                                                                                                                                 

  **Trimble Pro XR-**             1 Position Ave                                                                                               
  Post-processed                                                                                                                               

  **Trimble Pro XR-               1 Position Ave                                                                                               
  u**n-corrected                                                                                                                               

  **Trimble Pro XR**              5 Position Ave                                                                                               

  **Trimble Pro XR -** RT (PDOP   5 Position Ave 1                 2-3                 0.3-3.2             10                                  
  8, SNR 4) 1)                                                                                                                                 

  **Trimble Pro XR -** Post       5 Position Ave .5                2.9                                     8                                   
  Processing (PDOP 8, SNR 4) 1)                                                                                                                

  **Trimble Pro XR**              60 Position                      5.7                 2.2-7.0                                                 
                                  Ave                                                                                                          

  **Trimble Pro XR** - RT (PDOP   60 Position    1                 2-4                 6                   6                                   
  8, SNR 4) 1)                    Ave                                                                                                          

  **Trimble Pro XR -** Post       60 Position    .5                                    0.6-2.7             6                                   
  Processing (PDOP 8, SNR 4)      Ave                                                                                                          
  1**)**                                                                                                                                       

                                                                                                                                               

  **Trimble GeoExplorer 3**       1 Position Ave                                                                                               
  Uncorrected                                                                                                                                  

  **Trimble GeoExplorer 3**       1 Position Ave                                                                                               
  Post-Processed                                                                                                                               

  **Trimble GeoExplorer 3** RT    1 Position Ave                                                                                               
  -Corrected                                                                                                                                   

  **Trimble GeoExplorer 3**       1 Position Ave                                                                                               

  **Trimble GeoExplorer 3**       5 Position Ave                                                                                               

  **Trimble GeoExplorer 3 -** RT  5 Position Ave                   7-10                12                  24                                  

  **Trimble GeoExplorer 3 -**     5 Position Ave                                                                                               
  Post Processed                                                                                                                               

  **Trimble GeoExplorer 3**       60 Position                                          1-14                17                                  
                                  Ave                                                                                                          

  **Trimble GeoExplorer 3** - RT  60 Position                      8-13                2-10                14-19                               
                                  Ave                                                                                                          

  **Trimble GeoExplorer 3 -**     5 Position Ave 6                                                         36                                  
  internal, PDOP 6, SNR4, P-P 1)                                                                                                               

  **Trimble GeoExplorer 3 -**     5 Position Ave 2                                                         33                                  
  External Ant., PDOP 6, SNR4,                                                                                                                 
  P-P 1                                                                                                                                        

  **Trimble GeoExplorer 3** -     60 Position    6                                                         18                                  
  internal, PDOP 6, SNR4, P-P 1   Ave                                                                                                          

  **Trimble GeoExplorer 3 -**     60 Position    2                                                         13                                  
  External Ant, PDOP 6, SNR4, P-P Ave                                                                                                          
  1                                                                                                                                            

                                                                                                                                               

  1\) Mancebo & Chamberlain 2000                                                                                                               

                                                                                                                                               
  ------------------------------- -------------- -------------- -- -------------- ---- -------------- ---- -------------- ---- --------------- ----

**Section 4: Accuracy Reporting and Metadata**

Accuracy Reporting:

NSSDA provides a consistent means of reporting accuracy of GIS data. The
accuracy of GPS data used in the GIS data set is reported at the 95%
confidence level, in ground distances, in the units of the GIS.

The accuracy of agency GPS data may be reported by one of two
methods: 1) a report based on the expected accuracy of the GPS data
based on previously compiled accuracy tests of similar equipment used in
similar conditions, per the Table of Tested GPS Accuracies (Section 3),
or 2) a report of the accuracy of the GPS data set based on NSSDA
testing of the actual GIS data set.

Generally only horizontal GPS data is exported to the Forest Service
GIS; so only horizontal accuracy of GPS data is recorded in these
examples.

