PHOTOGRAMMETRIC ENGINEERING 
rotation thus calculated would be slightly 
different from that which would be obtained 
from the first method—besides, it would be 
directly affected by the errors in identification 
of the points common to the two models, and 
must therefore be considered as less reliable. 
The basic concept of the method of inde 
pendent models is that it is always better to 
make a measurement and derive a correction 
for it mathematically than to apply the cor 
rection manually. One is then justified in 
asking if it is really necessary to obtain a per 
fect orientation, and if it is not preferable to 
simply measure the residual parallaxes, the 
relative orientation being then refined by the 
calculations in the course of treatment of the 
data. To this concept, proposed by Inghilleri, 
one can object that the currently available 
instruments, even if they do permit measure 
ment of the parallax, are not generally 
equipped for automatic recording of this 
parameter so that it must be done manually, 
with consequent risk of error and waste of 
time. 
Pursuing this direction still further, one can 
eliminate relative orientation completely, and 
reduce the parallaxes, by means, for example, 
of By ,which can be recorded at the same time 
as the coordinates x, y, and z. These quantities 
define the directions of the two corresponding 
perspective rays and are equivalent to mea 
surements made on the stereocomparator: it 
is by this method (mentioned as a matter of 
interest) that the first trials of a so-called 
analytical method were carried out by I.G.N. 
in 1957. 
The Problem of Transferring Points in 
Analytical Methods 
The logical end result of this evolution is 
the measurement of photographic coordinates 
x,y for each control point on all the photo 
graphs upon wihch it appears, with the aid of 
a comparator. But it is not enough to make 
these measurements with the greatest pos 
sible precision; it is also necessary to be cer 
tain that exactly the same point is observed 
on all the photographs, because any error in 
identification will have exactly the same effect 
as a measurement error. It appears, then, 
that the transferring of control points is a 
critical problem in all instrumental proce 
dures and that this transferring should be 
done with a precision equal to that of the 
measurements. 
When they were first introduced, analytical 
methods were designed to duplicate mathe 
matically the operations that were normally 
done by plotting instruments, that is to say, 
the formation of the spatial model; this idea 
led to the use of stereocomparators which, by 
stereoscopic fusion, assured correct identifica 
tion of corresponding points in the stereo 
pair. But it is also necessary to be certain of 
the identification of points common to neigh 
boring stereopairs in the strip; stereocom 
parators having three plateholders have been 
designed for this purpose. But there is a less 
expensive solution, applicable to instruments 
having only two plateholders. In certain of 
these instruments, the movements of the 
plateholders are independent and the co 
ordinates x, y, and x 2 , y 2 are recorded directly. 
In passing to the next stereopair, it is easy to 
recover the previous coordinates on the com 
mon plateholder to assure identification, the 
stereoscopic setting then being accomplished 
by adjusting the other plateholder. However, 
experience shows that the execution of a 
setting with four independent movements is 
not very convenient. Other stereocomparators 
are provided with an xy-movement acting on 
both plateholders together and a differential 
movement dx, dy between them; it permits 
convenient setting but does not allow recov 
ery of a point already observed in the preced 
ing stereopair by simple resetting of the coor 
dinates. The IGN uses a stereocomparator 
designed especially to solve this problem; the 
recorded coordinates are indeed x, y, and x 2 , 
y 2 , but there is a differential control capable of 
adjusting the left or the right plateholder 
at will. 
There remains still the problem of identi 
fication of points on different strips, a prob 
lem which has taken on greater importance 
with the generalization of treatment in blocks 
by purely analytical methods. 
The simplest solution is evidently to mark 
all control points on the ground before taking 
the photographs, with targets visible on the 
photos. But this solution is not always possi 
ble; besides, it entails considerable work 
which is scarcely justified except for large- 
scale surveys, and it eliminates, at least par 
tially, the principal advantage ol aerotriangu- 
lation which is that it reduces the amount of 
field work. Another inconvenience, not neg 
ligible, is that it imposes difficult require 
ments of precision on the taking of the photo 
graphs if it is desired to have the points fall 
exactly at preplanned locations. Finally, 
experience indicates that pretargeted points 
are sometimes difficult to recover. 
On the other hand, pretargeting permits 
the use of a simple monocomparator, a com 
paratively low-cost instrument which gives 
perfectly independent measurements. 
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