U.S. National Map Accuracy Standards for Horizontal Accuracy

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: