Center for Spatial Analysis Technologies
|
During January 2001 the flood
project team continued to work on revising available DEMs (from NMD) by
topographic quadrangle for the Flint River Basin north of the town of Montezuma,
Georgia area as indicated in figure 1.
The revision is done by digitizing topographic contours from
7.5 minute topographic maps, gridding the area to suitable resolution,
and preparing new DEM’s for each map.
The new DEMs were inserted into the basinwide DEM,
constructed earlier. These new quadrangles were originally smoothed using methods proscribed by NMD. However, after comparing data re-gridded from the digitizing process to the smoothed data considerable differences were noted. Figure 2 shows these differences (revised minus the original DEM values) in the Ideal South Quadrangle. Difference values range from -32 meters to +29 meters. Note that negative values primarily occur in the flood plains of Whitewater and Buck Creeks, both tributaries to the Flint River where the original DEM elevations are on the average 20 meters higher than the original DEM elevations. These differences are extremely important in accurately defining the 100-year flood plain. There is also evidence of a patchwork pattern of errors indicative of the Gestalt Photomapper technique used in deriving the original DEM. The November 2000 flood inundation model indicates spotty flooding within the 100 year floodplain, in the vicinity of Reynolds, Georgia, as shown in figure 3. The revised flood inundation model run (February 12, 2001) produced a new 100 year floodplain for the Reynolds vicinity, as shown in figure 4. The boundaries of this revised floodplain model compare quite well with estimated base flood elevation data. The improved floodplain (figure 4) illustrates the importance of completing the process of DEM revision to produce a regionally accurate and consistent DEM for the river basin prior to beginning any comparative, qualitative analysis. The DEM revision work is being shared with NMD by the flood team and the Georgia Tech GIS Clearinghouse (GTCH) digital orthophotography team which is working on an active and complementary (USGS cooperative) project to prepare color DOQs for the state of Georgia. These DOQ’s will aid the flood team with the planned future flood inundation visualization capabilities. The ongoing liaison with NMD is needed to improve the process of DEM development in order to begin the organized support of hydrologic applications, such as flood inundation modeling. The plan is to begin to establish and to refine the process using the DEM’s of the Flint River Basin. NMD has offered to partner with the state of Georgia to improve available DEM’s. The flood team will meet in February with Georgia District Chief, Ed Martin, SE Regional SW Specialist, Larry Bohman, and Nick Faust of GTRI to discuss current progress and efforts for the coming year. |
 |
|
Figure 1: Status of DEM revision in the Flint River Basin, Georgia |
 |
|
Figure 2: Difference map depicting difference in elevation in meters of revised DEMs minus existing DEM elevations for the Ideal South Quadrangle. |
 |
|
Figure 3: Floodplain model (November 2000) in the vicinity of Reynolds, Georgia (north of Montezuma). |
 |
|
Figure 4: Current (February 12, 2001) floodplain model in the vicinity of Reynolds, Georgia |
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://csat.er.usgs.gov/cindi/jan2001.html
Page Contact Information: webmaster-ga@usgs.gov
Page Last Modified: Tuesday, 23-May-2006 10:57:06 EDT
