Federal Geographic Data Committee

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FGDC-STD-007.2-1998

Geospatial Positioning Accuracy Standards

Part 2: Standards for Geodetic Networks

Federal Geodetic Control Subcommittee

Federal Geographic Data Committee
2-ii

Federal Geographic Data Committee

Established by Office of Management and Budget Circular A-16, the Federal Geographic Data Committee

(FGDC) promotes the coordinated development, use, sharing, and dissemination of geographic data.

The FGDC is composed of representatives from the Departments of Agriculture, Commerce, Defense,

Energy, Housing and Urban Development, the Interior, State, and Transportation; the Environmental

Protection Agency; the Federal Emergency Management Agency; the Library of Congress; the National

Aeronautics and Space Administration; the National Archives and Records Administration; and the

Tennessee Valley Authority. Additional Federal agencies participate on FGDC subcommittees and

working groups. The Department of the Interior chairs the committee.

FGDC subcommittees work on issues related to data categories coordinated under the circular. 

Subcommittees establish and implement standards for data content, quality, and transfer; encourage the

exchange of information and the transfer of data; and organize the collection of geographic data to reduce

duplication of effort. Working groups are established for issues that transcend data categories.

For more information about the committee, or to be added to the committee's newsletter mailing list,

please contact:

Federal Geographic Data Committee Secretariat

c/o U.S. Geological Survey

590 National Center

Reston, Virginia 22092

Telephone: (703) 648-5514

Facsimile: (703) 648-5755

Internet (electronic mail): gdc@usgs.gov

Anonymous FTP: ftp://fgdc.er.usgs.gov/pub/gdc/

World Wide Web: http://fgdc.er.usgs.gov/fgdc.html
Federal Geographic Data Committee FGDC-STD-007.2-1998

Draft Geospatial Positioning Accuracy Standards

Part 2: Standards for Geodetic Networks

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CONTENTS

Page

2.1 Introduction .............................................................. 2-1

2.1.1 Objective ......................................................... 2-1

2.1.2 Scope ............................................................ 2-1

2.1.3 Applicability ...................................................... 2-1

2.1.4 Related Standards .................................................. 2-1

2.1.5 Standards Development Procedures ..................................... 2-2

2.1.6 Maintenance ...................................................... 2-2

2.2 Testing Methodology and Reporting Requirements ................................ 2-3

2.2.1 Accuracy Standards ................................................. 2-3

2.2.2 Accuracy Determination .............................................. 2-4

2.2.3 Accuracy Reporting ................................................. 2-5

2.3 References ............................................................... 2-6

Table

2.1 Accuracy Standards - Horizontal, Ellipsoid Height, and Orthometric Height ............. 2-3
Federal Geographic Data Committee FGDC-STD-007.2-1998

Draft Geospatial Positioning Accuracy Standards

Part 2: Standards for Geodetic Networks

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2.1 Introduction

2.1.1 Objective

This document provides a common methodology for determining and reporting the accuracy of

horizontal coordinate values and vertical coordinate values for geodetic control points

represented by survey monuments, such as brass disks and rod marks. It provides a means to

directly compare the accuracy of coordinate values obtained by one method (e.g., a classical lineof-sight traverse) with the accuracy of coordinate values obtained by another method (e.g., a

Global Positioning System (GPS) geodetic network survey) for the same point.

2.1.2 Scope

Geodetic control surveys are usually performed to establish a basic control network (framework)

from which supplemental surveying and mapping work, covered in other parts of this document,

is performed. Geodetic network surveys are distinguished by use of redundant, interconnected,

permanently monumented control points that comprise the framework for the National Spatial

Reference System (NSRS) or are often incorporated into the NSRS.

These surveys must be performed to far more rigorous accuracy and quality assurance standards

than those for control surveys for general engineering, construction, or topographic mapping

purposes. Geodetic network surveys included in NSRS must be performed to meet automated

data recording, submittal, project review, and least squares adjustment requirements established

by the Federal Geodetic Control Subcommittee (FGCS).

2.1.3 Applicability

Geodetic network surveys are often employed when large geopolitical area (e.g., county-level or

larger) mapping control is required, and where seamless connection with adjacent political areas

is critical. Accurate network control may also be required for controlling interstate

transportation corridors (highways, pipelines, railroads, etc.); long-span bridge construction

alignment; geophysical studies; structural deformation monitoring of dams, buildings, and

similar facilities.

2.1.4 Related Standards

Part 6: Point Profile, The Spatial Data Transfer Standard (FGDC, 1998) defines the format to be

used to transfer geodetic coordinate data, including the accuracy of the coordinate values,

between geographic information systems.

Part 3, National Standard for Spatial Data Accuracy (NSSDA), Geospatial Positioning Accuracy

Standards (FGDC, 1998)provides the statistical and testing methodology for estimating the

accuracy of point coordinate values produced from maps and other digital geospatial data with

respect to geo-referenced ground positions of higher accuracy.
Federal Geographic Data Committee FGDC-STD-007.2-1998

Draft Geospatial Positioning Accuracy Standards

Part 2: Standards for Geodetic Networks

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The public review draft of Part 4, Standards for A/E/C and Facility Management, Geospatial

Positioning Accuracy standards, uses the NSSDA for accuracy testing and verification. The

NSSDA may be used for fully geo-referenced maps for A/E/C and Facility Management

applicaitons such as preliminary site planning and reconnaissance mapping.

2.1.5 Standards Development Procedures

Draft accuracy standards for geodetic networks were developed by the FGCS Methodology Work

Group, Federal Geographic Data Committee. The draft accuracy standards were released for

public review through the FGCS and evolved into the final form presented in Table 2.1 of this

publication.

2.1.6 Maintenance

The U.S. Department of Commerce, National Oceanic and Atmospheric Administration,

National Ocean Service, National Geodetic Survey, maintains accuracy standards for geodetic

networks for the Federal Geodetic Control Subcommittee, Federal Geographic Data Committee. 

Address questions concerning accuracy standards for geodetic networks to: Director, National

Geodetic Survey, NOAA, N/NGS, 1315 East-West Highway, Silver Spring, Maryland 20910.
Federal Geographic Data Committee FGDC-STD-007.2-1998

Draft Geospatial Positioning Accuracy Standards

Part 2: Standards for Geodetic Networks

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2.2 Testing Methodology And Reporting Requirements

2.2.1 Accuracy Standards

Note that the following accuracy standards supersede and replace the accuracy standards found in

FGCC 1984 and FGCC 1988 (see Section 2.3). The classification standard for geodetic networks

is based on accuracy. Accuracies are categorized separately according to Table 2.1 for

horizontal, ellipsoid height, and orthometric height. Note: although the largest entry in Table

2.1 is 10 meters, the accuracy standards can be expanded to larger numbers if needed.

 Table 2.1 -- Accuracy Standards

 Horizontal, Ellipsoid Height, and Orthometric Height

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

 Accuracy 95-Percent 

 Classification Confidence 

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

 Less Than or

 Equal to:

 1-Millimeter 0.001 meters

 2-Millimeter 0.002 " 

 5-Millimeter 0.005 " 

 1-Centimeter 0.010 " 

 2-Centimeter 0.020 " 

 5-Centimeter 0.050 " 

 1-Decimeter 0.100 " 

 2-Decimeter 0.200 " 

 5-Decimeter 0.500 " 

 1-Meter 1.000 " 

 2-Meter 2.000 " 

 5-Meter 5.000 " 

 10-Meter 10.000 " 

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

When control points in a survey are classified, they have been verified as being consistent with

all other points in the network, not merely those within that particular survey. It is not

observation closures within a survey which are used to classify control points, but the ability of

that survey to duplicate already established control values. This comparison takes into account

models of crustal motion, refraction, and any other systematic effects known to influence survey

measurements.
Federal Geographic Data Committee FGDC-STD-007.2-1998

Draft Geospatial Positioning Accuracy Standards

Part 2: Standards for Geodetic Networks

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2.2.2 Accuracy Determination

 The classification standard for NSRS is based on Table 2.1.

 The procedure leading to classification involves four steps:

 l. The survey measurements, field records, sketches, and other documentation are

examined to verify compliance with the specifications for the intended accuracy of the

survey. This examination may lead to a modification of the intended accuracy.

2. Results of a minimally constrained, least squares adjustment of the survey

measurements are examined to ensure correct weighting of the observations and freedom

from blunders.

