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Elevation Data for Floodplain Mapping
summarizes FEMA’s five-year plan through 2010 for providing digital flood hazard data and maps for areas with flood risk.
The committee makes the following observations relative to FEMA’s Map Modernization process:
The committee assumes that the information contained in FEMA DFIRMs is of significant value and is based on sound logic, especially the floodplain zone delineations (e.g., 1% Zone, 0.2% Zone), Base Flood Elevation (BFE), and floodway areal extent.
Organization of the floodplain maps by “stream or coastal miles” is superior to organizing the FEMA DFIRM inventory according to typical “map sheets or panels.” This allows FEMA to monitor carefully how much of the approximately 4.2 million miles of streams and coastline have undergone FEMA modernization.
The decision not to digitize old paper FIRMS is correct. Digitization of paper FIRMs perpetuates historical error and does not generate a floodplain boundary consistent with best available elevation mapping or data.
5.1.2 FEMA Risk Determination and Mapping Prioritization
The committee concurs with FEMA’s desire to improve its method of flood risk determination and mapping prioritization. FEMA uses 10 logical geospatial risk factors analyzed in a GIS to prepare the National Flood Risk analysis. The system is based on Census block group information. The new risk assessment methods now correctly take into account population density and anticipated development in each county. Such risk assessment helps ensure that those geographic areas with the greatest population at flood risk are mapped first.
5.1.3 Best Practices and Processes
FEMA has provided product specifications, but a definition of best practices and processes required to achieve these specifications nationwide, resulting in accurate seamless elevation databases, would also be useful. The committee recommends that FEMA rely on not-for-profit organizations such as the American Society for Photogrammetry and Remote Sensing (ASPRS), the American Geophysical Union (AGU), and the American Congress on Surveying and Mapping (ACSM) to specify the best practices and processes.
5.1.4 FEMA’s Use of Elevation Data
The principal factor impacting the reliability of the floodplain boundary delineation is the quality of the input digital elevation information. The committee agrees that elevation