Maps are often used to influence legislation, promote a cause, or enlighten a particular group before decisions are made. In these cases, especially, map accuracy is of the utmost importance. There are many factors that influence map accuracy: the projection used, scale, base data, generalization, and so forth. The analyst/cartographer must be aware of these factors before map production begins. The accuracy of the map, in a large part, determines its usefulness. It is usually up to individual organizations to perform accuracy assessment and decide how those findings are reflected in the products they produce. However, several agencies have established guidelines for map makers.
US National Map Accuracy Standard
The United States Bureau of the Budget has developed the US National Map Accuracy Standard in an effort to standardize accuracy reporting on maps. These guidelines are summarized below (Fisher, 1991):
- On scales smaller than 1:20,000, not more than 10 percent of points tested should be more than 1/50 inch in horizontal error, where points refer only to points that can be well-defined on the ground.
- On maps with scales larger than 1:20,000, the corresponding error term is 1/30 inch.
- At no more than 10 percent of the elevations tested can contours be in error by more than one half of the contour interval.
- Accuracy should be tested by comparison of actual map data with survey data of higher accuracy (not necessarily with ground truth).
- If maps have been tested and do meet these standards, a statement should be made to that effect in the legend.
- Maps that have been tested but fail to meet the requirements should omit all mention of the standards on the legend.
USGS Land Use and Land Cover Map Guidelines
The USGS has set standards of their own for land use and land cover maps (Fisher, 1991):
- The minimum level of accuracy in identifying land use and land cover categories is 85%.
- The several categories shown should have about the same accuracy.
- Accuracy should be maintained between interpreters and times of sensing.
USDA SCS Soils Maps Guidelines
The United States Department of Agriculture (USDA) has set standards for Soil Conservation Service (SCS) soils maps (Fisher, 1991):
- Up to 25% of the pedons may be of other soil types than those named if they do not present a major hindrance to land management.
- Up to only 10% of pedons may be of other soil types than those named if they do present a major hindrance to land management.
- No single included soil type may occupy more than 10% of the area of the map unit.
Digitized Hardcopy Maps
Another method of expanding the database is by digitizing existing hardcopy maps. Although this may seem like an easy way to gather more information, care must be taken in pursuing this avenue if it is necessary to maintain a particular level of accuracy. If the hardcopy maps that are digitized are outdated, or were not produced using the same accuracy standards that are currently in use, the digitized map may negatively influence the overall accuracy of the database.