Inundaciones

Elevation Data for Floodplain Mapping
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). Under a funded mandate from Congress, FEMA is modernizing floodplain maps to better serve the program. To do so, however, FEMA needs land surface elevation datathat are about ten times more accurate than data currently available for most of the nation. New, high-accuracy digital elevation data should 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 otherapplications. he creation of floodplain maps is a fundamental part of FEMA’s National Flood Insurance Program. Floodplain maps define flood hazard zones and determine whether flood insurance is required for homes or buildings located near streams and rivers. Approximately $650 billion in insured assets are now covered under the program. FEMA, in collaboration with state and local partners is undertaking anambitious five-year program to replace existing paper Flood Insurance Rate Maps (FIRMs) with more accurate Digital Flood Insurance Rate Maps (DFIRMs). The effort is supported with Congressionally appropriated funds of more than $200 million per year. However, some concerns have been raised to Congress as to the adequacy of the map information available to support this task. This report focuses onthe adequacy of the two framework layers of floodplain maps: the “base map imagery” and the “elevation data.” 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, elevation data are not adequate. Most stateand local entities rely on the National Elevation Dataset that is maintained by the U.S. Geological Survey (USGS). Much of that dataset is more than 35 years old (and therefore out of

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Flood risks affect much of the nation. FEMA calculates flood risks by looking at 10 parameters including population growth, population density, housing rates, and declared disasters. Purple areas are at highestrisk followed by red, orange, yellow, and green at lowest risk. Source: FEMA

The three components of Digital Flood Insurance Rate Maps. Digital Flood Insurance Rate Maps comprise: 1) base map imagery, a digital orthophoto, that provides land surface information such as streams, roads, vegetation, and buildings; 2) base elevation data draped on top of the digital orthophoto to provide3-dimensional information, and 3) engineering analysis such as data and models of stream and river flows. The first two components are addressed in this report.

date due to natural and human-induced changes in the land surface over time), and it was not collected at the level of accuracy now needed for the flood insurance program. The report also reviews emerging technologies being used to generateelevation data to determine if they could or should be used to support flood map modernization. The report concludes that lidar technology, which is based on laser measurements collected from aircraft, should be used to update the nation’s base elevation data.

Two-dimensional “Base Map Imagery” Adequate to Meet FEMA Goals
Floodplain maps start with base map imagery that provide information aboutthe land surface such as location of streams, roads, and buildings. The base map imagery being used as part of floodplain mapping has taken a giant step forward in recent years with the use digital orthophotos to replace older paper maps containing only vector data (points, lines, and polygons). The report finds that FEMA’s base map image specification, which defines the required accuracy of...