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Elevation Data for Floodplain Mapping CHAPTER SIX Conclusions and Recommendations Floodplain mapping involves determining amount of flooding (hydrology), the height of flooding (hydraulics), and the land areas impacted by flooding. A floodplain or flood hazard map has two key inputs: (1) imagery and/or cartographic line work to provide land surface reference information (base map) and (2) a digital elevation model representing the earth’s surface or “terrain.” Responding to concerns expressed by the Congress, the National Academy of Sciences established this committee to respond to the following statement of task: Identify the current mapping technologies being used by the Federal Emergency Management Agency (FEMA) to develop flood hazard maps; Identify mapping technologies that are currently available; and Determine if newer technologies are appropriate and would be of additional benefit to floodplain mapping. 6.1 ADEQUACY OF BASE MAP AND ELEVATION INFORMATION Land surface reference information is used to identify the spatial relationship between the mapped floodplain and features such as roads, buildings, and administrative boundaries. This reference information in traditional Flood Insurance Rate Maps was supplied by conventional vector point, line, and area data layers, but in the more modern Digital Flood Insurance Rate Maps, the principal means for land surface reference has become digital orthophotos supplemented by some geographic information system (GIS) vector data layers for key features. The committee concludes that the nation’s existing digital imagery programs, supplemented by local cartographic data, are adequate to provide land surface reference information required for base maps in FEMA floodplain mapping. Land surface elevation data for flood management studies of individual streams and rivers has traditionally been derived by land surveying, but the very large areal extent of FEMA floodplain mapping, which covers nearly 1 million miles of the nation’s streams and shorelines, means that land surface elevation data for Flood Map Modernization are mostly derived from mapped sources, not from land surveying. FEMA floodplain mapping standards for detailed study areas call for elevation data of 2-foot equivalent contour accuracy in flat areas and 4-foot equivalent contour accuracy in rolling or hilly areas, but FEMA does not have a defined standard for approximate study areas. The corresponding
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Elevation Data for Floodplain Mapping root mean square errors of these elevation data are 0.61 feet (18.5 centimeters) for flat areas, and 1.22 feet (37.0 centimeters) for rolling or hilly areas. These standards apply to “bare-earth” elevation, that is, the land surface with buildings and vegetation removed. Accurate elevation data are needed for precise depiction of the shape of the land surface in the floodplain to support hydraulic engineering computation of floodwater elevation. Except for some special cases, FEMA does not generally support the cost of new elevation data collection. Some communities and a few states, most notably North Carolina, have undertaken elevation mapping programs that provide data of the required accuracy to meet floodplain mapping standards. Where locally or regionally collected high-accuracy elevation data are unavailable, floodplain studies rely on data from the U.S. Geological Survey (USGS) topographic maps of 1:24,000 scale. The average root mean square error of the National Elevation Dataset (NED) compared to National Geodetic Survey control points is 7.68 feet (2.34 meters). In other words, FEMA detailed floodplain mapping standards call for elevation data that are about 10 times more accurate than the NED, although existing elevation data coverage in many areas of the country is of significantly better quality. Of the approximately 1 million stream miles of floodplain mapping completed to date, base flood elevation (BFE) contours showing the expected height of the floodwater surface are shown for one-quarter of the streams, but they are omitted for the remaining three-quarters of the streams, where approximate studies have been done. One of the reasons approximate studies do not contain the computed water surface elevation is because the elevation data used to create the boundaries are not sufficiently accurate. FEMA has a floodplain boundary standard, applied to both detailed and approximate studies, that ensures the boundary line is accurately plotted in relation to the available elevation data, but this standard does not ensure the accuracy of the elevation data themselves. The determination of whether a building is in the 100-year flood hazard zone or not for flood insurance purposes is determined by intersecting the building footprint outline with the outline of the hazard zone on the floodplain map. If there is any overlap between the two, flood insurance is required if the property has a mortgage that is backed by the federal government. The committee concludes that within the limits of the available elevation data, the updated floodplain maps are adequate for this purpose. Rational floodplain management and flood damage estimation depend not only on how far the water spreads, but also on how deeply buildings are flooded and with what frequency flooding occurs. If the task of the nation’s flood management is looked at in this larger context, accurate land surface and floodwater surface elevation information is critical. This is so, for example, in the flood damage mitigation projects undertaken by the U.S. Army Corps of Engineers in collaboration with local communities, for which flood damage estimation requires knowing the first floor elevation of all flood-prone buildings.
