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Summary
Pages 1-9

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From page 1...
... During the annual appropriations hearings for Flood Map Modernization, concerns have been expressed to Congress that the underlying framework data used as input to the flood mapping process is not of adequate quality in much of the nation to properly support the new digital flood map creation. This study was commissioned by the National Academies for the purpose of informing Congress and the nation about this issue.
From page 2...
... The committee believes this base image mapping standard is satisfactory and the nation has adequate image mapping to support Flood Map Modernization through the National Digital orthophoto (http://www.ndop.gov) and National Agriculture Imagery (http://www.fsa.usda.gov/FSA/apfoapp?
From page 3...
... Land surface elevation data for flood management studies of individual streams and rivers have traditionally been derived by land surveying, but the very large areal extent of FEMA floodplain mapping, which covers nearly 1 million miles of the nation's streams and shorelines, means that land surface elevation data for Flood Map Modernization are mostly derived from mapped sources, not from land surveying. Land surface elevation information is combined with data from flood hydrology and hydraulic simulation models, to define the BFE, which is the water surface elevation that would result from a flood having a 1 percent chance of being equaled or exceeded in any year at the mapped location.
From page 4...
... of the approximately 1 million stream miles completed up to June 2005, one-quarter (or 247,000 stream miles) were mapped using detailed studies employing highresolution elevation data, and the resulting flood maps show the BFE of the floodwater surface as well as the spatial extent of the floodplain.
From page 5...
... Some communities undertaking Flood Map Modernization already have available elevation data of the required accuracy or pay to acquire such data as part of their contribution to the costs of floodplain mapping. Apart from exceptional circumstances, FEMA does not pay for the costs of elevation data acquisition in local communities.
From page 6...
... Lidar operates by projecting short laser pulses of light from an aircraft and measuring the time taken for these pulses to bounce back to the aircraft from the land surface. With appropriate processing, 1-foot to 2-foot equivalent contour accuracy can be achieved in the final bare-earth elevation data; this level of accuracy meets or exceeds FEMA elevation criteria for floodplain mapping in all areas.
From page 7...
... TINs represent sharp land surface features such as road embankments precisely, and they are the representation of choice for hydraulic analysis of floodplains, which defines floodwater surface elevations. A number of states and local communities are acquiring new elevation data on their own initiative and for various purposes, but these datasets frequently do not satisfy FEMA guidelines and specifications, for example, satisfying 10-foot equivalent contour accuracy rather than the 2- to 4-foot equivalent contour accuracy required by FEMA.
From page 8...
... 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.
From page 9...
... The committee suggests, for example, that analysis of a selection of updated flood maps could be useful to compare the quantitative effects of using lidar versus using conventional 10-meter or 30-meter NED information derived from USGS topographic maps to provide the elevation data. In a new, two-year study, beginning in early 2007, FEMA has separately requested the National Academies to undertake a distinct evaluation of flood map accuracy, including an examination of the whole range of uncertainty in flood mapping arising from uncertainty in flood hydrology and hydraulic modeling, as well as uncertainty in land surface elevation.


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