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photogrammetrically produce base maps. The established control network and the aerial photography were used to develop both the topographic and orthophoto base map products used in the GCES/GIS. Figure 2 is a schematic diagram of the various technological components involved in the development of the base map products. Production of the Large Scale Base Map Products Horizontal and vertical control points, a requirement for the production of accurate base maps using photogrammetric methods, were established using a combination of GPS and conventional surveying techniques. Very few reliable control points were already in existence in the bottom of the Canyon. Existing control on the rim was transferred down to river level, a distance of over one vertical mile, using GPS technology. Limited time, money, and the restrictive nature of the terrain prevented running standard survey level lines down existing trails. The river corridor is accessible only by boat, hiking, or helicopter. Logistic and administrative circumstances prevented helicopter transport of the survey party through the canyon. The targeting and traversing were accomplished by two crews, each on a large support raft, and a small, powered, Zodiac boat to move up and down river between each target. A Jet Ranger helicopter was used to transport the GPS crew between 46 existing NGS (National Geodetic Survey) control points on the rim area and to the river corridor to place two survey monuments on each of the 13 monitoring sites. As the survey crews carried their traverse downstream, they were able to tie the GPS points established for each monitoring site. The Department of Defense developed GPS to determine the geographical position of military vehicles, ships, planes, and personnel. Civilian application of the technology is rapidly expanding. Surveyors, scientists, city planners, and transportation planners are using GPS on a daily basis to locate important geographic features. By monitoring the time required for signals from a minimum of 3 satellites to reach a ground receiver (a distance of about 11,000 nautical miles in space), the determination of horizontal geographic location is possible. Ground receivers calculate the classical formula: D(distance) = R(rate) X T(time). With the position of each satellite known through the transmission of an emphemeris to the receiver, the latitude and longitude of the ground point is determined using 3-D (three- dimensional) spatial resection. The elevation of a point (3-D) can be calculated if the radio signals from at least 4 satellites are received. GPS control points were established at the beginning and end of each monitoring site. This project is the second time that GPS technology has been used in environments such as the Grand Canyon, where elevation differences between the rim and the base can be as great as 5,000 feet, and the available window for satellite acquisition is limited. Total stations, 4