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DIVISION 2 MEMORANDUM <br />2.5 Geometric Correction <br />Nineteen Landsat images were orthorectified using Digital Ortho Quarter Quadrangles (DOQQs), <br />1/24,000 USGS Quadrangles in Digital Raster Graphics (DRGs), and a 30 -meter Digital Elevation Model <br />(DEM). Figure 4.0 shows an index map of the Landsat frames processed for Division 2. Frames colored <br />in red correspond to multi -date data for the 2003 irrigated lands update in the Lower Arkansas River <br />Basin. Frames colored in orange are single -date additional images to complete coverage of the Division 2 <br />area. These data were projected to the UTM coordinate system, Datum NAD83, and Spheroid GRS 80. <br />The algorithm used for the geometric correction was the Landsat orthorectification model of ERDAS <br />Imagine. This model is derived by space resection based on co- linearity equations and requires elevation <br />information for removing relief displacement ( ERDAS Imagine 8.7 Online Documentation, 2003). <br />A sufficient number of Ground Control Points (GCP), between 22 and 32, was obtained on- screen for <br />each Landsat frame. For this purpose, tiled DOQQs and DRGs were displayed next to the Landsat frames <br />using the Image Geometric Correction Tool in ERDAS Imagine (Figure 5.0). The GCPs were selected <br />using highly visible features on the Landsat frame that are stable over time and can be positively <br />identified on both the Landsat and the DOQQs (e.g., road intersections). <br />The Root Mean Square Errors (RMSE) achieved in the georeferencing process were under 0.66 pixels or <br />less than 20 meters in all cases. Table 7.0 shows the number of ground control points selected for <br />Landsat scenes in frames 31/34, 32/34, and 33/33 and their corresponding RMSE errors. Table 8.0 shows <br />the number of ground control points selected for additional Landsat scenes in frames 31/33, 32/33, and <br />33/33 and their corresponding RMSE errors. <br />Page 11 of 23 P Riverside Technology, inc. <br />Water Resources Engineering and Consulting <br />