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Elevation Data for Floodplain Mapping
vertical datums across the nation, but this report does highlight land subsidence as an issue important to Flood Map Modernization; (3) mapping technologies other than airborne remote sensing—the committee considered in detail only photogrammetry, light detection and ranging (lidar), and interferometric synthetic aperture radar (IFSAR), which are all aerial mapping technologies, and did not consider land-based mapping technologies; (4) uncertainties in flood hydrology and hydraulics—the committee focused on issues related to base map and elevation data and not on hydrologic and hydraulic sources of uncertainty in flood risk assessment. A previous study examined uncertainty in flood hydrologic and hydraulic computations (NRC, 2000).
Besides the mapping technologies study presented in this report, FEMA has separately engaged the National Academies to undertake a longer-term flood map accuracy study within which the above issues will be addressed more fully.
This report presents the committee’s response to its statement of task. Two aspects of mapping need to be considered in this context—defining land surface reference information and land surface elevation.
BASE MAP INFORMATION
Land surface reference information describes streams, roads, buildings, and administrative boundaries that show the background context for mapping the flood hazard zone. The older paper FIRMs contain only vector data (points, lines, and polygons) to describe all land surface reference features. The new DFIRMs typically use a digital orthophoto as the base map, supplemented by planimetric vector data for key map features (e.g., roads needed for georeferencing building locations) and administrative boundaries (e.g., city or county boundaries) that cannot be observed in photography. An orthophoto is an aerial photograph from which all relief displacement and camera tilt effects have been removed such that the scale of the photograph is uniform and it can be considered equivalent to a map. FEMA’s requirement for an orthophoto base map is that it meet or exceed the U.S. Geological Survey (USGS) Digital Orthophoto Quarter Quadrangle (DOQQ) specification, which calls for a 1-meter pixel resolution, or ground sample distance (GSD), and the meeting of National Map Accuracy Standards at a scale of 1:12,000. The popularity of Google Earth (http://earth.google.com) has introduced into the public mind the idea of “image as base map” and reinforces the importance of regularly updated digital orthophotography covering the nation. The committee believes this base image mapping standard is satisfactory and the nation has adequate image mapping to support Flood Map Modernization through the National Digital Orthophoto (http://www.ndop.gov) and National Agriculture Imagery (http://www.fsa.usda.gov/FSA/apfoapp?area=home&subject=prog&topic=nai/) programs. The committee endorses the proposed Imagery for the Nation program, which seeks to create and maintain 1-meter GSD or better orthophoto products seamlessly across the United States