7832 - Laserfiche WebLink

Home Browse Search

Survey.-Horizontal and vertical control networks were required for the production of accurate base maps. The ground control network was established in the Grand Canyon using a combination of GPS (global positioning system) and conventional surveying. Limited reliable control points existed in the Grand Canyon prior to the GCES effort. Existing geographic control on the rim was transferred down to river level, a distance of over 1 vertical mile, using GPS technology and approved surveying techniques. Limited time, money, and the restrictive nature of the terrain prevented running standard survey level lines down existing trails. GPS control points were established at the beginning and end of each monitoring site. This study is the first time that GPS technology has been used in extreme environments such as the Grand Canyon. Theodolites and electronic distance measuring instruments were used to traverse along the river between the GPS control points. This process added intermediate control points which were used in producing the base maps and provides consistent ground control to be used by specific research projects. Aerial Photography.-Aerial photographs of the monitoring sites were collected for developing the orthophoto base maps. Prior to photography, 5- by 5-ft control panels were positioned on both sides of the river within the monitoring sites (fig. 5). The aerial photographs were collected at an elevation of 3,600 ft a.g.l. The monitoring sites were photographed in black and white at scales of 1:7,200 and 1:14,400 during June and July 1990. The Colorado River was at a constant flow release of 5,000 ft3/s during the time of photo acquisition. Consecutively, the entire river corridor of 291 miles was photographed at a scale of 1:4,800 using CIR (color infrared) photography for mapping natural resources. Coordinates of the horizontal and vertical control points were used to scale and level stereo models. Contour lines at 0.5-m intervals, with 1.0-m accuracy, and other features up to the old high water line on both sides of the Colorado River, were digitized and plotted as 1:2,400 scale base maps. Figure 6 at the back of this report shows a sample topographic product created for each monitoring site. The four base map products generated in this process are: 1. 1:2,400 hard copy topographic base maps with a contour interval of 0.5 m. 2. A digital topographic data set with a contour interval of 0.5 m in ARC/INFO format'. 3. 1:2,400 hard copy orthophoto maps with a contour interval of 0.5 m. 4. 1:2,400 hard copy orthophoto grids depicting only the reference tics and without contour lines. Hard copy map products were plotted on 4-mil mylar. All map products are in the Arizona State Plane Coordinate System (meters) with a Transverse Mercator projection. Base map products are a valuable tool to georeference data for survey and GIS purposes, including aerial photography and tabular data. The base map products are used by GOES, Remote Sensing and Geographic Information Group, the contributing agencies, and GCES researchers for the development of particular monitoring products. ' ARC/INFO is the geographic information software produced by ESRI, Inc. Mention of this or any other commercial product or vendor is not to be construed as an endorsement by the U.S. Government. 6