U.S. National Map Accuracy Standards for Horizontal Accuracy

Explains federal standards for determining horizontal ground accuracy of geospatial data

According to the United States National Map Accuracy Standards of the National Mapping Program (NMP) , the standard for horizontal accuracy is as follows:

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 1/30th inch, measured on the publication scale; for maps on publication scales of 1:20,000 or smaller, 1/50th inch.

Written another way:

For maps of larger scale than 1:20,000, at least 90% of the tested points must be within 1/30 inch (.03333 inch). For maps of 1:20,000-scale or smaller, at least 90% of the tested points must be within 1/50th inch (.02 inch).

Depending on the scale of the map, the actual ground distance represented by 1/30th and 1/50th of an inch will vary. To determine the minimum standards for horizontal accuracy in actual ground meters, the following calculation must be performed.

If larger than 1:20,000-scale, use this calculation:

.03333 x scale x 2.54 / 100 = ground meters.
If 1:20,000-scale or smaller, use this calculation:

.02 x scale x 2.54 / 100 = ground meters.

Examples:

For 1:100,000-scale maps:

.02 x 100,000 x 2.54 / 100 = 50.8 meters (or 166 ft)
For 1:24,000-scale maps:

.02 x 24,000 x 2.54 / 100 = 12.2 meters (or 40 ft)

Conversion table for each scale:

Map Scale	Ground Meters
1:12,000	10.2
1:24,000	12.2
1:100,000	50.8
1:250,000	127
1:500,000	254
1:2,000,000	1,016

Related content: General Standards & Policies for Geographic Data

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