Elevation Highlighted in New NRC Study
Adapted from NRC website
Base Map Inputs for Floodplain Mapping is the title of a new study about to be released by the National Research Council. The committee focuses on good elevation data and calls for a new Elevation for the Nation program that would meet that need.
The study reaches the following conclusions and recommendations:
1) Elevation for the Nation should employ LIDAR as the primary technology for digital elevation data acquisition. LIDAR is capable of producing a bare-earth elevation model with 2-foot equivalent contour accuracy in most terrain and land cover types; a 4-foot equivalent contour accuracy is more cost-effective in mountainous terrain, and a 1-foot equivalent contour accuracy can be achieved in very flat coastal or inland floodplains.
2) A seamless nationwide elevation model has application beyond the FEMA Modernization program.
3) The new data collected in Elevation for the Nation should be disseminated to the public as part of an updated National Elevation Dataset (http://ned.usgs.gov/).
4) The Elevation for the Nation database should contain the original LIDAR mass points and edited bare-earth surface, as well as any breaklines required to define essential linear features.
5) In addition to the elements proposed for the national database, secondary products including triangulated irregular networks, hydrologically corrected digital elevation models, and hydrologically corrected stream networks and shorelines should be created to support FEMA floodplain mapping. Standards and interchange formats for these secondary products do not currently exist and should be developed.
An early version of that report is available via a link at http://dels.nas.edu/besr/reports.php. Readers may read the report online or order a bound copy.
Floodplain maps serve as the basis for determining whether homes or buildings require flood insurance under the National Flood Insurance Program run by the Federal Emergency Management Agency (FEMA). Approximately $650 billion in insured assets are now covered under the program. Under a funded mandate from Congress, FEMA is modernizing floodplain maps to better serve the program. However, concerns have been raised to Congress as to the adequacy of the "base map" information available to support floodplain map modernization. The National Research Council initiated this study to advise Congress and the nation on this issue. The report concludes that there is sufficient two-dimensional "base map imagery" available from digital orthophotos (aerial and satellite photographs similar to those viewed on Google Earth) to meet FEMA’s flood map modernization goals. However, the three-dimensional "base elevation data" that are needed to determine whether a building should have flood insurance are not adequate. FEMA needs land surface elevation data that are about ten times more accurate than data currently available for most of the nation. The report recommends that new, high-accuracy digital elevation data be collected nationwide using laser measurements from aircraft (LIDAR technology). The new data should be input into the National Elevation Dataset that the U.S. Geological Survey maintains for use in support of flood map modernization and other applications.