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Aerial photography 
The value of aerial photography in landslide investigations has been 
reported by many investigators (4 ,5 ,7 513,14). The effectiveness 
of this technique was well demonstrated by Maruyasu and others (8.5 
in analyzing a mountainous region where landslides were common in 
Japan. Using photo-interpretation techniques, they identified 365 
landslides. After the region was field checked, an additional 68 
landslides were identified, resulting in an 84 percent accuracy in 
overall identification. An accuracy of 96 percent was reported in land- 
slide identification of areas not covered by trees and shrubs, such as 
in paddy fields. 
As previously indicated, aerial photography is used in all phases of a 
landslide investigation. Small scale photography is used in analyzing 
large regional areas; medium scale photography is used for recognizing 
geologic units and landforms, and large scale photography aids in 
identifying the vulnerable locations. Thus, typically, more than one 
scale of photography is needed for a landslide investigation.  Avail- 
able small and medium scale photography are usually used for eval- 
uating regional patterns and identifying landforms, while new photo- 
graphy at medium and large scales are obtained for the detailed 
evaluation of the landslide susceptible areas and vulnerable locations. 
Vertical, aerial photography providing stereoscopic coverage is the 
usual format obtained. For the detailed, large-scale coverage, the 
vertical photography is frequently supplemented with oblique aerial 
views. The obliques provide a more unobstructed view of the valley 
walls and unstable slopes. 
Aerial photographs typically used for landslide studies include pan- 
chromatic and infrared black-and-white films, and natural color and 
color infrared films. Panchromatic black-and-white film is the most 
common type used because it is low cost, convenient to handle, and 
readily available, but natural color and color infrared films are now 
being more extensively used. Color photography is especially valuable 
for outlining differences in moisture, drainage, vegetation conditions, 
and soil and rock contacts. The stratification in exposed soils and 
rocks is most easily recognized on natural color film. Color infrared 
films are most helpful for delineating the presence of water on the 
surface and for giving clues to subsurface water conditions by show- 
ing the vigor of the surface vegetative cover. This has made color 
infrared film especially valuable for locating the presence of seepage 
zones at or rzar the surface. 
Although no clear-cut recommendations can be made to fit all situations 
or localities, some of the desirable conditions for acquiring new 
aerial photography for landslide investigations are discussed below: 
l. Optimum soil-moisture contrasts. The optimum contrast of soil 
moisture occurs under two differing conditions: (a) when the soils are 
wet but not totally saturated or (b) when the moisture levels are low. 
The former occurs in the spring or early summer because of high water 
conditions. Wet areas and seepage zones present in unstable or poten-