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EXPLANATION . Very high hazard zone . High hazard zone Moderate to low hazard zone 100-year floodplain of the Uncompahgre River Fans associated with gullies and small basins ~ ~ cfl Directidn of flow o I 400 800 I FEET Figure 16. Detail from the Colorado Geologicai Survey map showing generalized hazard zones for debris fans in the Ouray, Colorado area Uochim, 1986). Map scale should be consistent with standard topographic base maps and aerial photographs. Creating maps at scales different from existing maps requires the meticulous transfer of topographic and geologic informa- tion from other maps-thus consuming a great deal of time and money and possibly introducing serious errors. One of the most useful mapping techniques for plan- ners and public officials is the superposition of landslide information on orthophotographic base maps or aerial photographs. The ability to identify actual land or man- made features heightens awareness of the landslide hazard. Base maps A base map is necessary to undertake environmental geology or land-use planning investigations. Topographic maps are the best base maps for most purposes. They show contour lines, slope, relief, and landform characteristics, as well as roads, buildings, and other cultural features. The most common and highest quality topographic maps of Colorado are those published and sold by the U.S. Geological Survey. Very high quality 71f2 minute (1:24,000) maps and (1:50,000) county series maps have been made of the entire state. U.S. Geological Survey maps also come in other scales: 1:62,500 quadrangles: 1:100,000 (30 x 60 min.) quadrangles; and 1:250,000 (lOx 2") sheets. A 1:500,000 topographic-political map of ,the entire state is also available. There may be cases that require a special base map. In such instances, aerial photographs can be used to prepare a topographic map at any desired scale, detail, and accuracy. Black-and-white photo coverage exists for nearly all of Colorado and is available from public agen- cies or private photogrammetric consultants. Natural color and infrared photograpby are also available for some areas. For more information on the avai1ability of aerial photography for Colorado contact either; Rocky Mountain Mapping Center National Cartographic Information Center U.S. Geological Survey Mail Stop 504, Box 25046 Denver Federal Center Denver, CO 80225 Telephone: (303) 236-5829 Walk-in service for the U.S. Geological Survey is in Building 25 of the Denver Federal Center. or Division of Technical Services (D-435) Bureau of Land Management ~ 27