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Aerial Photography Aerial photographs of the GIS sites were collected for the mapping efforts by a photogrammetric contractor. Flight operations required an extensive amount of coordination and briefing by the NPS. Air tours and sightseers cause high density air traffic over the Canyon, prompting the FAA (Federal Aviation Administration) to designate the area as controlled air space. The aerial photography pilots were informed of operations by the NPS coordinator at Grand Canyon National Park and by the FAA. All logistics for the operation had to be planned prior to the aerial mission, because of the Canyon's effect on radio communications between river crews, flight crews, and air traffic control. Prior to photography, control panels were positioned on both sides of the river within the monitoring sites and were used as reference points for mapping (fig. 5). The 5- by 5-foot panels were designed to be recognizable on aerial photographs taken from 3,600 feet above ground level. The monitoring sites were photographed with 1:7,200 and 1:14,400 (used to produce the orthophotos) scale black and white aerial photographs during June and July, 1990 and 1991, at a constant flow release of 5,000 fOls. In addition, the entire river corridor of 291 miles was photographed with 1:4,800 scale CIR (color infrared) for mapping natural resources. Glass diapositives were produced from the 1:7,200 scale black and white film for use in an analytical stereo plotter. Coordinates of the horizontal and vertical control points were used to scale and level the stereo models. Contour lines at 0.5-meter intervals, with 1.0- meter 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. The base map products generated in this process are: 1. 1:2,400 hard copy topographic base maps with a contour interval of 0.5 meter. 2. A digital topographic data set with a contour interval of 0.5 meter in ARC/INFO format. ARCANFO is the geographic information software produced by ESRI, Inc. 3. 1:2,400 hard copy orthophoto maps with a contour interval of 0.5 meter. 4. 1:2,400 hard copy orthophoto grids depicting only the reference tics and not contour lines. All hard copy map products are plotted on four-mil mylar. Digital as well as the hard copy map products are in Arizona State Plane Coordinate System (meters) with a Transverse Mercator projection. It is important to note that an extensive QA/QC program has been developed between Reclamation and the photogrammetric contractor to produce the best map products available. Preliminary topographic map sheets are sent to the Remote Sensing and Geographic Information Section for a cartographic evaluation to check for scale errors and cartographic 7