of flood hydraulics, and in the transformation of the modeled water surface elevation onto a map. This report deals only with the influence of framework information on determination of the floodplain; a forthcoming National Research Council study on flood map accuracy will examine the totality of these uncertainties in much greater detail.

From the viewpoint of hydrology and hydraulics, the concern with precise framework information is for the flood hazard zone in and around a stream. However, when constructing a floodplain map for a county, it is impractical to construct framework information for just those locations—it is more practical to prepare maps with base and elevation data for the entire county or by complete drainage basins.

FEMA Map Modernization seeks to create a floodplain map such as that shown for Bexar County in most counties in the United States, as shown in Figure 2.5. This involves flood modeling for several million stream reaches, and logistical considerations require the use of automated hydrologic and hydraulic methods as described in Chapter 3. As a result, FEMA Map Modernization relies heavily on nationally available framework data sources. The task of this report is to examine whether these data sources are of adequate quality to support this effort.

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