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Elevation Data for Floodplain Mapping
Lidar is the most robust and cost-effective technology to address FEMA’s needs for elevation data to support floodplain mapping. However, sources of error and the statistical modeling of these errors are not known with the same degree of rigor as they are for photogrammetry, which places a greater burden on the purchaser of lidar data to test and validate deliverables to gain confidence that project specifications have been met. Professional organizations with elevation mapping and surveying experience should be called on to assist public agencies such as FEMA, the USGS, NOAA, states or local entities, and academia to define guidelines for professional practice and mapping specifications for application domains, such as floodplain mapping. Specifications should address FEMA’s requirements for 4-foot equivalent contour accuracy in complex, hilly terrain and 2-foot equivalent contour accuracy elsewhere, as well as the 1-foot equivalent contour accuracy in vulnerable coastal or very flat inland floodplains as recommended for the nation by this committee.
IFSAR is not capable of achieving FEMA’s accuracy requirements for bare-earth elevation data in dense urban areas and dense vegetation encountered in many areas of interest to the floodplain mapping program. Exceptions to the general statement on the unsuitability of IFSAR data for floodplain mapping are (1) the State of Alaska, where perpetual cloud cover, extreme terrain, inaccessibility of services, and hazards to small, low-flying aircraft pose serious limitations to both photogrammetry and lidar; and (2) some rugged, barren areas of the western United States.
5.4.4 National Orthoimagery and Elevation Data Initiatives
The USGS has established liaisons with state and federal agencies to foster data-sharing partnerships. A consortium of federal and state agencies, led by the USGS, comprises NDEP, whose mission is to establish digital elevation standards and requirements. The governance infrastructure for national elevation data, led by NDEP and currently implemented as the NED, is analogous to the NDOP model for digital orthophoto imagery. The USDA is a major stakeholder in NDOP because of its use of imagery to administer farm and conservation programs. NDOP generally follows the guiding principles set forth by the Imagery for the Nation concept. A parallel governance model for elevation data could be created that (1) embraces FEMA as a major stakeholder due to its need for current, accurate data to support the NFIP; (2) uses NDEP to drive development of standards and specifications; and (3) follows an Elevation for the Nation concept.