5 
in no way inferior to black-and-white photos as regards their photogrammetric properties 
and the structure of the developed layers; on the other hand, they do not induce so much 
eye strain when used in stereoscopic instruments. That is why they are used both for 
interpretation and photogrammetric purposes. 
It would be premature to make any definite statements on the economical effectiveness 
of aerial colour photography. The negative process calls for little, if any, additional ex- 
penses; it is the positive process, that raises the cost, colour-paper being much more 
expensive and the procedure more laborious. The increase in the total cost of an aerial 
survey must be about 10-20 per cent. 
II. The use of aerial colour photographs. 
In forest and tundra studies spectrozonal aerial photos are most commonly adopted. 
As a result, the greater part of interpretation process has become office work. On spectro- 
zonal photos not only the kind of wood but also in some cases the age of the trees is 
identifiable. Fir forest stands show dark green, pine — green, birch — greenish-yellow, oak 
— yellowish-brown, aspen — brownish red, etc. The vegetation in clearings and areas left 
after wood-fire, as well as in swamped places is quite distinguishable. Moreover, the surface 
of the earth that is under the canopy of tree foliage stands out better on spectrozonal photos 
due to the difference in colouring between the surface vegetation and the tree-tops. This is 
also true of streams and damp places. Tundra landscapes produce a multicoloured effect on 
spectrozonal pictures: the polar birch shows greenish brown, the willow — dull brown, moss 
looks dark green, lichens come out light green, while moss-and-peat bogs are bright green, 
and grass marshes yellow-green. 
Aerial photography is executed for forestry purposes on several scales of which the 
most common is 1 : 25,000. 
The greater cost of colour pictures is offset by both an improvement in the quality 
attained and by a reduction in ground work required. For instance, when mapping forest 
and tundra areas that are difficult of access the use of spectrozonal films enables the 
amount of interpretation in the field to be greatly reduced as compared with the cases when 
black-and-white pictures are employed; it should be noted that the cost of field interpre- 
tation is three or even four times as high as that of aerial photography proper. 
By employing spectrozonal photos in forestry a good deal of money was saved as 
against the estimate that had provided for the use of black-white films. 
In geological investigations both the CN and the SN-2 films are used, but the former 
is more frequently preferred for the reason that it is much easier to identify the kind of 
rock or soil on prints where their natural colours are reproduced. 
Summer landscapes in sand deserts are characterized by a lack of colours; so when 
dealing with them three-layer films are preferable, for details that are hardly discernible 
on black-and-white photos stand out fairly well on colour prints. Brownish-greys and 
greyish-yellows are characteristic of different kinds of sand. The optimum zone for 
sandstone is the visible part of the spectrum within 520-600 m. 
Sparsely growing shrubs show as greenish dots. In general, colour photographs enable 
the morphological type and the consolidation of the sand to be identified as well as shrubs, 
semi-shrubs (possible of classification into groups) and surface details, e.g. paths. For 
unwooded mountainous areas as well as alpine landscapes three-layer films can be used to 
advantage. Not only differently tinted outlines of bare rocks and grass covered slopes or 
the structure of ice-field surface are identified more easily and in fuller detail on these 
films, but also paths and features obscured by shadows can be recognized. These latter are 
generally lost on spectrozonal photographs because the application of dense filters, which 
is unavoidable with such films, prevents them from being affected by the short-wavelength 
region of the spectrum, which is the very one characteristic of shadows. For wooded moun- 
tainous areas the CH-2 film is employed.