> 1\) Reporting expected accuracy of GPS-derived GIS data based upon
> Forest Service or other NSSDA published receiver test.
>
> When the GPS accuracy values listed in the Table of Tested GPS
> Accuracies are used, report accuracy using the following "compiled to
> meet" statement:
>
> Report accuracy at the 95% confidence level for data *produced
> according to procedures that have been demonstrated to produce data
> with particular horizontal accuracy values* as:
>
> Compiled to meet \_\_\_\_ (meters, feet) horizontal accuracy at 95%
> confidence level
>
> 2\) Reporting tested accuracy of GPS-derived GIS data based upon your
> own NSSDA test results.
>
> When the GPS accuracy is directly tested by the user, report accuracy
> using the following \"tested\" statement:
>
> Report accuracy at the 95% confidence level for data *tested* for
> horizontal accuracy as:
>
> Tested \_\_\_\_ (meters, feet) horizontal accuracy at 95% confidence
> level

Metadata Reporting:

The NSSDA method of stating accuracy is consistent with the FGDC
metadata requirements. The reported horizontal accuracy values can
travel with the GIS dataset as metadata. Accuracy reporting is an
important element of the Federal Geographic Data Committee, Content
Standard for Digital Geospatial Metadata, FGDC STD-001-1998. Listed
below are specific examples of accuracy reporting from that document:

For GPS data, report the accuracy value in digital geospatial metadata
(Federal Geographic Data Committee, 1998, Section 2), as appropriate to
dataset spatial characteristics:

(Data_Quality_Information/Positional_Accuracy/Horizontal_Positional_Accuracy/Horizontal_Positional_Accuracy_Assessment/Horizontal_Positional_Accuracy_Value)

Enter the text "National Standard for Spatial Data Accuracy" for these
metadata elements (Federal Geographic Data Committee, 1998, Section 2),
as appropriate to dataset spatial characteristics:

(Data_Quality_Information/Positional_Accuracy/Horizontal_Positional_Accuracy/Horizontal_Positional_Accuracy_Assessment/Horizontal_Positional_Accuracy_Explanation)

Provide a complete description on how the accuracy values were
determined in metadata, as appropriate to dataset spatial
characteristics, (Federal Geographic Data Committee, 1998, Section 2):

(Data_Quality_Information/Positional_Accuracy/Horizontal_Positional_Accuracy/Horizontal_Positional_Accuracy_Report)

Accuracy of existing or legacy spatial data and maps may be reported, as
specified, according to the NSSDA or the accuracy standard by which they
were evaluated.

In addition to the accuracy statement, the following information should
be provided to the theme metadata compiler:

-   Coordinate system & Datum used: Generally UTM in NAD27

-   Projection: Generally UTM

-   Units.

-   Attribute Description (GPS data dictionary; features, attributes and
    attribute values)

-   Source -- Receiver type, antenna type, receiver settings (SNR, PDOP,
    logging rates RATES, Elevation Mask, etc), Number of positions per
    point feature, Correction method (if applicable), and any field
    other relevant field procedures,

**References:**

1.  Federal Geographic Data Committee, Geospatial Positioning Accuracy
    Standards, Part 3: National Stand for Spatial Data Accuracy,
    FGDC-STD-007.2-1998.

2.  Federal Geographic Data Committee, Geospatial Positioning Accuracy
    Standards Part 4, Engineering, Construction, and Facilities
    Management, FGDC-STD-007.4-2002.

3.  Federal Geographic Data Committee, Content Standard for Digital
    Geospatial Metadata, FGDC STD-001-1998.

4.  Federal Geographic Data Committee, A Proposal for a National Spatial
    Data Infrastructure Standards Project.

5.  Executive Order 12906, Coordinating Geographic Data Acquisition and
    Access: the National Spatial Data Infrastructure.

6.  Forest Service, Geometronics Service Center, Guidelines for Digital
    Base Map Updates, EM 7140-24.

7.  National Map Accuracy Standard, U.S. Bureau of the Budget, 1947.

8.  Positional Accuracy Handbook, Using the National Standard for
    Spatial Data Accuracy to measure and report geographic data quality,
    October 1999, Land Management Information Center, Minnesota
    Planning.

9.  Forest Service GIS Data Dictionary, March 1, 2002.

10. Ministry of Forests Procedures and Guidelines for Operational Forest
    Resource Survey and Mapping Using Global Positioning System
    Technology, February 2001, Version 3, Province of British Columbia.

11. Global Positioning System Standard Positioning Service Performance
    Standard, October 2001, Department of Defense.

**Appendix 1: Application of the GPS Data Standard for GIS**

The following examples illustrate some applications of the GPS Data
Standard in a GIS project:

**A GPS Mapping Project:**

You are assigned to map heritage resource site point features for
inclusion as corporate data in the forest GIS. The GIS data steward has
decided that the theme will be mapped to meet NMAS per direction in the
GIS Data Dictionary. The project is in heavy canopy conditions in
western Washington. You have a Garmin eTrex, a Trimble GeoExplorer3 ,
and a Pathfinder Professional GPS receiver available on the district.
Which receiver should you use to log the data?

