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Elevation Data for Floodplain Mapping
the framework of requirements for the FEMA Map Modernization process. Chapter 6 summarizes the committee’s conclusions and recommendations. Because of the technical nature of some of this report, the reader is referred to an extensive glossary and list of acronyms in Appendixes C and D, respectively.
FEMA creates floodplain maps for riverine and coastal flooding using similar framework data but different methods for modeling flood inundation. Since the nation possesses more than 4.2 million miles of streams and rivers, but only about 60,000 miles of coastlines, the main focus in this report is on riverine flooding. Coastal flooding will be considered in more detail in the forthcoming National Resource Council study on flood map accuracy. Particular issues of concern with coastlines are the effects of land subsidence, discussed in Chapter 3, and the very flat slope of many coastal zones, discussed in Chapter 5.
The committee notes that the report uses a mixture of U.S. and metric units because that is the practice in this field of study. For example, topographic maps typically have contour intervals measured in feet, but the aerial mapping companies preparing the elevation data underlying these maps usually specify the accuracy of these data in centimeters or meters. Where important, measurements in both systems of units are given.
LIMITATIONS OF THIS STUDY
This study was conducted in a short period of time and the committee held one public meeting, described in Appendix B. Limitations of time, and the narrow focus of the statement of task, meant that this study did not focus in detail on the following issues:
Coastal flooding—this involves a different methodology than riverine flooding and since the nation has 60,000 miles of coastlines and about 4.2 million miles of rivers and streams, the committee focused on riverine flooding because that makes up the bulk of Flood Map Modernization. This study has highlighted, however, the very flat slope of the coastal areas of the Gulf of Mexico and the eastern seaboard, which require particularly precise elevation information.
Geodetic control—the actual elevation of a point on the land surface is defined using its height above the geoid, which is a surface of constant gravitational potential approximating mean sea level and defined over the nation. The National Geodetic Survey is conducting a Height Modernization program (http://www.ngs.noaa.gov/heightmod/) to facilitate direct use of Global Positioning System-derived elevations and to revise vertical datums used for elevation mapping. The committee did not consider variations in the precision of definition of the survey control points or vertical datums across the nation, but this report does highlight land subsidence as an issue important to Flood Map Modernization.
Mapping technologies other than airborne remote sensing—the committee