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Elevation Data for Floodplain Mapping FEMA also requires that the flood depth at structures be known for detailed study areas when flood insurance is obtained. The flood insurance rate for detailed study areas is based on the height of the first finished floor with respect to the BFE. The committee concludes that rational flood management for the nation requires that the problem be viewed in three dimensions, quantifying flood depth throughout the floodplain, not as a two-dimensional problem of defining the extent of a floodplain boundary on a flat map. Moreover, it is shown in this report that when the slope of the NED is computed, it has a zero slope in 11 percent of the continental United States and Alaska. These locations of very flat terrain occur primarily along the Gulf coast, in Florida, along the eastern seaboard, and at several places in the interior of the nation. Very flat terrain zones along the coasts are particularly flood-prone because of potential storm surge from the oceans. The committee concludes that elevation data of at least 1-foot equivalent contour accuracy should be acquired in these very flat areas, rather than the 2-foot equivalent contour accuracy data that the FEMA floodplain mapping standards presently require for flat areas. FEMA floodplain mapping standards require elevation data preferably measured during the last seven years to account for the effects of land development on flood elevations. The nation’s existing elevation data derived from topographic maps are, on average, more than 35 years old. Based on these considerations, the committee concludes that the nation’s land surface elevation data need to be modernized and mapped more accurately to properly support FEMA Map Modernization and the nation’s flood mapping and management needs. 6.2 AVAILABLE MAPPING TECHNOLOGIES The committee examined three technologies for supplying elevation information: photogrammetry, light detection and ranging (lidar), and interferometric synthetic aperture radar (IFSAR). Photogrammetry is a technique by which sequences of overlapping vertical aerial photographs taken from an aircraft are interpreted by automated and/or manual means to produce orthoimagery, elevation data in a variety of forms, and/or planimetric information (e.g., the location of building footprints, road centerlines, stream centerlines). Some information about the land surface elevation is routinely obtained during the orthophoto production process but is not sufficient to create a high-quality digital elevation model. If elevation data meeting FEMA’s specifications are required, obtaining these data using photogrammetry is a labor-intensive, time-consuming, and therefore, relatively expensive process. It is particularly difficult to view the earth below the canopy in forested areas, and photogrammetry requires that the same point on the ground point be viewed in two photographs taken from different angles so that its elevation can be computed correctly. The
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Elevation Data for Floodplain Mapping committee concludes that at the national scale of the FEMA Map Modernization program, photogrammetry is the best technique for acquiring the orthoimagery needed for land surface reference information in floodplain base maps, but it is not the most appropriate technique to be used for acquiring bare-earth elevation information. Lidar is a technique by which a laser system onboard an aircraft or spacecraft transmits laser pulses toward the terrain at rates up to hundreds of thousands per second. The laser energy interacts with the terrain and some of the energy is reflected back toward the aircraft receiver. By knowing the position and orientation of the aircraft and lidar instrument, and the range to the ground, the elevation of the ground surface can be determined accurately. A typical lidar mission collects millions to billions of elevation measurements. Lidar has the advantage in mapping ground covered by vegetation that some pulses reflect off the vegetation while others penetrate holes in the canopy to reach the ground surface. It is important to note that only a single lidar pulse through a hole in the canopy is required to obtain an accurate ground surface elevation measurement. Such a large number of pulses are emitted overall that a sufficient number reach the ground surface to identify its elevation separately from the overlying vegetation. The committee concludes that at the scale of the Flood Map Modernization program, lidar is the most cost-effective technology to acquire elevation information over large regions to support floodplain mapping to FEMA accuracy standards. IFSAR is a remote sensing technique that makes use of radio detection and ranging (radar) technology whereby pulses of microwave electromagnetic energy are transmitted from an aircraft or spacecraft toward the ground. The transmitted energy interacts with the terrain and a portion is scattered back toward the aircraft (referred to as backscatter). IFSAR remote sensing systems typically use two antennas that operate at the same time onboard the aircraft or spacecraft. The backscattered energy can be processed to create a three-dimensional map of the surface. IFSAR is nearly as accurate when flown at high altitude as at low altitude, and radar penetrates cloud cover, so large areas can be mapped relatively quickly and inexpensively. Indeed, the Shuttle Radar Topography Mission (SRTM), operated by the National Aeronautics and Space Administration in 2000, resulted in a significantly improved map of global topography. Depending on the wavelength of the radar used, IFSAR can either be reflected off vegetation or partially penetrate it, but the problem of accurately detecting the bare-earth elevation distinct from overlying vegetation cannot be solved with IFSAR nearly as accurately as with lidar. FEMA has sponsored several evaluations of IFSAR data and found them unsuitable due to the effects of vegetation in nearly all cases, particularly for heavily vegetated riparian areas along stream banks where bare-earth elevation data are critical for hydraulic modeling. The committee concludes that IFSAR may have utility for elevation mapping in low-risk, rugged, barren areas of the western United States and in Alaska where perpetual cloud cover limits the application of lidar.