Use the following steps to apply the GPS Data Standard for GIS. Convert
NMAS to GPS Data Standard using the formulas in Appendix 2, or use the
accuracy values in Section 1. Select a receiver from the Table of Tested
GPS Accuracy in Section 3 that meets the 13.9 meter accuracy in heavy
canopy. One choice would be a Pathfinder ProXR. Log data using the same
configuration listed in the table: number of positions per feature,
receiver settings (PDOP and SNR), data processing method, etc. Export
the data to GIS. Report accuracy at the 95% confidence level for data
produced according to procedures that have been demonstrated to produce
data with particular horizontal accuracy values as:

*Compiled to meet* 13.9 (meters) horizontal accuracy at 95% confidence
level

Report the metadata: Horizontal_Positional_Accuracy_Value = 13.9 meters,
Hororizontal_Positional_Accuracy_Explanation = National Standard for
Spatial Data Accuracy, Horizontal_Positional_Accuracy_Report = List the
receiver type, settings, software used, etc. per Section 4.

**Testing a GPS receiver Accuracy:**

You have mapped burned areas for a fire recovery team with a new model
Garmin GPS receiver. There is no test data available in the Table of
Tested GPS Accuracy for this model. The fire area is in open pine
forests in western Montana. The GIS data steward needs to report the
accuracy of the GIS layer. How should the accuracy be reported?

The Standard allows two methods: 1) Deduce that the receiver performance
will be similar to other recreation / navigation type GPS receivers used
in open canopy. 2) Test the receiver using the GPS Data Standard.

To test the new receiver, use a set of known check points positioned to
a higher accuracy standard. In this case, the Lubrecht Test network is
nearby and has similar canopy conditions. Use the Test Guidelines in
Section 2 to log data at the test course using the same data logging and
receiver settings that were used for the fire mapping. Log data four
times at each of the seven control points. You will now have a data set
of 28 check points. Export this data to a spreadsheet, enter the known
coordinates of the checkpoints, and then use the formulas shown in
Appendix 2 to make a NSSDA accuracy calculation. Per Section 2, document
the test procedures.

For example, assuming your result is 11 meters, report accuracy at the
95% confidence level for data produced according to procedures that have
been demonstrated to produce data with particular horizontal accuracy
values as:

*Tested* to meet 11 (meter) horizontal accuracy at 95% confidence level.

Report the metadata: Horizontal_Positional_Accuracy_Value = 11 meters,
Hororizontal_Positional_Accuracy_Explanation = National Standard for
Spatial Data Accuracy, Horizontal_Positional_Accuracy_Report = List the
receiver type, settings, software used, etc. per Section 4 under
reporting.

**Appendix 2: National Standard for Spatial Data Accuracy Spreadsheet**

![](media/image1.png){width="5.998611111111111in"
height="6.9527777777777775in"}

**Appendix 3: Formulas**

**Horizontal Accuracy using NSSDA formulas**:

Determine the radial Root Mean Square Error (RMSE) for the GPS data set:

RMSE~r~ = sqrt\[Σ ((Xdata - Xcheck)^2^+(Ydata -- Xcheck)^2^)/n\]

where:

Xdata , Ydata are the coordinates of the check point in the GPS data, i
data, i

Xcheck , Ycheck are the coordinates of the check point in GPS test
network.

Modify RMSE error to 95% probablitiy:

Accuracy~r~ = 1.7308 \* RSME~r~

An alternate formula for RSMEr is:

= sqrt\[(Σ(Ex^2^ + Ey^2^))/n)^2^+(Std Deviation of Error)^2^\]

National Map Accuracy Standard (NMAS 1947) Conversion to National
Standard for Spatial Data (NSSDA)

If error is normally distributed in each the x- and y-component and
error for the x-component is

equal to and independent of error for the y-component, the factor 2.146
is applied to compute

circular error at the 90% confidence level (Greenwalt and Schultz,
1968). The circular map

accuracy standard (CMAS) based on NMAS is:

