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

and hilly areas, which corresponds to a root mean square error of 18.5 centimeters
for flat areas and 37.0 centimeters for rolling and hilly areas, respectively. Flat and
hilly are not defined quantitatively in the current guidelines; they are subjective terms
that are to be interpreted during the scoping phase of a flood study. In other words,
the detailed floodplain mapping standards call for elevation data that are about 10
times more accurate than the NED, although existing elevation data coverage in
many areas of the country is of significantly better quality. This means that the
existing NED, and the topographic contour information upon which it is based, are
not adequate to support Flood Map Modernization, except where new high-accuracy
elevation data have been added from state or local sources.
ELEVATION FOR NATION METHOD
Elevation for the Nation implies not simply a new data measurement initiative,
but also a change in the way the nation’s elevation data are archived. In order to
support all forms of subsequent interpretation, all of the measured LIDAR points
should be stored. The points defining the bare-earth elevation are combined with
break lines defining the boundaries of water features to produce a digital terrain
model that is capable of several forms of output representation, including traditional
contours, regularly girded digital elevation models, or a better approach called a
triangulated irregular network (TIN), in which individual points and lines are
combined into a triangular mesh that continuously spans the land surface of an area.
TINs represent sharp land surface features such as road embankments precisely,
and they are the representation of choice for hydraulic analysis of floodplains, which
defines floodwater surface elevations.
SUMMARY OF METHODOLOGY
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