1.1 Aerial Photography
The photographic camera and the television camera, which operate in the visible
range between ultra-violet and infra-red are still the most important systems
in remote sensing for pollution studies (Schneider, 1972). For many decades
ihe vertical aerial photograph has been an invaluable aid in many fields of
research and operationa! use. It provides a rapid, objective over-view of the
environment. The aerial photograph or photogrephic image contains a bigh
information content and has very good spatial resolution. It also provides an
exactly-timed documentation of the environment for comparative studies of changes
and development, The observation that objects photographed in the visible
region of the electro-magnetic spectrum do not have the same degree of reflection
led to experimentation with various film filter combinations, using multibaud
cameras which simultaneously exposes in different regions. The Fairey Surveys!
camera system, made up of four Vinten cameras is shown in Figure 1. Many
investigations have shown that vegetation under stress responds differentiy
from healthy plants in various parts of the electromagnetic spectrum. Of the
three most usuei iypes of photographs taken by a multiband camera (viz. black
and white panchromatic, colour and colour infra-red) the colour infra-red film
has been found to be the most single useful image type for vegetation damage
analysis. However, normal colour film is able to detect discolouration of
water caused by sediments and effluents, which are invisible on other film types.
Therefore, a multiband camera system is often the optimum sensor in pollution
studies.
As most forms of vegetation have high reflectance in the near infra-red region,
they take on a variety of red tones. This method of enhancement has aided the
detection of vegetation and its quality in industrial and urban areas, in turn
determining environmental quality. With a normal colour film, vegetation
suffering pollution damage can only be separated from healthy vegetation when
visible crown symptoms exist (Stellingwerf, 1969). However, with a colour iufra-
red film damage effects may be determined with or without visible crown symptomi
in nature, Unhealthy, damaged or dying vegetation tends to deviate from the red
colour towards cyan (Kirby, 1977). Another important advantage of infra-red
film is the easy differentiation of conifers and deciduous species of trees. As
conifers are usually more sensitive to atmospheric pollution, this is an
important extra asset of false colour film.
In addition, atmospheric scattering of visible and near infra-red wavelengths,
due to reflection of dust particles, smoke, water vapour, sulphur dioxide, etc.,
does not affect colour infra-red film to ihe same degree as other photographic
sensors, thus giving colour infra-red a strong haze and smog penetration ability.
so valuable in studying industrial areas. This capability occurs as atmospheric
scattering takes place mainly in the short blue-light wavelengths, which are
filtered out of the colour infra-red images.
Infra-red and visible radiation are often reflected and transmitted quite
differently by vegetation and this has led to ihe development of vegetation keys
used in the interpretation of aerial photography. Figure 2 shows an example of
such a key used in a project of an industrial area of Teeside, N.E. England (van
Genderen, 1974). The key allows one to understand how and why the various
vegetation types, both healthy and under stress, are recurded as they appear.
Other keys have been developed by Murtha, (1976) ana Stoliinmwerf, (1969),
1.2 Thermal Infra-red Scanner
Airborne thermal mappers or line scanners are used to measure the infra-red
radiation from the Farth!s surface and thus to measure the temperature and
emissivity. The infra-red atmospheric windows of 3 to 5 and 8 to 14 microns
are extensively used and the sensor can measure temperature differences in the
order of 0.59 to 19C. Infra-red scanners can operate during the day or night
and through haze and smoke.
