Page 138 - Introduction to Mineral Exploration

6: REMOTE SENSING 121

ment differs on two adjacent photographs. It 5 Platform velocity – changing speed of the
can be seen that the parallax of the base at aircraft during film exposure (Fig. 6.11e). This
the object is less than the parallax of the top applies particularly to satellite imagery.
of the object. Thus, a difference in elevation, 6 Earth rotation – see section 6.2.3 (Fig. 6.11f).
in addition to producing radial displacement,
also produces a difference in parallax. It is
this difference in parallax that gives a three- 6.4.5 Photointerpretation equipment
dimensional effect when stereopairs are viewed The two main pieces of photointerpretation
stereoscopically. Allum (1966) gives a good equipment that exploration geologists use are:
description of relief and parallax. (i) field stereoscopes capable of being taken
into the field for quick field checking, and (ii)
mirror stereoscopes which are used mainly
6.4.3 Photographic resolution
in the office and can view full 23 cm × 23 cm
The resolution of a photographic film is influ- photographs without overlap. These pieces of
enced by the following factors: equipment and their use are fully described by
1 Scale, which has already been discussed. Moseley (1981) and Drury (2001).
2 Resolving power of the film. A 25 ISO film Extra equipment which can be utilized in an
(a slow film) has a large number of silver halide exploration program includes a color additive
grains per unit area and needs a long exposure viewer and an electronic image analyzer:
time to obtain an image. A 400 ISO film (a fast 1 A color additive viewer color codes and
film) has relatively fewer grains per unit area superimposes three multispectral photographs
but requires less exposure time. The 25 ISO to generate a false color composite. Multispec-
film has better resolution, while the 400 ISO tral photographs need three or four cameras
film tends to be grainy. taking simultaneous images in narrow spectral
3 Resolving power of the camera lens. Com- bands, e.g. 0.4–0.5, 0.5–0.6, 0.6–0.7 µm. Color
pare the results from a high quality Nikon lens is far easier to interpret because the human eye
with those of a cheap camera lens. can differentiate more colors than gray tones.
4 Uncompensated camera motion during 2 The electronic image analyzer consists of a
exposure. closed circuit TV scanner which scans a black
5 Atmospheric conditions. and white image and produces a video digital
6 Conditions of film processing. image. The tape is then processed by computer
The effect of scale and resolution is ex- and the results are shown on a TV monitor.
pressed as ground resolution distance (GRD). This brings us back to the digital image pro-
Typically GRDs for panchromatic film vary cessing described in section 6.2.6.
from centimeters for low flown photography to High resolution scanners mean that aerial
about a meter for high flown photography. photographs can now be converted into digital
images. To some extent these can be treated in
a similar way to the satellite digital imagery.
6.4.4 Problems with aerial photographs and Low altitude aerial photography can also be
flying
acquired in a high resolution digital form,
1 Drift – edges of the photograph are parallel to with numerous spectral bands. These high
the flight line but the plane drifts off course resolution images can be combined with lower
(Fig. 6.8c). resolution satellite digital imagery to preserve
2 Scale – not uniform because of parallax, the best features in both. For example, a high
terrain elevation, aircraft elevation (Fig. 6.11a), spatial resolution aerial photography can be
and the factors described below. combined with high spectral resolution satel-
3 Tilt – front to back or side to side, causing lite imagery (Department of Land Information
distortions of the photograph (Fig. 6.11b,c). 2003). This produces a product that can dis-
4 Yaw (or crab) – the plane turns into wind to tinguish roads, cultural and urban features
keep to the flight line, resulting in photographs derived from the aerial photograph, and also
whose edges are not parallel to the flight line provide information on vegetation cover and
(Fig. 6.11d). geology derived from the satellite imagery.