3. Local and network accuracy measures computed by random error propagation determine

the provisional accuracy. In contrast to a constrained adjustment where coordinates are

obtained by holding fixed the datum values of the existing network control, accuracy

measures are computed by weighting datum values in accordance with the network

accuracies of the existing network control.

4. The survey accuracy is checked by comparing minimally constrained adjustment results

against established control. The result must meet a 95 percent confidence level. This

comparison takes into account the network accuracy of the existing control, as well as

systematic effects such as crustal motion or datum distortion. If the comparison fails,

then both the survey and network measurements must be scrutinized to determine the

source of the problem.

Users with specialized applications that require more exacting accuracy estimates at the CORS

sites should contact NGS. It is not necessary to directly connect to a CORS to compute the

network accuracy of a control point. However, it is necessary that the survey be properly

connected to existing NSRS control points with established network accuracy values.

By supporting both local accuracy and network accuracy, the diverse requirements of NSRS users

can be met. Local accuracy is best adapted to check relations between nearby control points. For

example, a surveyor checking closure between two NSRS points is mostly interested in a local

accuracy measure. On the other hand, someone constructing a Geographic or Land Information

System (GIS/LIS) will often need some type of positional tolerance associated with a set of

coordinates. Network accuracy measures how well coordinates approach an ideal, error-free

datum.

Thus, for control points in the NSRS, both local accuracy and network accuracy will be reported

for each geodetic component (horizontal control, ellipsoidal height, and orthometric height).
Federal Geographic Data Committee FGDC-STD-007.2-1998

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Part 2: Standards for Geodetic Networks

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2.2.3 Accuracy Reporting

When providing geodetic point coordinate data, a statement should be provided that the data

meets a particular accuracy standard for both the local accuracy and the network accuracy. For

example, these geodetic control data meet the 2-centimeter local accuracy standard for the

horizontal coordinate values and the 5-centimeter local accuracy standard for the vertical

coordinate values (heights) at the 95-percent confidence level. A similar statement should be

provided for these same data reporting the network accuracy.

Note: In the above statement the data may comply with one accuracy value for the horizontal

component and a different accuracy value for the vertical component. If a dataset does not

contain elevation data, label it for horizontal accuracy only; conversely, when a dataset does not

contain horizontal data, label it for vertical accuracy only.

It is preferred that accuracy value(s) be reported in metric units; however, feet shall be used when

the dataset coordinates are in feet (i.e., State Plane Coordinates in feet). The number of

significant digits for the accuracy value(s) shall be consistent with the number of significant

digits for the dataset point coordinates. For most geodetic control network applications,

centimeters should be used for reporting local accuracy and network accuracy values.
Federal Geographic Data Committee FGDC-STD-007.2-1998

Draft Geospatial Positioning Accuracy Standards

Part 2: Standards for Geodetic Networks

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2.3 References

Federal Geodetic Control Committee, 1984, Standards and Specifications for Geodetic Control Networks:

Silver Spring, Maryland, National Geodetic Survey, National Oceanic and Atmospheric

Administration, 29 p.

Federal Geodetic Control Committee, 1988, Geometric Geodetic Accuracy Standards and Specifications

for Using GPS Relative Positioning Techniques, Version 5.0, reprinted with corrections, August

1, 1989: Silver Spring, Maryland, National Geodetic Survey, National Oceanic and Atmospheric

Administration.

Federal Geodetic Control Subcommittee, 1994, Input Formats and Specifications of the NGS Data Base,

Vol. I, Horizontal: Silver Spring, Maryland, National Geodetic Survey, National Oceanic and

Atmospheric Administration.

Federal Geographic Data Committee, 1998, Part 1, Reporting Methodology, Geospatial Positioning

Accuracy Standards, FGDC-STD-007.1-1998: Washington, D.C., Federal Geographic Data

Committee, 10 p.

Federal Geographic Data Committee, 1998, Part 3, National Standard for Spatial Data Accuracy,

Geospatial Positioning Accuracy Standards, FGDC-STD-007.3-1998: Washington, D.C., Federal

Geographic Data Committee, 28 p.

Federal Geographic Data Committee, 1998, Part 6, Point Profile, Spatial Data Transfer Standard, 

FGDC-STD-002.6: Washington, D.C., Federal Geographic Data Committee.