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 |