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<br />00292' <br /> <br />APPENDIx A <br /> <br />Methods of Landslide Evaluation <br /> <br />Aerial photographY: The use of aerial photography has proven to be an <br />effective technique for recognizing and delineating landslides. It provides a <br />three-dimensional overview of large areas and allows the viewer to evaluate <br />the interrelations of topography, 'drainage; surface cover, geologic materials, <br />and human activities (Rib and Liang, 1978). Mapping can be performed directly <br />on the photos which can then be taken into the field to be checked. <br /> <br />The types of landslide features that can be recognized on aerial photographs <br />include: 1) small isolated ponds, lakes, and other closed depressions, 2) <br />natural springs, 3) abrupt and irregular changes in slope and drainage <br />patterns, 4) hummocky and irregular surfaces, 5) smaller landslide deposits <br />that are commonly younger and form within older and larger landslide deposits, <br />6) steep curved scarps at the upper edge of the deposit, 7) irregular soil and <br />vegetation patterns, 8) disturbed vegetation, and 9) interspersed, <br />discontinuous flat areas (USGS, 1981). <br /> <br />Aerial photgraphs typically used for landslide studies are panchromatic and <br />infrared black-and-white, natural color, and color infrared. Black-and-white <br />photos are the most common since they are the least expensive. However, <br />natural color photography is more useful for determining differences in soil <br />moisture content, drainage, and vegetation, and color infrared is especially <br />valuable for locating seepage zones. <br /> <br />The scale of the photography greatly influences the ability to identify <br />landslides. Small-scale photographs (1:40,000 or smaller) may be necessary to <br /> <br />view the full extent of very large landslides. Whereas small landslides would <br /> <br />virtually disappear at scales of 1:75,000. Photographs of scales of 1:4,800 <br />or larger work best on slides only a few hundred feet wide (Rib and Liang. <br />1978; USGS. 1981). <br /> <br />When available. old photos should be compared with recent ones. This can <br />pinpoint the initiation of slide movement and show its development over time. <br />Old photos are also essential in areas that have become urbanized and where <br />all. or most of the features necessary for recognizing landslides have been <br />destroyed by development or covered by vegetation. <br />