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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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