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: