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Since most landslides are much smaller than the landslide shown in
Figure 3, they are not directly identifiable on satellite imagery.
However, the value of satellite imagery, as noted by Alfoldi (1) and
Gagnon (3), is that the landslide susceptibility of an area can be
determined indirectly from some of the features that are identifiable
at those scales. Regional physiography, geologic structure, and most
landforms as well as land-use practices and distribution of vegetation
are evident on the satellite imagery. These features in conjunction
with the tonal pattern present on the imagery provide clues to the type
of surface materials present, the surface moisture conditions, and the
possible presence of buried valleys. Correlating these factors to
geology and topography and using local experience in a region make it
possible to rate the susceptibility of various areas to sliding. For
example, Alfoldi noted in his study of landsliding in eastern Ontario
that on the satellite image the clay plains were easy to spot because
they are almost 100 percent cultivated: the till plains were recogniz-
able because they form a poorer agricultural area and field and forest
sections are intermixed; and the elevated sand plains of the old Ottawa
River delta (which overlay the clay plains) are kept mostly in forest.
An additional advantage noted by Alfoldi for satellite imagery was the
frequent coverage available. Seasonal changes in vegetative cover and
moisture levels--as indicated by tonal changes--can be evaluated to
increase the accuracy of interpretation of terrain conditions. Also,
any changes noted during the year in the landslide-susceptible zones,
such as urban expansion, clear-cutting of forest, forest fires, and
draining of swamps, might presage renewed or new landslide activities.
This could alert the interpreter to the necessity for a more detailed
investigation in these areas.
Satellite photography and imagery for large parts of the world are
available from the EROS Data Center of the U.S. Geological Survey,
Sioux Falls, South Dakota 57198. They can be ordered by providing the
geographic coordinates of the area of interest (latitude and longitude).
Experience has indicated that band 5 (0.6 to 0.7 um) or the infrared
color composite is the most beneficial for landslide investigations.
Infrared imagery
Infrared imagery offers some unique information that cannot be obtained
directly from the analysis of aerial photography. The combination of
aerial photography and infrared imagery provides a more accurate and
complete portrayal of terrain conditions than can be obtained from either
system alone. Infrared imagery provides the following types of supple-
mental information that is valuable for evaluating existing landslide and
landslide-susceptible terrain:
l. Surface and near-surface moisture and drainage conditions;
2. Indication of the presence of massive bedrock or bedrock at
shallow depths;
3. Distinction between loose colluvial materials that are present
on steep slopes and are susceptible to landslides, and the massive bed-
rock that is more stable on steep slopes; and
