MAP ACCURACY STANDARDS
National Map Accuracy Standard - NMAS (U.S. Bureau of the Budget, 1947)
1. Horizontal accuracy. For maps on the publication scales of 1:20,000 or larger, not more than 10% of the points tested shall be in error by more than 1/30 inch.
2. Vertical accuracy. Not more than 10% of the elevations tested shall be in error more than one-half the contour interval.
The Reference Guide Outline (The Photogrammetry for Highways Committee, 1968)
A. Contours - Ninety percent of the elevations determined from solid-line contours of the topographic maps shall have an accuracy with respect to true elevation of one-half (1/2) contour interval or better and the remaining ten (10) percent of such elevations shall not be in error by more than one contour interval.
B. Planimetric Features - Ninety percent of all planimetric features which are well-defined on the photographs shall be plotted so that their position on the finished maps shall be accurate to within at least one-fortieth (1/40) of an inch of true coordinate position.
C. Spot Elevations - Ninety (90) percent of all spot elevations placed on the maps shall have an accuracy of at least on-fourth (1/4) the contour interval, and the remaining ten (10) percent shall be not in error by more than one-half (1/2) the contour interval.
The above listed standards are still most commonly used in specifications for mapping projects.
American Society for Photogrammetry and Remote Sensing (ASPRS) Accuracy Standards for Large-Scale Maps
ASPRS Specifications and Standards Committee, 1990, provide accuracy tolerances for maps at 1:20,000 scale or larger "prepared for special purposes or engineering applications." RMSE is the statistic used by the ASPRS standards.
Accuracy is reported as Class 1, Class 2, or Class 3. Class 1 accuracy for horizontal and vertical components is discussed below. Class 2 accuracy applies to maps compiled within limiting RMSE's twice those allowed for Class 1 maps. Similarly, Class 3 accuracy applies to Federal Geographic maps compiled within limiting RMSE's three times those allowed for Class 1 maps.
ASPRS Accuracy Standards for Large-Scale Maps (ASPRS Specifications and Standards Committee, 1990) evaluates positional accuracy for the x-component and the y-component individually. Positional accuracy is reported at ground scale.
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Vertical map accuracy is defined by the ASPRS Accuracy Standards (ASPRS Specifications and Standards Committee, 1990) as the RMSE in terms of the project's elevation datum for well defined points only.
For Class 1 maps according to the ASPRS Accuracy Standards, the limiting RMSE is set at one third the contour interval. Spot elevations shall be shown on the map with a limiting RMSE of one-sixth the contour interval or less.
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