CMAS = 2.1460 \* RMSEx = 2.1460 \* RMSEy

= 2.1460 \* RMSE~r~ /1.4142

= 1.5175 \* RMSE~r~

Where,

RMSEx is the Root Mean Square Error in the X coordinate

RMSEy is the Root Mean Square Error in the Y coordinate

The CMAS ( 90% confidence) can be converted to accuracy reported at
NSSDA Accuracy~r~ , (95% confidence)

Accuracy~r~ = 1.1406 \* CMAS~r~

Therefore, NMAS horizontal accuracy reported according to the NSSDA is:

1.1406\* \[S \* (1/30\")/12\"\] feet, or 0.0032 \* S, for map scales
larger than 1:20,000

1.1406\* \[S \* (1/50\")/12\"\] feet, or 0.0019 \* S, for map scales of
1:20,000 or smaller

where S is the map scale denominator.

National Standard for Spatial Data Accuracy (95% confidence) from
Circular Error Probable (50% confidence).

NSSDA accuracy = (1.7308\*(RMSEr=5 meter CEP/0.83)).

NSSDA accuracy = 10.4 meters

####### Appendix 4: USFS GPS Test Networks

**Powell, Idaho GPS Test Course**

This GPS test site is on the Powell Ranger District of the Clearwater NF
near Highway US 12 at the Idaho Highway Maintenance area about 12 miles
west of Lolo Pass and about 55 miles SW of Missoula, MT. The test site
is located near the Lochsa River. Mountains on the north and south sides
of the course obstruct the view at an angle of 10 degrees. The canopy
consists of large (24" to 42" d.b.h.) old growth cedar and spruce trees
in a flat valley bottom with only a small amount of understory. At most
stations, the canopy would be considered heavy. The course has 11
turning points or stations that outline a 12.019 acre polygon. The
polygon can be divided to produce 2 smaller polygons.

For further information contact: Dick Karsky, Missoula Technology
Development Center, 406-329-3921. email: dkarsky@fs.fed.us

**Lubrecht, Montana Test Course**

The Lubrecht Test Course is located at the Lubrecht Experimental Forest
about 40 miles northeast of Missoula, MT. This course is a polygon with
seven turning points (stations) and is located on gentle terrain, under
a mixed lodgepole and ponderosa pine canopy. The trees are about 19
meters tall with a minimal understory. The canopy would probably be
classified as a light to medium. Station B-31 is located in the open,
with a clear view of the sky down to an angle of 15 degrees.

For further information contact: Dick Karsky, Missoula Technology
Development Center, 406-329-3921 email: dkarsky@fs.fed.us

**Clackamas, Oregon Test Course**

The Clackamas Test Course is located on the Clackamas River Ranger
District of the Mount Hood National Forest about 70 miles southeast of
Portland Oregon. The course is located in Cascade Mountains of western
Oregon. The course is on gentle terrain with no terrain obstructions
above 15 degrees. The canopy at the course consists of heavy second
growth Douglas-fir and western hemlock (trees about 24 to 40 inches
d.b.h.) with a vine maple and red alder understory. At most stations,
the canopy would be considered heavy. The course has 13 stations that
outline a of 7.20 acre polygon.

For further information contact: Ken Chamberlain, 503-808-241, email:
kchamberlain@fs.fed.us.

**Bedford, Indiana Test Course**

The Bedford Test Course is located on the Hoosier National Forest, 11
miles from Bedford, Indiana. The site has gentle terrain under a dense
canopy of uneven-aged oak, beech, and hickory trees typical of eastern
hardwood forests, with canopy tops 31 to 37 meters (100 to 120 feet)
above the ground. The test course has seven turning points located on
two finger ridges with one point in the bottom of the draw between the
ridges. The polygon defined by the seven turning points contains 3.32
acres.

For further information contact: Dale Weigel, 812-277-3597, email:
<dweigel@fs.fed.us>

**Newtown Square, PA**

This GPS Test Site is located in Ridley Creek State Park about 16 miles
west of Philadelphia. This course consists of two polygons with 10
turning points (stations) under a heavy deciduous canopy dominated by
large diameter poplar, oak, and beech. There are two additional control
points (accurate to within 1.5 cm) located between the polygons in an
open field. Ridley Creek State Park is characterized by gently rolling
forested hills and meadows.

