New digital, high-resolution land elevation maps created using a proven remote-sensing laser technology are needed to support the Federal Emergency Management Agency's effort to modernize the nation's floodplain maps, says a new National Research Council report requested by Congress. The floodplain 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.
FEMA has been partnering with state and local governments in a $200 million-per-year modernization effort to replace paper floodplain maps with digital ones. FEMA sets accuracy requirements for the maps, but it is generally up to state and local governments to provide the data upon which the maps are based. Congress requested the report because of concerns that underlying base map information currently available for much of the nation is not adequate to support the new digital maps.
The committee that wrote the report focused on two layers of floodplain maps: base map imagery and base map elevation. It concluded that there is sufficient two-dimensional imagery available from digital "orthophotos" -- aerial and satellite photographs -- to meet FEMA's standards for mapping landmarks such as streams, roads, and buildings that show the context necessary for mapping flood hazard areas. The committee also endorsed a program known as Imagery for the Nation, a joint federal-state effort to keep orthophoto databases current.
However, there 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.
In addition, FEMA requires that elevation data in the new digital maps be about 10 times more accurate than most existing data in the National Elevation Dataset, which are used by many states and local communities partnering with FEMA to produce new flood maps. By last summer, digital floodplain maps had been prepared for about 1 million miles of the nation's 4.2 million miles of rivers and streams, but only 247,000 miles had been mapped using high-resolution elevation data.
The committee called for a new elevation mapping program, which it named Elevation for the Nation to parallel the existing Imagery for the 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.
Elevation for the Nation's first focus should remain on that part of the nation where flood risk to the population justifies collecting new data, the committee said. Existing local and regional data may be used if they are sufficiently accurate and complete, the report adds.
The committee emphasized that a seamless nationwide elevation dataset would have many applications beyond FEMA's flood insurance maps, although it acknowledged that the cost of creating such a dataset will be significant. Data collected in Elevation for the Nation should be disseminated to the public as part of an updated National Elevation Dataset.
The report was completed in a short time so that Congress could consider its findings and recommendations during the upcoming appropriations process. Meanwhile, FEMA has requested a separate, more comprehensive Research Council study of flood map accuracy, which is expected to take two years to complete.
"The advent of lidar technology represents a major step forward in the effort to improve the accuracy of the nation's flood maps," said Robert Detlefsen, NAMIC's vice president of public policy. "The challenge now is for policymakers to use the knowledge generated by this new technology to enact meaningful reforms to the National Flood Insurance Program, as well as to consider appropriate land use and conservation measures."
The report was sponsored by the National Academies, which is made up of the National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council. They are private, nonprofit institutions that provide science, technology, and health policy advice under a congressional charter. The National Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. A committee roster follows.
Source: The National Academies news release, NAMIC
Posted: Friday, February 02, 2007 12:00:00 AM. Modified: Friday, February 02, 2007 2:20:26 PM.
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