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Elevation Data for Floodplain Mapping
in a GIS to prepare the National Flood Risk analysis. Such risk assessment helps ensure that those geographic areas with the greatest population at flood risk are mapped first.
The committee found orthoimagery to be one of the most useful and important components of the FEMA DFIRM. Orthoimagery to be used as a FEMA DFIRM base map may be obtained by using passive or active remote sensing systems. Passive systems using aerial photography can be used to generate multiple products, including orthophotos, planimetric maps, and digital elevation models. While the creation of true orthoimagery from traditional digital orthoimagery is ideal, traditional digital orthoimagery derived from aerial photography is sufficient for most FEMA raster base maps. In contrast to passive systems, active orthoimagery systems can be used in conditions with cloud cover. However, radar orthoimagery can contain serious geometric errors that are not found in traditional optical orthoimagery, and the committee recommends that radar orthoimagery for FEMA base mapping applications be collected only when obtaining traditional passive optical orthoimagery is not possible.
Several sources of orthophotography maintained at different federal agencies can be used for FEMA floodplain base mapping. The committee endorses the Imagery for the Nation concept whereby aerial photography for the nation is updated on a predictable, systematic basis. In general, the committee concludes that existing orthoimagery and associated vector mapping data being employed in FEMA base mapping are of acceptable accuracy.
5.4.3 Digital Elevation Technologies
FEMA’s DFIRM specifications call for elevation data of 2-foot equivalent contour accuracy in flat areas and 4-foot equivalent contour accuracy in hilly areas, with elevation mapped during the last 7 years is preferred. The committee’s study of available elevation data has shown that the average age of the USGS topographic map sheets is 35 years and their equivalent contour accuracy does not meet FEMA flood mapping standards. A new initiative of elevation data for the nation is needed. The committee evaluated three operational technologies—photogrammetry, lidar, and IFSAR—to serve FEMA’s need for elevation data to support its mission of floodplain mapping.
Photogrammetry is a mature technology capable of meeting FEMA’s accuracy requirements for elevation data in most types of terrain and vegetation. However, extraction of bare-earth elevation models is still a very labor intensive, time-consuming, and therefore, relatively expensive process. Thus, photogrammetry alone is not cost or time effective enough to support the current demand for accurate, up-to-date elevation to support the FEMA floodplain mapping mission.