5 
ACQUISITION OF DATA IN AEROTRIANGULATION 
In the absence of pretargeting, one can use 
natural images, such as the corner of a build 
ing, the centerlines of crossroads, etc., but 
such details are rarely distinct and sharp, and 
they may have different appearances on 
different photos, making the measurements 
uncertain. Recourse to stereoscopic vision, 
and consequently to a stereocomparator, is 
quite indispensable, but this alone is not 
enough to assure the transfer of a control 
point from one strip to another. 
An attractive-appearing solution is the 
marking of the points on the plates, using an 
instrument like the Wild PUG or the Zeiss 
Snap Marker. There are two ways in which 
they can be used: 
1. Each point can be marked on only one 
plate of each strip, the pointing being done 
stereoscopically in a stereocomparator. But it is 
not very practical to make a precise stereo 
scopic pointing if one of the points is already 
marked, unless, as pointed out by Prof. E. H. 
Thompson, the shape and dimensions of the 
stereoscopic index are exactly adapted to those 
of the mark; in fact, it is well known that point 
ing by superposition of identical images is far 
less precise than pointing by bisection or by 
framing. 
2. The second manner of operating consists in 
marking all the points on all the plates. To carry 
out this operation, it is necessary to associate 
one plate, on which the point has been chosen, 
successively with all the other plates upon which 
the point appears, and to make a very precise 
stereoscopic pointing on each, the first plate 
being marked last. This amounts to the same 
thing as in the case of pretargeted points, and 
the measurements can be made very rapidly on 
the stereocomparator. Prof. Thompson has 
criticized this method on accuracy and economic 
grounds. It is certain that the fact that there are 
in reality two pointings, one stereoscopic and 
the other monocular, reduces the precision to 
some extent, to say nothing of the errors intro 
duced in marking the points or of the greater or 
lesser suitability of the mark made in the emul 
sion. On the other hand, regarding the economic 
considerations, one can hardly dispute the fact 
that the marker-monocomparator combination 
is appreciably less costly than most of the mono 
comparators now on the market, especially since 
a considerable number of markers can be associ 
ated with a single monocomparator. Prof. 
Thompson has justly stated that the slowest and 
most demanding operation is the association of 
the plates, two at a time, and their arrangement 
for stereoscopic examination: once this has been 
done, it is not much more difficult to measure 
the coordinates on the stereocomparator than to 
do the marking. But the difficulty is that if one 
adopts this method, the measurements must be 
made point by point rather than plate by plate; 
consequently, each plate must be put in place 
for measuring as many times as the number of 
points it contains, and, each time, it is necessary 
to point and to measure the coordinates of at 
least two fiducial marks, so that the measure 
ments can be related to the same system of axes. 
One can perhaps avoid these repeated measure 
ments if the positioning of each plate on the 
plateholder can in some way be recaptured with 
a precision on the order of a micron. But one 
cannot escape the manipulations necessary to 
associate the plates two at a time, which is a 
source of time loss incompatible with the 
efficient utilization of so expensive an instru 
ment. 
The method of photographic recording ap 
pears capable of solving the irritating problem 
of how to transfer points in aerotriangulation. 
It is on the Nistri ТАЗ stereocomparator 
that one finds the principal application of 
this technique; this instrument allows one 
to photograph on 35-millimeter film the 
image seen through the oculars—that is, the 
point with the reticle superimposed. This idea 
has been adopted in the SOM stereocom 
parator, with the difference that a reversed 
positive made from the film can be introduced 
in the instrument, examined in one or the 
other of the oculars, and fused stereoscopi 
cally with the corresponding area on the plates 
being measured. This resolved, at least 
theoretically, the problem of transferring 
from one strip to another. But, as a matter 
of fact, this possibility has never been utilized 
in practice at the IGN because of the com 
plications introduced in the operational pro 
cedure, the insufficient capacity of the 35- 
millimeter film magazine, the difficulty of 
obtaining a correct exposure, and the addi 
tional burden imposed on the photographic 
laboratories. The differences in scale between 
strips, entailing inequalities in the apparent 
diameters of the photographic mark and the 
instrumental mark, would probably have re 
vealed difficulties that appear more serious, al 
though not insurmountable. However, aban 
doned by the IGN, the idea was momentarily 
picked up by the U.S. Geological Survey. Ac 
cording to information submitted by Marvin 
B. Scher, consideration was given to applying 
the idea to an instrument of the monocom 
parator type; but this project, unfortunately, 
does not seem to have been brought to frui 
tion. 
A novel and somewhat surprising solution 
has been experimented with in the last few 
years by the Directorate of Overseas Surveys; 
it consists of using as control points natural 
details in the photographic image, the novelty 
lying in the fact that these details have very 
small dimensions (a few tens of microns). 
They are selected according to a somewhat 
complex procedure in which, after stereo 
scopic examination under moderate enlarge 
ment, the final choice is made under very 
great enlargement, but without precise stereo 
scopic fusion. The points are then marked by