Method

Comments

Flood study

Agencies other than FEMA may have elevation information that may not appear on the FIRM or the Flood Insurance Study (FIS) report. These include

 

  • Federal sources of floodplain studies, technical information, and design manuals (e.g., Army Corps of Engineers, Tennessee Valley Authority, U.S. Geological Survey, National Resources Conservation Service, Federal Highway Administration)

  • State agencies (e.g., environmental agencies, departments of transportation, state geological surveys, state floodplain management agencies)

  • Local or regional agencies (e.g., river basin commissions, flood control districts, local and county planning commissions, public works departments, utility companies and agencies, dam commissions)

Preliminary Flood Insurance Study

Communities have discretion in using data from studies and maps that are in progress and have not yet been given final approval or adopted and published. The information from draft or preliminary studies may be the “best-available” data in areas with only approximate A zones

Profiles from a Flood Insurance Study

This involves comparison of the location of the site on the FIRM to cross-section lines, and then utilizing that relationship to read a BFE on the appropriate profile sheet included in the FIS report

Floodway data tables from a Flood Insurance Study

The tables identify the BFE with and without the computed floodway at each cross section for a stream reach. Rather than reading the profiles, the floodway data table provides the BFE at the cross section, eliminating interpolation or profile reading errors

FIRM

While the FIRMs may indicate BFEs, they are graphical depictions of the observations and computations reported in the FIS report and are not as accurate or precise as information within the report. Aside from graphical approximations or errors in transferring information from the report to the map, BFEs on FIRMs are shown to whole feet, while information within the FIS report is shown to one-tenth of a foot, a big difference

aNational map accuracy standards are available at <http://rockyweb.cr.usgs.gov/nmpstds/nmas.html>. “Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error more than one-half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale.”



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