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.
Subcommittee for Base Cartographic Data (retired)
Standards Working Group
|Draft||1996-12||FGDC Standards Working Group|
|Comment adjudication log||1998-04||FGDC Steering Committee|
||1998||FGDC Steering Committee|
|Proposal for revision||2003-11-17||FGDC Standards Working Group|
For more information
- Principles of Error Theory and Cartographic Applications
- National Map Accuracy Standard (1947) - Defines accuracy standards for published maps, including horizontal and vertical accuracy, accuracy testing method, accuracy labeling on published maps, labeling when a map is an enlargement of another map, and basic information for map construction as to latitude and longitude boundaries.
- The Minnesota Governor's Council on Geographic Information, with support from the Minnesota Land Management Information Center (LMIC), prepared the Positional Accuracy Handbook: Using the National Standard for Spatial Data Accuracy to measure and report geographic data quality, October 1999 (33 pages, 920K, PDF) to facilitate use of the FGDC standards. Printed copies of the Handbook are also available. See the LMIC site for more information. This handbook describes how positional accuracy can be measured and reported for databases that contain geographic features like roads, rivers, and property lines. Five practical examples walk you through the process using databases developed at Minnesota Departments of Transportation and Natural Resources, the City of Minneapolis, Washington County, and Lawrence Mapping.