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Appendix A: Methods for Estimating Base Flood Elevations in Approximate Studies
Pages 107-108

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From page 107...
... onto a topographic map at the same scale can produce an estimated BFE if the floodplain boundary generally conforms to the contour lines along the flooding source in question Contour interpolation This method is similar to the topographic study approach, but the SFHA crosses contour lines method In riverine areas, the difference between the water surface elevations on opposite banks of a flooding source must be within one-half of the map contour interval to meet national map accuracy standards.a In these cases, the approximate BFE will be equal to the elevation of the lower of the two bank elevations plus one-half the contour interval. This method should be performed at each structure location In lacustrine areas, the difference between the highest and lowest determined water surface elevations around the flooding source must be within one-half of the map contour interval to be acceptable (FEMA, 1995)
From page 108...
... Rather than a Flood Insurance Study 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 .

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