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TROPICAL VEGETATION AND CROPS, MILLER 127 
laden winds from the Sahara, which bring their effects almost to the coast. More unusu- 
ally I have seen a similar pall reach nearly to the equator in western Uganda. In these parts 
of the tropies conditions during the rainy seasons virtually preclude any extensive photo- 
graphy. As a result very little cover has been taken, in the growing period, and the available 
air photos mostly depict the vegetation and fields at a time when contrasts are probably at 
a minimum. At least, however, there is some uniformity in the photography, whereas the 
fragmentary cover, which is often all that is obtainable in the humid tropies, may have 
intervals of weeks or months between runs, and so depict the vegetation, and especially 
short-term crops, at many different stages of growth. 
With the high altitudes which the sun attains in the tropies vertical aerial photo- 
graphs taken there are especially liable to the blemish variously known as “hot-spot”, 
“shadow point” or “no shadow point”. This occurs when a point within the photograph 
is directly in line with the camera-station and the sun. Objects at and immediately 
around this point almost completely screen their shadows from the camera. In the case 
of tall trees standing on the outer side of the hot-spot the camera is able to “see under” the 
crowns and to record the shadow on the near side of the crown image. This reversed 
shadow, when viewed with the normally-positioned shadow on the adjacent photograph, 
causes such ocular confusion that the effects of parallax are lost and it is impossible to 
assess or measure the true heights of the trees concerned. It is significant that the best 
description and explanation of this phenomenon comes from Sims [17] in Australia where 
tall, open forest is common. Such stands are rare in the tropics where either there is a 
continuous canopy or the trees are too wide-crowned and short-boled for shadow reversal 
to occur. However the frequent presence of haze over tropical forest brings out another 
effect of hot-spot. Haze does not scatter light equally in all directions and there is gen- 
erally a peak of dispersion directly back along the path of the incident light [2]. As the 
hot-spot marks the point at which the camera is directly between the subject and the 
object on the photograph, the back-scatter of light by haze is also at a maximum at this 
point. Thus the reduction in contrast due to the loss of shadows on the photographs is 
aggravated by reflection from the haze. Tropical air photographs often show a strong 
difference between average density on opposite sides, an effect which is also due to the 
differential scattering of light by haze. Fortunately electronic printing can now overco- 
me most of the deleterious effects of unequal scattering by haze, but even this ingenious 
system cannot help with missing or reversed shadows. 
Probably because of the overwhelming need to obtain general purpose cover suitable 
for topographic mapping there has not been much research into the use of special types 
of photography in the tropics. The Belgians have gone furthest in investigating different 
combinations of film and filter, and have selected a type of “modified infra-red” as best 
for small-scale photography of rural areas with plenty of natural vegetation in the 
Congo [16]. The only samples of this photography which I have seen covered mainly 
savanna areas; they were of high quality and showed more distinct tonal contrasts 
amongst the tree vegetation than did some older panchromatic photographs of the same 
area. The Belgians also make the point that infra-red film is less susceptible to the 
effects of dry haze, and that the use of this film extends the range of conditions in 
which photography can be undertaken — a quality which is obviously valuable when 
extensive cover is required within a limited season. British efforts at testing the possi- 
bilities of infra-red film in the tropics have not been nearly so successful. In every 
attempt so far the infra-red film has proved more liable to static electricity markings, 
a trouble which the Ordnance Survey in Britain also experienced when they first used 
infra-red film for photography of tidal flats. On small-scale photographs of conifer and 
eucalypt plantations in Kenya infra-red prints undoubtedly gave better separation of 
species than was possible with panchromatic photographs: this advantage did not persist 
in really large-scale photographs when the details of crown structure rather than tone 
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