Below is a brief summary of the standards, followed by the official government-speak jargon. The vertical accuracy standard requires that the elevation of 90 percent of all points tested must be correct within half of the contour interval. The USGS systematically tests the accuracy of its maps. Presently, accuracy testing is performed on 10 percent of the mapping projects at each contour interval as a method of controlling overall quality. It is rare for a 7.5-minute map to fail the test.
Exceptions involve areas covered by dense woodland or obscured by fog or clouds.
In those areas, aerial photographs cannot provide the detail needed for precise mapping. United States National Map Accuracy Standards 1. These limits of accuracy shall apply to positions of well-defined points only. Similarly, features not identifiable upon the ground within close limits are not to be considered as test points within the limits quoted, even though their positions may be scaled closely upon the map. This class would cover timber lines and soil boundaries.