National Map Accuracy Standards (NMAS)

The U.S. National Map Accuracy Standards (NMAS) define a relationship between map scale and map accuracy. The NMAS were published by the U.S. Bureau of the Budget in 1941 (revised in 1947). U.S. maps that meet the standards contain the text “This map complies with National Map Accuracy Standards.” GIS projects in the United States often use base maps or digital databases with error estimates based on the NMAS.

Map scale is the ratio of distance on a paper map to distance on the surface of the earth. For example, if the distance between points on a map is 1 unit and the equivalent distance on the earth is 24,000 units, then the map scale is 1:24,000. The smallest size point or line width that can be practically drawn, correctly placed, or easily seen on a paper map is about 0.5 mm (1/50th of an inch). The scale of a map will determine the size on the earth of a 0.5 mm misplacement, point diameter, or line width on the map, and so the limit of potential accuracy for any map can be estimated from the map scale. Thus, on a map with a scale of 1:24,000, a 0.5 mm misplacement would result in a 12 m misplacement on the earth. For any map, the potential accuracy in meters can be approximated by multiplying the scale denominator by 0.0005 m (0.5 mm). For a 1:24,000 scale map, 24,000 × 0.0005 m equals 12 m, the potential accuracy for the map.

[Page 305]Potential accuracy is precision, not accuracy. Actual map accuracy can be determined only by comparing measurements on a map to measurements on the ground. This is what the NMAS were designed to do: relate map scale, or potential accuracy, to actual accuracy through testing.

For horizontal accuracy, the NMAS divide maps up into two groups: those with scales larger and smaller than 1:20,000. The NMAS require that for a map of smaller scale than 1:20,000, objects should have errors of less than 1/50th of an inch. For a 1:24,000 scale map, that is 480 inches (12.2 m) on the ground. For larger-scale maps, errors should be less than 1/30th of an inch (0.85 mm). A 1:10,000 scale map should have a tested accuracy of 333 inches (8.5 m).

It would be impractical to test every feature on a map. The NMAS require that 90% of “well-defined” points be within the required accuracy. These are “points that are easily visible or recoverable on the ground, such as the following: monuments or markers, such as benchmarks, property boundary monuments; intersections of roads, railroads, etc.; corners of large buildings or structures (or center points of small buildings); etc.”

The NMAS also define requirements for vertical accuracy by requiring that 90% of tested map elevations be at least as accurate as one half the contour interval. For a USGS map with 20-foot contour intervals, the mapped elevations must be accurate to within 10 feet (about 3 m) to meet the standards.