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Figure 5. - 5- by 5-ft control panel. IV The orthophoto map is a photograph showing images of objects in true orthographic positions. The orthophoto map was rectified and georeferenced for accurate control. The GCES orthophoto products are used to define the coordinates of known point, line, or polygon data to within an initial accuracy of 2.0 meters. The GCES orthophotos are used to establish common control between aerial photography (current or historical) and base maps in the transfer of data to the GCES/GIS data base. By using the historical aerial photography, the Grand Canyon researchers are able to collect and analyze surface data from the past. The topographic product is used in the development of volumetric, slope/aspect data and as an accurate reference tool for surveys and monitoring. A digital orthophoto was developed for each site to expedite and facilitate the dissemination of these useful data sets to the contributing agencies and researchers. GCES/GIS data base design.-The design and strategy were developed by representatives of the USGS (U.S. Geological Survey), GCES, NPS, and Reclamation's Remote Sensing and Geographic Information Group in Denver in 1991. The data base design contains two primary components: "structure" and "content." "Structure" includes the layout of computer directories; for example, grouping data by theme (i.e., topography) or by spatial location (i.e., river reach). "Content" includes category and subcategory designations such as "geology and formation" or "hydrology and flow velocity." All ARC/INFO coverages or data layers are stored in a computer directory tree structure under one master directory called GCES. Two types of sub-directories are found under the home directory GCES. One directory contains basin-wide data such as sampling sites and climatic monitoring stations. The other contains 14 directories dividing the data base by geomorphic 7