For further information contact: Richard A. McCullough, 610-557-4081,
email: rmccullough01@fs.fed.us

**Appendix 5: Existing Standards**

**Federal Standards:**

***United States National Map Accuracy Standards***

With a view to the utmost economy and expedition in producing maps which
fulfill not only the broad needs for standard or principal maps, but
also the reasonable particular needs of individual agencies, standards
of accuracy for published maps are defined as follows:

**1. Horizontal accuracy.** For maps on publication scales larger than
1:20,000, not more than 10 percent of the points tested shall be in
error by more than 1/30 inch, measured on the publication scale; for
maps on publication scales of 1:20,000 or smaller, 1/50 inch. These
limits of accuracy shall apply in all cases to positions of well-defined
points only. Well-defined points are those that are easily visible or
recoverable on the ground, such as the following: monuments or markers,
such as bench marks, property boundary monuments; intersections of
roads, railroads, etc.; corners of large buildings or structures (or
center points of small buildings); etc. In general what is well defined
will be determined by what is plottable on the scale of the map within
1/100 inch.

Thus while the intersection of two road or property lines meeting at
right angles would come within a sensible interpretation, identification
of the intersection of such lines meeting at an acute angle would
obviously not be practicable within 1/100 inch. Similarly, features not
identifiable upon the ground within close limits are not to be
considered as test points within the limits quoted, even though their
positions may be scaled closely upon the map. In this class would come
timber lines, soil boundaries, etc.

**2. Vertical accuracy,** as applied to contour maps on all publication
scales, shall be such that not more than 10 percent of the elevations
tested shall be in error more than one-half the contour interval. In
checking elevations taken from the map, the apparent vertical error may
be decreased by assuming a horizontal displacement within the
permissible horizontal error for a map of that scale.

**3. The accuracy of any map may be tested** by comparing the positions
of points whose locations or elevations are shown upon it with
corresponding positions as determined by surveys of a higher accuracy.
Tests shall be made by the producing agency, which shall also determine
which of its maps are to be tested, and the extent of the testing.

**4. Published maps meeting these accuracy requirements** shall note
this fact on their legends, as follows: "This map complies with National
Map accuracy Standards."

**5. Published maps whose errors exceed those aforestated** shall omit
from their legends all mention of standard accuracy.

**6. When a published map is a considerable enlargement** of a map
drawing (manuscript) or of a published map, that fact shall be stated in
the legend. For example, "This map is an enlargement of a 1:20,000-scale
map drawing," or "This map is an enlargement of a 1:24,000-scale
published map."

**7. To facilitate ready interchange and use of basic information for
map construction** among all Federal mapmaking agencies, manuscript maps
and published maps, wherever economically feasible and consistent with
the uses to which the map is to be put, shall conform to latitude and
longitude boundaries, being 15 minutes of latitude and longitude, or 7.5
minutes, or 3-3/4 minutes in size.

*Issued June 10, 194l* U.S. BUREAU OF THE BUDGET

Revised April 26, 1943

Revised June 17, 1947

**Executive Order 12906, Coordinating Geographic Data Acquisition and
Access: the National Spatial Data Infrastructure**

(Clinton, 1994, Sec. 4. Data Standards Activities, item d), "Federal
agencies collecting or producing geospatial data, either directly or
indirectly (e.g. through grants, partnerships, or contracts with other
entities), shall ensure, prior to obligating funds for such activities,
that data will be collected in a manner that meets all relevant
standards adopted through the FGDC process."

Federal Geographic Data Committee ( FGDC ) Geospatial Positioning
Accuracy Standards

FGDC Geospatial Positioning Accuracy Standards Part 3, NATIONAL STANDARD
FOR SPATIAL DATA ACCURACY, FGDC-STD-007.3-1998, (NSSDA) implements a
testing and statistical methodology for positional accuracy of fully
georeferenced maps and digital geospatial data, in either raster, point,
or vector format, derived from sources such as aerial photographs,
satellite imagery, and ground surveys. The NSRS is the framework that
references positions to the national datums. Positional accuracy of
geodetically surveyed points in the National Spatial Reference System is
reported according to Part 2, Standards for Geodetic Control Networks,
Geospatial Positioning Accuracy Standards. NSRS points may also be
selected as an independent source of higher accuracy to test positional
accuracy of maps and geospatial data according to the NSSDA. The lead
agency is the Department of the Interior, U.S. Geological Survey,
National Mapping Division. The responsible FGDC unit is the Subcommittee
on Base Cartographic Data.

**Forest Service GIS Data Standards**

**1.3.1.1 The Forest Service GIS Data Dictionary, March 1, 2002**.

The Forest Service GIS Data Dictionary the Spatial Source Code and
Horizontal accuracy of most of the themes listed; e.g. roads and trails,
water, wildlife, etc as:

> **Spatial Source Code:** Best available source with a target scale of
> 1:24000 for Continental U.S., Puerto Rico, and Hawaii and 1:63360 for
> Alaska.
>
> **Horizontal Accuracy:** Targeted to National Map Accuracy Standards
> (NMAS)

In addition some layers list in the DOMAIN TABLE a field for SOURCE
CODE. These use the same codes and GPS terms defined below under 2
Guidelines for Digital Base Map Updates EM 7140-24: i.e. GPS 2-5 meter
3D etc

**Guidelines for Digital Base Map Updates EM 7140-24:**

This document list Source Code codes for data used in PBS/SEQ updates.
Relevant GPS sources are:

02 GPS 25 Meter Receiver, 3-D.

03 GPS 25 Meter Receiver, 2D.

04 GPS Survey Grade and Sub-meter.

> Survey Grade.
>
> Sub-meter.

**Note** that these are 50% or Circular Error Probable error estimates
(Appendix 3 Formulas)

**Guidelines for Digital Base Map Updates. EM 7140-24:**

This document uses NMAS for the accuracy standard.

This document list source codes for data used in PBS/SEQ updates.
Relevant GPS "Source Codes" are:

02 **GPS 25 Meter Receiver, 3-D. The** GPS data collected such that
sufficient satellites are simultaneously visible at base and remote
instruments to enable continuous 3-D collection. Positional Dilution of
Precision (PDOP) value should be no greater than 8, and the signal-
level mask setting should be no less than 6. All data must be
differentially processed using a base station that is within the
allowable range for 2 to 5 meters or better accuracy. The base station
instrument must occupy a point whose coordinates are established using
third-order or better survey techniques. All data collected should
comply with NMAS for appropriate map scale 20 feet horizontal on the
1:24,000- scale PBS/SEQ map, 106 feet on the Alaska 1:63,360-scale
PBS/SEQ. Data collected must be referenced to the appropriate coordinate
system and datum. Differences between NAD 83 and NAD 27 can cause
positional errors of a few centimeters to more than 100 meters. No
source code memo is required as long as these parameters are met.

03 **GPS 25 Meter Receiver, 2D.** The GPS data collected such that
sufficient satellites will be simultaneously visible at base and remote
instruments. PDOP value should be no greater than 8, and signal-level
mask setting should be no less than 6. All data must be differentially
processed using a base station closer than 300 miles to the remote
instrument. The base station instrument must occupy a point whose
coordinates are established using third-order or better survey
techniques. For linear features such as trails, 2-D data collection
should not be used if more than 40 feet of change in relief occurs along
the feature being collected. All data collected should comply with NMAS
for appropriate map scale 40 feet horizontal on the 1:24,000-scale PBS/
SEQ map, 106 feet on the Alaska 1:63.360-scale PBS/SEQ. Data must be
referenced to the appropriate coordinate system and datum. Differences
between NAD 83 and NAD 27 can cause positional errors of a few
centimeters to more than 100 meters. No source code memo is required as
long as these parameters are met.

04 **GPS Survey Grade and Sub-meter.**

**Survey Grade.** The GPS data collected by survey (L1) and geodetic
(L1, L2) grade receivers processed and network adjusted to a standard
deviation of 2 sigma (95 percent) at a maximum of 3 centimeters
horizontal and 10 centimeters vertical or better of ground position.
*Section Deleted -- Not Applicable to these standards.*

**Sub-meter.** The GPS data collected and processed using carrier phase
(L1 C/A code) receivers and processing using Trimble Navigation's MCORR
400 or Phase Processor for static point. When using phase processing,
base lines should not exceed 50 kilometers for positional accuracies of
less than 0.3 meter and should not exceed 120 kilometers for positional
accuracies of less than 1 meter. When using MCORR 400, baseline lengths
should not exceed 500 kilometers from a carrier phase reference station
to achieve positional accuracies of less than 1 meter. Data must be
collected in 3-D mode only. Data collected must be referenced to the
appropriate coordinate system and datum. Differences between NAD 83 and
NAD 27 can cause positional errors of a few centimeters to more than 100
meters. No source code memo is required as long as the above parameters
are met.