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Flood Mapping Accuracy: USGS DEM Elevation Data vs. LiDAR Elevation Data
Following is a brief discussion of the comparison of two data sources that can be used for defining the flood inundation effects: USGS DEM (Digitatal Elevation Model) elevation data and; LiDAR (Light Detection And Ranging) elevation data.
If LiDAR data is available for an area it will yield much more accurate results that USGS DEM data if processes properly. The main problem is that at this time (October 2012) the areas covered by LiDAR are very limited, whereas USGS DEMs are available for the entire USA.
1. The primary benefit is greatly increased horizontal and vertical accuracy. USGS Digital Elevation Models (DEMs) were developed as a topographic maps at 1:24,000 scale with 40′ contour intervals (with some 20′) that were based on photography in the 1970’s. Elevation accuracy at any given point was stated as within 20′ of true elevation. In other words the interpolation of an elevation point every 10meters (for the DEM) is a very generalized product. It densifies the elevation points by interpolation.
LiDAR on the other hand is developed scale independent, with a grid of ground elevations that occur about 10-12 per square foot (or meter), with an usual accuracy within 1 cm of actual elevation. This yields an extremely accurate elevation dataset that is very detailed regarding the actual ground shape.
2. LiDAR also is able to penetrate trees and gives an accurate “bare earth” elevation dataset, whereas USGS DEMs where developed by photogrammetric techniques that could not “see” the ground and so created “best guess” contours in areas obscured by trees.
3. LiDAR elevation data reflects current terrain conditions (as of the acquisition date), whereas the USGS DEM is very outdated information from the mid 1970’s that does not show any grading activities that have occurred since that time. Presumably there have been new roads constructed, road widening projects, new bridges, new building pads graded, perhaps even very large subdivision or commercial development.
4. The benefit of re-mapping the flood limits using LiDAR when available are three fold:
a) The absolute elevation of building foundations, bridges and other areas of interest can be confidently identified;
b) The detail of the inundation limit will be very precise as it relates to very accurate contour lines and spot elevations; and
c) In general, Dam Operators and EMA’s (Emergency Management Agencies) will be able to have a greater degree of confidence in the extent of the flood effects, especially in areas of relatively flat topography.
There are certainly other benefits, but increased confidence of the risk (or lack of risk) for any location in or near the flood zone would be the greatest in our opinion.