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Elevation Data for Floodplain Mapping
FEMA also requires that the flood depth at structures be known for detailed study areas when flood insurance is obtained. The flood insurance rate for detailed study areas is based on the height of the first finished floor with respect to the BFE. The committee concludes that rational flood management for the nation requires that the problem be viewed in three dimensions, quantifying flood depth throughout the floodplain, not as a two-dimensional problem of defining the extent of a floodplain boundary on a flat map.
Moreover, it is shown in this report that when the slope of the NED is computed, it has a zero slope in 11 percent of the continental United States and Alaska. These locations of very flat terrain occur primarily along the Gulf coast, in Florida, along the eastern seaboard, and at several places in the interior of the nation. Very flat terrain zones along the coasts are particularly flood-prone because of potential storm surge from the oceans. The committee concludes that elevation data of at least 1-foot equivalent contour accuracy should be acquired in these very flat areas, rather than the 2-foot equivalent contour accuracy data that the FEMA floodplain mapping standards presently require for flat areas.
FEMA floodplain mapping standards require elevation data preferably measured during the last seven years to account for the effects of land development on flood elevations. The nation’s existing elevation data derived from topographic maps are, on average, more than 35 years old.
Based on these considerations, the committee concludes that the nation’s land surface elevation data need to be modernized and mapped more accurately to properly support FEMA Map Modernization and the nation’s flood mapping and management needs.
AVAILABLE MAPPING TECHNOLOGIES
The committee examined three technologies for supplying elevation information: photogrammetry, light detection and ranging (lidar), and interferometric synthetic aperture radar (IFSAR).
Photogrammetry is a technique by which sequences of overlapping vertical aerial photographs taken from an aircraft are interpreted by automated and/or manual means to produce orthoimagery, elevation data in a variety of forms, and/or planimetric information (e.g., the location of building footprints, road centerlines, stream centerlines). Some information about the land surface elevation is routinely obtained during the orthophoto production process but is not sufficient to create a high-quality digital elevation model. If elevation data meeting FEMA’s specifications are required, obtaining these data using photogrammetry is a labor-intensive, time-consuming, and therefore, relatively expensive process. It is particularly difficult to view the earth below the canopy in forested areas, and photogrammetry requires that the same point on the ground point be viewed in two photographs taken from different angles so that its elevation can be computed correctly. The