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126 Commission VII Invited paper
The Interpretation of Tropical Vegetation and Crops
on Aerial Photographs
by R. G. MILLER.
At one time it seemed to me that “interpretation” was an odd way to describe the
process of extracting information from aerial photographs. With more experience I have
come to realise that it is in fact a very apt term. The aerial photograph records its in-
formation in signs and symbols which have to be translated by each interpreter into the
language of his own study. Any specialist observer is likely to pick out clues ignored by
others to whom their significance is not apparent, but many common features will be
used by interpreters with different interests to produce different sets of information.
The techniques of examination used by most of them will by very similar, and the special
nature of the information they may individually obtain will be due to the different back-
grounds of knowledge and experience possessed by each of them.
The interpretation of vegetation and crops on aerial photographs of tropical areas
is not in any way a different process from the interpretation of similar features on
photographs taken in other parts of the world. However the tropies have their own par-
ticular characteristics, and they present the air photo interpreter with some novel and
interesting material.
Before leaving general considerations it is worth recalling that the photograph may
be an incomplete or even a false record of the conditions. No combination of camera and
film will produce an exactly truthful record in all conditions. Colwell has shown in various
papers (most recently in [5]) how intelligent planning can help to keep the photographic
recording system as close as possible to the path of rectitude. Such planning requires pre-
knowledge both of the elements to be recorded and of the response of the photographic
system to them. In the tropies the required knowledge is often not available ahead of
photography or there are too many diverse users to justify giving special consideration
to any one aspect. For the most part the requirement is still multi-purpose rather than
specialist photography. Fortunately the quality of photography obtained by modern, wide-
angle cameras generally brings satisfaction to specialist users as well as to cartographers.
It is usual to think of the tropics as a region of bright clear sunshine and to expect
that aerial photography should be an easy matter in these conditions. The climate in the
tropics is, however, far from uniform, and is often extremely awkward for photographic
purposes. The scarcity of landing grounds, of meteorological records and of recording
stations add to the difficulties. In the truly equatorial belt (extending roughly 10° North
and South of the equator) clear weather at low elevations is very rare, and le Ray’s
description of conditions in the Ivory Coast [12] is widely applicable. When there is no
cloud there is often a moist haze near the ground through which it is impossible to obtain
good, sharp photographs. It is sometimes very noticeable in these conditions how the
quality of photography improves over hills or plateaux which rise sufficiently above the
general level of the country to emerge from the low-lying mists.
Many of the oceanic islands of the tropics rise abruptly from the sea to considerable
heights. Their peaks are strongly cloud-forming, and the plume of cloud may persistently
cover much of the land area. When the cloud is absent or of small extent, the conditions
can be almost ideal and excellent photography may then be obtained.
The plateaux of central and eastern Africa and the outer zones of the continental
tropics have distinct dry seasons, in the early part of which there are usually clear skies
and a clean atmosphere offering good conditions for aerial photography. By the middle
of the dry season, however, visibility is liable to be impaired by the smoke from grass
and bush fires. In West Africa the latter part of the dry season is also marked by dust-
