[T]here is inadequate elevation information available to map the shape of the land surface in three dimensions, which is critical in determining the likely direction, velocity, and depth of flood flows, the committee said. In fact, most of the publicly available elevation data is more than 35 years old, with 1970 being the average date of origin in the U.S. Geological Survey's National Elevation Dataset. Land development and urban expansion since then have significantly altered the surface. New road embankments and flood drainage structures also affect expected floodwater depths, as does land subsidence, which is particularly significant in coastal areas.
The committee called for a new elevation mapping program, which it named Elevation for the Nation to parallel the existing Imagery for e Nation concept. The program should employ a technology known as light detection and ranging, or "lidar," to acquire elevation data. Lidar operates by projecting short laser pulses of light from a low-flying aircraft and measuring the time it takes for the light to bounce back from the surface. Lidar is the only technology to produce elevation data that are accurate within one to two feet in most terrain, including the bare-earth terrain beneath vegetation, and that meet FEMA's elevation accuracy requirements. The committee found a striking level of agreement among representatives of several federal agencies that lidar is the current technology of choice for measuring surface elevation.
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