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Maps were geo-referenced in the field during July and September of 1996, using a <br />portable Global Positioning System (GPS). GPS relies on the availability of navigation <br />satellites within the visible horizon. In narrow canyons, satellite signals may become <br />distorted by bouncing off canyon walls; narrow windows of sky may contain too few <br />satellites to calculate a position. To assure accurate measurements while in the field <br />Pathfiner software was used before each mapping field trip to calculate times and locations <br />within the canyon where abundant satellites would be visible. GPS accuracy is also <br />influenced by a preinstalled error in the satellite signal know as selective availability (SA). <br />SA is a mechanism employed by the U.S. Department of Defense that limits the accuracy of <br />GPS data to 30-100 meters. GPS positions collected in the field were then compared to <br />data collected at a base station of known location during the same time period, and field <br />positions were corrected to an accuracy of 2-5 meters. <br />Geo-referenced maps were digitized and entered into a geographic information <br />system using Arc/Info. All linear features and polygons were assigned attributes, and <br />lengths and areas were calculated. This process has been completed for the Cedar Ridge <br />reach and is on-going for the three other reaches. <br />Photographic Analysis <br />The photograph has long been used as a tool for describing changes in <br />geomorphology and vegetation that have occurred over time. The images included with <br />this report are matched pairs that illustrate such changes along the Green River in <br />Desolation Canyon. <br />The black-and-white images were made using a Crown Graphic 4x5 Field View <br />Camera. Field copies of the original photographs were made to facilitate locating the site <br />where the photographs were taken. Landmarks in the foreground and background helped <br />to achieve precise replication of the image. Each site was then benchmarked for future <br />studies. Notes detailing observations were made in the field for each photograph