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<br />002471 <br /> <br />contributed significantly to defining the EDZ boundaries. Because we did not use these <br />photographs, the EDZs calculated for Glen Canyon may be expected to be somewhat <br />smaller than in the reaches downstream from Lees Ferry. <br /> <br />Accuracy of the Surficial Geologic Maps <br />The surficial geologic mapping described above has many sources of error and <br />uncertainty. Errors may be the result of (1) errors in mapping and aerial photograph <br />interpretation, (2) the uncertainty resulting from the width of the hand-drawn line on the <br />aerial photograph, (3) digitizing errors due to operator error, and (4) scale transformation <br />errors resulting from distortion in the aerial photographs. Because of the length of river <br />mapped and the number of years of repeat mapping, errors of the first type are inevitable. <br />These errors were located and edited by an iterative process using two error-checking <br />routines. The other sources of error are addressed in an accuracy assessment that <br />determines confidence levels for the maps. <br />The first of the error-checking routines is the inspection of erosion-deposition <br />maps for changes in deposits that are either unlikely or impossible. Erosion-deposition <br />maps are made by evaluating every possible change in map unit categories from one year <br />to the next and assigning a new attribute to each of these combinations describing that <br />change. For example, an attribute of "deposition" would be applied in the event the level <br />of a deposit changed from a low-elevation category to a high-elevation category. An <br />erroneous result would occur if a change from a low-elevation deposit to a higher deposit <br />were identified for a period during which no floods occurred that could have inundated a <br />deposit at that level. Unrealistic results of this type were checked by inspecting each of <br />the maps and the original mapping on the photographs. <br />The second error-checking routine is the analysis of a longitudinal profile for each <br />deposit level. These longitudinal profiles were developed within the GIS by assigning a <br />downstream distance and an elevation to every mapped alluvial deposit. The downstream <br />distance was calculated as the distance from the upstream end of the study area (the base <br />of Glen Canyon Dam) to the centroid of each mapped deposit projected onto the river <br />centerline. The elevation for each deposit was calculated as the average elevation of the <br /> <br />21 <br />