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As many pollutants are discharged with warm water, infra-red line scanners have 
been widely used to map rivers, estuaries and coastlines to study the distribu- 
tion of this type of pollution. The most obvious application of infra-red 
scanners is to map the movement and mixing of cooling water being discharged 
from power stations: As many power stations are sited on estuaries with rapidly 
varying -tidal conditions, the airborne infra-red technique has a great advantage 
over ship methods in being able to map large areas quickly. 
For exact thermal observations, the simple qualitative reference of 'cool is 
dark, warm is iigbt!, does not suffice. When absolute temperature measurements 
are required, calibration elements (black bodies) need to be utilised in the 
infra-red line scanners. Thermal imagery taken without internal calibration 
needs additional measurements from an infra-red radiometer/thermometer and the 
co-ordination of the curves with the thermal imagery. 
1.3 Side Looking Airborne Radar 
Radar has a particular application in bad weather areas, where cloud cover reduces 
the opportunity for conventional air photography. As the spectral band vtilised 
is in the microwave it has the ability to penetrate through clouds and ir can also 
operate at night. Radar systems do not, however, have the accuracy ard resolu- 
tion in terms cf minimum target size discrimination that aerial photography has 
and they are mainly used to acquire small scale imagery of large areas such as 
for oil pollution along coastal waters. Due to the microwave back-scatter 
properties of land and water surfaces and the oblique view given by imaging radar 
systems, they are sensitive in the depiction of surface roughness: this is 
especially useful in the detection of oil slicks where the oil film flattens 
out the normal wave patterns of the sea surface. Slicks appear on the radar 
imagery as dark or low signal return areas, as most of the microwave pulse is 
reflected away from the detector. 
2. Air Pollution and the Remote Sensing of its effecis on 
Vegetation 
Air pollution is defined as the contamination of the atmosphere with undesirable 
solids, liquids and gases. In a strict sense, air may be considered polluted 
when any substances, foreign or additional to its normal composition, is added. 
This definition is much too wide, however, for the purposes of air pollution 
control and the term 'air pollution! is usually restricted to those conditions 
in which the general atmosphere contains substances in concentrations which are? 
harmful, or likely to be harmful, to man or his environment (Mellanby, 1972). 
Air oxidants, including sulphur dioxide, ozone, fluorides, nitrous dioxide, 
peroxacetyl nitrate (PAN) and copper oxides cause foliage discolouration, dropping 
of leaves and eventual death. Some of these symptoms are subtle and affect the 
same species selectively. An apparently healthy individual can be growing 
adjacent to a neighbour of the same species in its last stages of decline. Air 
oxidant pollution is frequently slight at first and can easily be overlooked if 
the wrong sensor system is used. Therefore, it is extremely important that the 
various remote sensing techniques, as described in the previous section, are 
evaluated for specific problems. 
2.1 Smoke and Dust 
The burning of fossil fuels, particularly coal and oil, is the greatest cause of 
air pollution in the form of dust and smoke. It has long been recognised that 
smoke in the air of towns and eround industrial sites has an adverse effect on 
photosynthesis, by reducing the light intensity and hours of bright suushine per 
day. Quality, as weil as quantity, vf light is affected since the absorption 
and scattering of radiation is greatest in tbe ultra-violet range. This fact 
makes the use of infra-red film, which cuts out these shorter wavelengths, 
extremely desirable in areas with high levels of atmospheric pollution.