Skip to main content

Mapping the Zone Improving Flood Map Accuracy (2009) / Chapter Skim
Currently Skimming:

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

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.


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)
Read the entire page →
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 .
Read the entire page →

Key Terms

  • contour interval
  • water control
  • base flood
  • flood insurance
  • accuracy standards

  • flooding source
  • control structure
  • map contour
  • stream reach
  • surface elevations

  • floodway data
  • cross section
  • water surface
  • contour lines
  • map accuracy

  • horizontal error
  • anational map
  • national map
  • elevations tested
  • geological survey

  • lines method
  • profile reading
  • profile sheet
  • national resources
  • flood elevation

  • contour interpolation
  • gages measure
  • planning commissions
  • study approach
  • topographic maps


    • This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
    More information on Chapter Skim is available.