# U.S. National Map Accuracy Standards for Horizontal Accuracy

Explains federal standards for determining horizontal ground accuracy of geospatial data

According to the National Map Accuracy Standards

• 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