128 TROPICAL VEGETATION AND CROPS, MILLER 
values could be used as guides to recognition. Our biggest disappointment was over the 
photography of stands of Pinus carıbaea in British Honduras. Contrary to the behaviour 
of conifers in Europe and North America this Pine is imaged in a lighter tone than 
neighbouring broadleaved trees on panchromatic photographs. On infra-red photographs 
| the tones of the broadleaved trees were lightened whereas the tone of the Pines was 
little altered, and the tonal difference between the two classes of tree was reduced 
rather than increased. 
Nowhere in the tropies has it proved possible to recognise conifers from other species 
in mixed forest by their tones on small-scale panchromatic film. I have hunted unsuc- 
cessfully for species of Agathis, Podocarpus and Juniperus. 1 cannot help wondering if 
the usual tonal contrasts between conifers and other trees in the northern hemisphere 
are mainly due to the differences in the maturity of the foliage. In the mixed forests of 
the tropics conifers are associated with evergreen trees so that there is no systematic 
| difference in the maturity of the foliage on any type of tree. I refer to the maturity 
rather than the age of the foliage for I have noticed that spruce and larch may also be 
indistinguishable by tonal characters on British photographs taken in mid-summer. I 
believe that in the natural forests of New Zealand, too, the coniferous trees show no 
| | consistent tonal differences from the many other evergreen species. 
The question of the scale of photography most suitable for any particular project is 
| one which can only be decided after consideration of many factors, not least of which. is 
the economic one. As most tropical photography so far has been taken primarily for 
topographie mapping purposes, much of it has been on scales of 1/30,000 or smaller. In 
studying forest vegetation on such photographs it is always tempting to think that 
larger-scale photographs would be much more helpful, and in particular that they would 
facilitate the identification of species. In fact this has very seldom proved to be the case 
[4, 10, 18]. Most tropical forest types are so mixed that it is not possible to describe or 
identify them on the basis of one or even a few species. In the relatively rare cases of 
gregariousness or single species dominance this character is reflected in the structure of 
the forest and can usually be recognised even on small scale photographs. As there is 
little variation in the canopy density of these forests and few opportunities for individual 
| | | height measurements, large-scale photographs have little advantage for ordinary type- 
mapping, especially since small-scale photography nowadays is capable of such high defi- 
nition. I find on the contrary that there is often advantage in the concentration of char- 
acters on small-scale photographs, and that, in using a stereoscope with variable magni- 
fication, the bulk of the work may be carried out at low power. After all it is one of the 
features of aerial photographs that they provide a bird's-eye view, which can reveal an 
order or pattern that is not always apparent at close quarters. There are obviously other 
limiting factors, but for the interpretation of tropical vegetation it is, I believe, especi- 
ally necessary to envisage each type as a whole congregation rather than to treat each 
one on the basis of its component species. 
Of course such other considerations as the amount of detail which can be recorded 
and the scale of mapping required to portray it must be taken into account. When the 
fine patterns of land use and cultivated crops are being investigated, medium or large- 
scale photography may be needed to show the essential detail and provide the space in 
which to delineate it. 
Optical magnification of the photographs under examination is a valuable aid in 
interpretation, but it may reveal more than the pen of the observer can cope with. The 
ability of the photogrammetrist to produce maps at scales considerably larger than that 
of the photography presents other users with the problem of using the photographs to 
the same intensity. Enlarged prints are one solution, but I am surprised that there has 
not been more development of examination stereoscopes capable of handling them. 
In the forefront of any photo-interpreter's equipment must be his store of know-