5.4 Floodplain Mapping: How Accurate Should It Be?

Mapping requirements vary based on the use of the map. The accuracy of the map is determined by comparing the vertical and horizontal distances (tolerances) between objects on the map to the actual distances between the same two objects measured in the field (USACE, 1994).

For general flood control project planning, floodplain mapping, and flood control studies, map scales typically range from 1 in. = 400 ft to 1 in. = 1000 ft, with a feature (horizontal) tolerance ranging from 20-100 ft. The elevation tolerance may be from 0.2-2 ft with contour intervals ranging from 2-5 ft.

For flood insurance studies, the normal map scale is 1 in. = 400 ft with a feature location tolerance of 20 ft. The elevation tolerance is 0.5 ft with a normal contour interval of 4 ft.

For the actual siting of structures, such as navigation locks or storage reservoirs, map scales may range from 1 in. = 20-50 ft with a feature location tolerance of 0.05-1 ft. Vertical tolerances of 0.01-0.05 ft. with contour intervals of 0.5-1 ft are typical.