Geology Reference
In-Depth Information
Figure 4.6 Route
of Ching Cheung
superimposed on
1949 aerial
photograph (after
Hencher, 1983c;
Hudson &
Hencher, 1984).
Even with little training, the importance of air photographs can
be immediately clear, as in
Figure 4.6,
which is an air photograph
from 1949 on to which has been marked the route of the Ching
Cheung Road in Hong Kong, constructed in 1963. Various ground
hazards are evident in the photo (landslides and deep gulleying) and
it is no surprise that these led to later problems with the road,
as addressed in
Chapter 7
and discussed by Hudson & Hencher
(1984).
Systematic interpretation of air photographs for determining
geotechnical hazards has been carried out in several countries.
For example, the whole of Hong Kong was mapped, in terms of
perceived geotechnical hazard, from air photographs in the 1980s
at a 1:20,000 scale and locally at 1:2,500 and, whilst never
intended for site-speci
cinterpretation,thesewereveryusefulfor
urban planning (Burnett et al., 1985; Styles & Hansen, 1989). Air
photos can be used for detailed measurement by those trained to do
so.
Figure 4.7
shows displacement vectors for the slow-moving rock
landslide at Pos Selim, Malaysia. The 3D image was prepared from
as-built drawings and oblique air photographs taken from a heli-
copter and linked to surveyed control points. The vectors produced
(up to 15m drop in the rear scarp) are considered accurate to about
0.2m. Topographic surveys can also be carried out using terrestrial
or airborne LIDAR surveys and these can be repeated to monitor
ongoing movements in landslides or in volcanic eruptions
(e.g. Jones, 2006). In some situations, especially for remote sites
lacking good air photo coverage, satellite images may be helpful,
although often the scale is not large enough to provide the detailed
interpretation required and stereo imagery is impossible
-
unlike for
purpose-
own aerial photograph sequences. Use of false spectral
