High Resolution 1:10,000 scale Mapping Strategy of Multi ... - NDMA

elevation that would result from a flood having a 1 percent chance of being equaled
or exceeded in any year at the mapped location. A floodplain map is created by
tracing the extent of inundation of the landscape by water at the BFE.
Use of the maps to regulate land development in floodplains by local
communities typically requires the first floor elevation of a building to be at or above
the BFE if that building is to be constructed within the floodplain. The governing
criterion used is thus often stated as: Is the first floor elevation above the BFE? In
some communities, a safety margin such as 1 foot of elevation is added to the BFE
in order to take into account allowable encroachments into the floodplain that may
raise the water surface elevation by 1 foot. This criterion, based on vertical rather
than horizontal criteria, is better than that used in flood insurance determinations.
Rational floodplain management and flood damage estimation depend not
only on how far the water spreads, but also on how deeply buildings are flooded and
with what frequency. If the task of the nation’s flood management is observed in this
larger context, accurate land surface and floodwater surface elevation information
are critical. For example, in the flood damage mitigation projects undertaken in
collaboration with local communities, flood damage estimation requires knowing the
first floor elevation of all flood-prone buildings. The rational flood management for the
nation requires that the problem be viewed in three dimensions, quantifying flood
depth throughout the floodplain, not as a two-dimensional problem of defining the
extent of a flood plain boundary on a flat map.
ELEVATION DATA
The elevation data of 0.5 m equivalent contour accuracy by National Map
Accuracy Standards in flat areas and 1.0 m equivalent contour accuracy in rolling
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