The Accuracy and Precision Revolution Continued from page 35 Figure 3 Hillshade and hydrography displayed at 1:24,000 scale. Note how hydrography matches terrain (hillshade) in most areas. A red square surrounds the magnified areas in Figures 3 and 4 that show where flaws in the data become painfully apparent. For the most part, the hydrography follows the terrain in Figure 3 at a scale of 1:2,400 (about 1-meter resolution, which is the pixel size of the bare earth lidar data). Figure 4 uses a hillshade of the bare earth lidar returns from 1-meter lidar data. In this figure, one can see how poorly the hydrography matches the terrain at 1-meter resolution. The hydro linework does not follow the drainages very closely. There are errors of omission. Where there should be line work, there is none, and there are errors where the line work simply is wrong. Analytic Methods Analytic methods will need to change as we learn to use data with greater detail and intensity. Processes that might have worked at 10-meter resolution will now need modification. Figure 5 illustrates the type of problems encountered when we attempted to automate stream generation on a half-meter digital elevation model (DEM) derived from lidar. The red line represents the stream drainage that should have been generated, while the darker blue line is what the program produced. The increased accuracy and precision caused the stream delineation program to send the course of the stream along the roadbed. Special programming had to be added to make automated watercourse line generation successful. (Note the white line represents 36 ArcUser Winter 2010 Figure 4 Enlarged area displayed at 1:2,400 shows slight discrepancies between hydrography and terrain. www.esri.com