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Elevation Data for Floodplain Mapping 6.3 ELEVATION FOR THE NATION The committee concludes that the nation’s information for land surface elevation is inadequate to support FEMA’s Map Modernization and that new national digital elevation data collection is required. The committee proposes that this program be called Elevation for the Nation to parallel the existing Imagery for the Nation concept. The committee recommends the following: Elevation for the Nation should employ lidar as the primary technology for digital elevation data acquisition. Lidar is capable of producing a bare-earth elevation model with 2-foot equivalent contour accuracy in most terrain and land cover types; a 4-foot equivalent contour accuracy is more cost-effective in mountainous terrain, and a 1-foot equivalent contour accuracy can be achieved in very flat coastal or inland floodplains. A seamless nationwide elevation database created at these accuracies would meet FEMA’s published requirements for floodplain mapping for the nation. The first focus of this program should be on remapping the elevation of the 65 percent of the nation that contains 92 percent of its population, where flood risk justifies the required data collection. The program can use newly acquired data or existing local and regional data if the existing data are reasonably up-to-date. A seamless nationwide elevation model has application beyond the FEMA Map Modernization program; many local and state governments are acquiring lidar data at these accuracies or better. For example, in 2007, the Florida Division of Emergency Management will be acquiring lidar data satisfying 1-foot equivalent contour accuracy of shorelines for storm surge modeling and hurricane evacuation planning. As part of Elevation for the Nation, federal, state, and local mapping partners should have the option to request data that exceed minimum specifications if they pay the additional cost of data collection and processing required to achieve higher accuracies. The new data collected in Elevation for the Nation should be disseminated to the public as part of an updated National Elevation Dataset. The Elevation for the Nation database should contain the original lidar mass points and edited bare-earth surface, as well as any breaklines required to define essential linear features. In addition to the elements proposed for the national database, secondary products including triangulated irregular networks, hydrologically corrected digital elevation models, and hydrologically corrected stream networks and shorelines should be created to support FEMA floodplain mapping. Standards and interchange formats for these secondary products do not currently exist and should be developed. Comprehensive standards for lidar data collection and processing are also needed.
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Elevation Data for Floodplain Mapping Professional societies and federal agency consortia are appropriate entities to lead development of these standards; funding to support these efforts should be considered as part of a nationwide effort. The committee reached its conclusion that Elevation for the Nation is needed for two main reasons: first, for the nation as a whole the existing elevation data are so old, and the gap between their accuracy and the accuracy required for floodplain mapping is so great, that the need for new elevation data is clear; and second, the required elevation mapping technology exists and has been commercially deployed such that implementing Elevation for the Nation is technically feasible. Regardless of whether “best-available” elevation data are used or new elevation data are acquired for a flood study, informed judgments must be made about the appropriateness of these datasets and their influence on flood data computations. The committee recognizes that Elevation for the Nation will involve significant expense, perhaps as much as the existing Flood Map Modernization program. It is for Congress and others to determine whether this expense is justified in the context of national spending priorities. Certainly the data arising from Elevation for the Nation will have many beneficial uses beyond floodplain mapping and management. The current study was conducted in a short time to address very specific questions about the mapping technologies used to produce floodplain maps. As such, the committee did not have the resources or scope to examine in detail many important issues related to flood map accuracy. The committee suggests, for example, that analysis of a selection of updated flood maps could be useful to compare the quantitative effects of using lidar versus using conventional 10-meter or 30-meter NED information derived from USGS topographic maps to provide the elevation data. In a new, two-year study, beginning in early 2007, FEMA has separately requested the National Academies to undertake a distinct evaluation of flood map accuracy, including an examination of the whole range of uncertainty in flood mapping arising from uncertainty in flood hydrology and hydraulic modeling, as well as uncertainty in land surface elevation. The committee hopes that the present report provides solid input to the upcoming study and helps to further objective examination of the most cost-effective methods needed to support the nation’s floodplain mapping and management.
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