tially unstable areas are directly visible outcropping on hillsides or 
are evident by the presence of luxuriant vegetative growth over those 
wet areas. The latter occurs during dry periods in late summer or fall 
or over disrupted areas. During dry periods, vegetation in wet areas 
or seepage zones has more growth and color than that in areas that have 
normal moisture levels. Areas disrupted by cracks and fissures are 
better drained and thus drier, and vegetation located over those areas 
shows evidence of stress earlier.  Contrasting vegetation and moisture 
conditions are best noted on color infrared photography. 
2. Optimum ground cover. A minimal cover of snow and tree foliage is 
desirable. In humid, temperate climates, this usually occurs in early 
spring after the snow melts and before the deciduous trees leaf out or in 
late fall after the leaves fall and before snow falls. 
3. Optimum shadow conditions. Photographs taken when the sun angle is 
high and shadows on the hillsides and slopes are minimal are the best 
for interpretation. However, in some special cases, such as in areas 
of low topography, small slope failures are enhanced by the presence of 
long shadows produced when the photography is taken with a low sun angle 
(either early in the morning or late in the afternoon). 
These optimum conditions are not always present at one time, and a 
compromise may be required to obtain as many of the ideal conditions as 
possible. Some detailed investigations may require photographs to be 
taken several times during the year. 
Diagnostic patterns of landslides on aerial photography 
Experience has demonstrated that landslides are more prevalent in 
certain landforms and occur most frequently at certain vulnerable loca- 
tions. The technique for identifying landforms on aerial photography 
is beyond the scope of this paper. A discussion of this topic, with 
inclusion of stereoscopic examples of landslide susceptible landforms, 
is found in Chapter 3 of the new landslide manual published by the 
Transportation Research Board (12). 
An investigator already familiar with the appearance of landslides on 
the ground should become oriented to the aerial view of landslides by 
an examination of photographs of some known examples. The difference 
between an aerial view and a ground view results chiefly from the fact 
that the former gives a three-dimensional perspective of the entire 
slide area, but at a rather small scale. Ground photographs, on the 
other hand, show only two dimensions, but on a larger scale. 
The following features discernible on aerial photography are typical of 
landslides or landslide-susceptible terrain, but not all features are 
evident for each landslide. Most of these features are illustrated in 
Figure 1. The items numbered in Figure 1 correspond to the specific 
features listed below: