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Elevation Data for Floodplain Mapping
root mean square errors of these elevation data are 0.61 feet (18.5 centimeters) for flat areas, and 1.22 feet (37.0 centimeters) for rolling or hilly areas. These standards apply to “bare-earth” elevation, that is, the land surface with buildings and vegetation removed. Accurate elevation data are needed for precise depiction of the shape of the land surface in the floodplain to support hydraulic engineering computation of floodwater elevation. Except for some special cases, FEMA does not generally support the cost of new elevation data collection. Some communities and a few states, most notably North Carolina, have undertaken elevation mapping programs that provide data of the required accuracy to meet floodplain mapping standards.
Where locally or regionally collected high-accuracy elevation data are unavailable, floodplain studies rely on data from the U.S. Geological Survey (USGS) topographic maps of 1:24,000 scale. The average root mean square error of the National Elevation Dataset (NED) compared to National Geodetic Survey control points is 7.68 feet (2.34 meters). In other words, FEMA detailed floodplain mapping standards call for elevation data that are about 10 times more accurate than the NED, although existing elevation data coverage in many areas of the country is of significantly better quality.
Of the approximately 1 million stream miles of floodplain mapping completed to date, base flood elevation (BFE) contours showing the expected height of the floodwater surface are shown for one-quarter of the streams, but they are omitted for the remaining three-quarters of the streams, where approximate studies have been done. One of the reasons approximate studies do not contain the computed water surface elevation is because the elevation data used to create the boundaries are not sufficiently accurate. FEMA has a floodplain boundary standard, applied to both detailed and approximate studies, that ensures the boundary line is accurately plotted in relation to the available elevation data, but this standard does not ensure the accuracy of the elevation data themselves.
The determination of whether a building is in the 100-year flood hazard zone or not for flood insurance purposes is determined by intersecting the building footprint outline with the outline of the hazard zone on the floodplain map. If there is any overlap between the two, flood insurance is required if the property has a mortgage that is backed by the federal government. The committee concludes that within the limits of the available elevation data, the updated floodplain maps are adequate for this purpose.
Rational floodplain management and flood damage estimation depend not only on how far the water spreads, but also on how deeply buildings are flooded and with what frequency flooding occurs. If the task of the nation’s flood management is looked at in this larger context, accurate land surface and floodwater surface elevation information is critical. This is so, for example, in the flood damage mitigation projects undertaken by the U.S. Army Corps of Engineers in collaboration with local communities, for which flood damage estimation requires knowing the first floor elevation of all flood-prone buildings.