Development of a Federal Geographic Data Committee (FGDC) National Standard for Spatial Data Accuracy
Subcommittee for Base Cartographic Data (SBCD), FGDC
The objective is to facilitate sharing and interoperability of geospatial data by providing a flexible and inclusive standard for testing and reporting accuracy of maps and geospatial data.
The National Standard for Spatial Data Accuracy (NSSDA) implements a well-defined statistic and testing methodology for positional accuracy of maps and geospatial data derived from sources such as aerial photographs, satellite imagery, or maps. Accuracy is reported in ground units. The testing methodology is comparison of data set coordinate values with coordinate values from a higher accuracy source for points that represent features readily visible or recoverable from the ground. While this standard evaluates positional accuracy at points, it applies to geospatial data sets that contain point, vector, or raster spatial objects. Data content standards, such as FGDC Standards for Digital Orthoimagery and Digital Elevation Data, will adapt the NSSDA for particular spatial object representations.
The standard insures flexibility and inclusiveness by omitting accuracy metrics, or threshold values, that data must achieve. However, agencies are encouraged to establish "pass-fail" criteria for their product standards and applications and for contracting purposes. Ultimately, users must identify acceptable accuracies for their applications.
Ground coordinates of points established according to Federal Geodetic Control Subcommittee (FGCS) draft Standards for Geodetic Control Networks and process specifications are used in the National Spatial Reference System (NSRS) control network. NSRS ground control may be used to reference project control surveys to a common georeference system. The accuracy of spatial data derived from control surveys is expressed using the NSSDA. The NSSDA may also be related to FGCS Standards for Geodetic Networks by using NSRS points to test the accuracy of geospatial data. Both the NSSDA and Standards for Geodetic Networks will be integrated into a multipart FGDC Geospatial Positioning Accuracy Standard.
As stated in Executive Order 12906, Coordinating Geographic Data Acquisition and Access: the National Spatial Data Infrastructure, the FGDC will coordinate the Federal Government's development of the National Spatial Data Infrastructure (NSDI). The NSSDA supports the development of the NSDI by providing a common accuracy testing and reporting methodology to facilitate sharing and interoperability of geospatial data.
The NSSDA is intended to replace the 1947 National Map Accuracy Standard (NMAS). 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.
Starting with the existing draft developed by SBCD, the FGDC will invite wide participation from across the map and geospatial data user and producer communities.
The NSSDA was developed by the FGDC ad hoc working group on spatial data accuracy to replace the NMAS. The American Society for Photogrammetry and Remote Sensing (ASPRS) Accuracy Standards for Large-Scale Maps formed the basis for revision of the NMAS (ASPRS Specifications and Standards Committee, 1990). The NSSDA, in its former version as the draft National Cartographic Standards for Spatial Accuracy (NCSSA), extended the ASPRS Accuracy Standards to map scales smaller than 1:20,000. The NCSSA were released for public review through the Federal Geographic Data Committee and were substantially rewritten as a result. Principal changes included omission of a defined accuracy specification in favor of a requirement for reporting tested or expected accuracy; a composite statistic for horizontal accuracy instead of component (x,y) accuracy, and alignment with emerging FGCS accuracy standards (FGCS, 1995). The NCSSA was renamed the National Standard for Spatial Data Accuracy to emphasize its applicability to digital spatial data as well as graphic maps.
Related standards include the following:
Geospatial Positioning Accuracy Standard (approved FGDC standards project) - Federal
Geodetic Control Subcommittee (FGCS)
Standards for Geodetic Control Networks (draft) - FGCS
The National Map Accuracy Standard of 1947
ASPRS Accuracy Standards for Large-Scale Mapping
FGDC data content standards, including Standard for Digital Elevation Data and Standard for Digital Orthoimagery
The SBCD submitted the draft National Standard for Spatial Data Accuracy to Standards Working Group through a 09/20/96 memo signed by Kathy Clement, Chair, SBCD, for evaluation to recommend for public review and comment (Step 6, Review and Evaluate, of the Standards Reference Model).
The SBCD has provided development resources for the preparation of the committee draft and will provide resources to adjudicate comments from the public review and approval process.
The primary participants are the members of the SBCD; however, development of the standard has been coordinated with FGCS standards activities (refer to RELATED STANDARDS, above). The public review and comment period will include an aggressive program of outreach to ensure a broad level of participation from the geospatial data community. Particular emphasis will be given to involving the American Society of Photogrammetry and Remote Sensing, the American Congress on Surveying and Mapping, and State GIS councils.
Target Authorization Body
The FGDC Steering Committee is the target authorization for this standard.