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Report calls for updating flood maps

WASHINGTON -- Maps being used to calculate flood danger in the United States rely on data that are decades out of date, according to a report that calls for a new national program to remap land levels.

The National Research Council yesterday proposed an "Elevation for the Nation" program to produce up-to-date data.

The most immediate need is for the Federal Emergency Management Agency's flood-mapping program. Those maps are used by mortgage companies and FEMA's National Flood Insurance Program to determine whether property owners should be required to purchase flood insurance.

The report notes that FEMA has been working with state and local governments in a $200-million-a-year effort to replace paper flood plain maps with digital ones.

The new report was requested by Congress so that it could consider the recommendations in upcoming appropriations decisions. FEMA has requested a more comprehensive study of flood map accuracy.

While two-dimensional map data are available and accurate, most of the elevation information dates to 1970s data from the US Geological Survey.

Since then, there have been significant changes, including subsidence in coastal areas, urban expansion, and land development. Such changes affect floodwater movement and depth.

The National Research Council panel recommended the use of lidar to develop the new elevations. Lidar sends short light pulses from an aircraft and measures the time it takes them to bounce back, allowing it to calculate variations in ground level.