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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