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<br />00.2473 <br /> <br />Some of the analyses applied to the surficial geologic maps involve creating <br />overlays between two separate maps and calculating areas of change in attribute labels. <br />The estimate of standard error described above characterizes the uncertainty in the <br />determination of the total area of map units, but does not address the error of individual <br />polygons. We therefore used a method that estimates uncertainty by assuming uniform <br />error around the entire perimeter of every polygon. Sondossi and Schmidt (2001) <br />estimated positional accuracy of mapping similar to that conducted in this study by <br />comparing digitized locations of 56 points with the actual locations of the same points on <br />orthophotograph base maps. These data indicate a mean positional error of 1.6 m, which <br />is approximately the same as the error that would be expected due to the thickness of a <br />0.3 mm pencil line at the scale of the aerial photographs. We approximated the error for <br />individual polygons as the product of this mean position error and the perimeter of each <br />polygon. The maximum error in a map consisting of a set of individual polygons was <br />then approximated as the sum of the errors of the individual polygons. This error was <br />then expressed as a percentage of the total area of those polygons. Because these <br />percentage errors are large for small polygons, polygons smaller than 500 m2 were <br />excluded from consideration in the erosion-deposition maps. Applying this method to <br />each of the erosion-deposition overlay maps, the estimated error ranges from 5 to 14% <br />with a gross average of 9%. For simplicity, we applied the maximum estimated error, <br />rounded up to 15% to all erosion-deposition comparisons. <br /> <br />RESULTS <br /> <br />Rate and Mat!nitude of Bed Det!radation <br />The onset of degradation and the channel cleaning flows <br />Degradation of the bed in the first 10 km downstream from Glen Canyon Dam <br />began between 1956 and 1959, and by 1965 had progressed downstream to Lees Ferry. <br />During the period of dam construction, degradation rates were high at cross-sections <br />within 5 km from the dam, and cross-sections more than 10 km downstream were mostly <br />stable (Figure 5). The rate of bed degradation accelerated dramatically after 1959, and it <br /> <br />23 <br />