Appendix C – National Standard for Spatial Data Accuracy
National Standard for Spatial Data Accuracy
In 1941, the U.S. Bureau of the Budget established national map accuracy standards so that federally produced maps fulfill the broad needs of standardization and are produced expediently and economically. The current standards, which were revised in 1947, 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 and railroads, etc.; corners of large buildings or structures (or center points of small buildings); etc. In general, what is well-defined will also be determined by what is plottable on the scale of the map within 1/100 inch. Thus while the intersection of two roads 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 riot 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 Tests shall be made by the producing agency, which shall also determine which of its maps are to be tested, and the extent of such testing.
4. Published maps meeting these accuracy requirements shall note this fact in their legends, as follows: “This map complies with the national standard map accuracy requirements.”
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, of 7 1/2 minutes, or 3 3/4 minutes in size.
The Proposed NSSDA Standards
At the time of completing the 2005 revision of the Definitions of Surveying and Associated Terms, the U.S. National Map Accuracy Standards, as outlined in this appendix, remained current. However, just like the proposed geodetic control survey standards, map accuracy standards have also been moving toward standards based on positional accuracy.
New map accuracy standards were defined in “FGDC Geospatial Positioning Accuracy Standards, Part 3: National Standard for Spatial Data Accuracy“. The proposed new standards for spatial data were at the final stage of revision in 2005.
The National Standards for Spatial Data Accuracy apply to 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. It provides a common language
for reporting accuracy to facilitate the identification of spatial data for geographic applications.
This standard is classified as a “Data Usability Standard” by the Federal Geographic Data Committee Standards Reference Model. A Data Usability Standard describes how to express “the applicability or essence of a dataset or data element” and includes “data quality, assessment, accuracy, and reporting or documentation standards” (FGDC, 1996, p. 8).
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, and 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 NSSDA for particular spatial object representations.
The standard ensures 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. Data and map producers must determine what accuracy exists or is achievable for their data arid report it according to NSSDA.
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 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.
Use NSSDA to evaluate and report the positional accuracy of maps and geospatial data produced, revised, or disseminated by or for the Federal Government. According to 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.”
Accuracy of new or revised spatial data will be reported according to NSSDA. Accuracy of existing or legacy spatial data and maps may be , reported, as specified, according to NSSDA or the accuracy standard by which they were evaluated.
Data producers may elect to use conformance levels or accuracy thresholds in standards such as the National Map Accuracy Standards of 1947 (U.S. Bureau of the Budget, 1947) or Accuracy Standards for Large-Scale Maps [American Society for Photogrammetry and Remote Sensing (ASPRS) Specifications and Standards Committee, 19901, if they decide that these values are truly applicable.
Federal Geographic Data Committee
Geospatial Positioning Accuracy Standards
Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.
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