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6: REMOTE SENSING 117

11 bits per pixel (2048 gray tones). Specialist These films make use of different parts of
image processing software is required to pro- the spectrum, e.g. visible light (0.4–0.7 µm)
cess these images. Ikonos instruments have and photographic near infrared (0.7–0.9 µm).
both cross- and along-track viewing capabilit- Panchromatic film produces a print of gray
ies, which allows frequent revisit capability, tones between black and white in the visible
e.g. 3 days at 1 m resolution. part of the spectrum. These are by far the most
The high resolution imagery satellite common and cheapest form of aerial photogra-
QuickBird was launched in October 2001 phy. Color film produces a print of the visible
(DigitalGlobe 2004). It provides a panchro- part of the spectrum but in full natural color. It
matic channel with 0.6 m resolution (0.4– is expensive but very useful in certain terrains.
0.9 µm) and four channels of multispectral, By contrast, color infrared film records the
stereoscopic data (0.45–0.52, 0.52–0.60, 0.63– green, red, and near infrared (to about 0.9 µm)
0.69, and 0.76–0.90 µm) with a resolution of parts of the spectrum. The dyes developed in
2.44 m. QuickBird also collects 11 bits per each of these layers are yellow, magenta, and
pixel. The satellite operates in a 450 km 98 cyan. The result is a “false color” film in which
degrees sun-synchronous orbit, with each blue areas in the image result from objects
orbit taking 93.4 minutes. Each scene covers reflecting primarily green energy, green areas
16.5 × 16.5 km. in the image result from objects reflecting
primarily red energy, and red areas in the image
result from objects reflecting primarily in the
6.3.4 Hyperspectral airborne systems
photographic near infrared portion of the
Hyperspectral airborne systems acquire spec- spectrum. Vegetation reflects IR particularly
tral coverage in the 0.4–2.4 µm range. These well (Fig. 6.6) and so IR is used extensively
sensors are capable of acquiring any band where differences in vegetation may help in
combination ranging from an optimal number exploration. Aerial photographs are generally
between 10 and 70 multispectral bands to a full classified as either oblique or vertical.
hyperspectral data set of 286 bands (Spectrum
Mapping 2003). Using a band configuration tool
the user can define a band file for the desired Oblique photographs
number of bands and individual bandwidth Oblique photographs can be either high angle
for each band. This band file is then selected oblique photographs which include a horizon
in-flight using flight operations software. or low angle oblique photographs which do
not include a horizon. Oblique photographs
are useful for obtaining a permanent record of
6.4 PHOTOGEOLOGY cliffs and similar features which are difficult to
access, and these photographs can be studied at
Photogeology is the name given to the use of leisure in the office. Similarly, studies can be
aerial photographs for geological studies. To made of quarry faces to detect structural prob-
get the best out of photographs geologists must lems, to plan potential dam sites, etc.
plan the photogeological work in the office and
in the field. A typical scheme is:
1 annotation of aerial photographs; Vertical photographs
2 compilation of photogeology on to topo- Vertical photographs are those taken by a cam-
graphical base maps; era pointing vertically downward. A typical
3 field checking; aerial photograph is shown in Fig. 6.7. The
4 re-annotation; principal point is the point on the photograph
5 re-compilation for production of a final that lies on the optical axis of the camera. It is
photogeological map. found on a photograph by joining the fiducial
The common types of aerial photos are: marks. Normally a title strip includes bubble
panchromatic black and white photographs balance, flight number, photograph number,
(B&W), B&W taken on infrared (IR) sensitive date and time of the exposure, sun elevation,
film, color photographs, and color IR. flight height, and camera focal length.