curvilinear lines. The darker tones are usually due to the better 
growth of vegetation along the fracture zones where it is easier for 
the roots of plants to grow and where moisture levels are usually higher. 
In delineating fracture zones, care must be taken not to interpret man- 
made features, such as fence lines or field boundaries, as fracture 
zones. Generally, features having straight lines, right-angle inter- 
sections, and standard geometric patterns are man-made. 
A further advantage of aerial photography is the speed in which a land- 
slide can be evaluated once it occurs. A light aircraft can be rented 
to perform a rapid reconnaissance of the slide, and a hand-held, 35-mm 
camera can be used to obtain a record of the conditions existing. A 
plane with a mapping camera can then be brought in as soon as practical 
to obtain the necessary coverage for mapping the slide and planning 
corrective measures. 
Time is frequently of importance in investigating a recent landslide. 
The occurrence of a landslide does not mean that final adjustments to 
the unstable conditions have occurred and no further movement will occur. 
In many cases, especially in unconsolidated deposits, the materials 
present in the scarp face remain in an unstable condition because they 
are on a very steep slope. The scarp face rapidly retrogrades uphill 
by continued slumping until a more stable condition occurs. Thus, a 
new landslide should be investigated as soon as possible not only to 
determine corrective measures but also to look for evidence of possible 
continued movement. The most significant sign of possible further 
instability is the presence of cracks on the crown of the slide. 
Figure 2 (left) shows some telltale signs at a recent slide. Figure 2 
(right) shows the slide area 5 months later. Additional movement 
occurred in the area where the telltale signs were evident in the 
earlier photograph. 
Satellite imagery 
Satellite coverage of the United States, most of Canada and many other 
areas of the world have been collected since 1972. Initially, coverage 
was obtained on an 18-day cycle, but with the launching of a second 
satellite in 1975, imagery is now collected on approximately a 9-day 
cycle. Each gatellite scene covers an area of approximately 33 000 km“ 
(10 000 miles“) and has a resolution of about 80 m (250 ft). The 
satellite scene can be provided as a 70-mm film at a scale of 
1:3 369 000; as a 185 x 185-mm (7.3 x 7.3-in) black-and-white print of 
each of the four bands collected, or as a color composite at a scale of 
1:1 000 000; and on computer-compatible tapes for computer processing. 
The scenes can also be provided on 35-mm slides. 
At the small scale of satellite imagery, only extremely large land- 
slides can be identified directly. The area enclosed within the 
rectangle in Figure 3 illustrates such an example. The scalloped edges 
of the scarp slopes and the hummocky topography of this large slide 
are evident on this satellite scene.