ACQUISITION OF DATA IN AEROTRIANGULATION 
coordinates to better than 5 microns. 
It is not a vain exercise to emphasize the 
importance of the plan for taking the photo 
graphs. One is usually satisfied with parallel 
strips with the longitudinal and lateral over 
lap as small as possible for reasons of econ 
omy, but this small overlap makes it neces 
sary to select the pass points at the extreme 
edges of the format, in the areas where the 
resolution is poorest. Besides, this plan has 
the disadvantage of not permitting the cal 
culation of a block in the air, without using 
any known points: the transmission of lateral 
strip tilt transversely from one strip to an 
other is not really assured, since the tie points 
between the strips are nearly alined. The 
vertical control network on the ground should 
then necessarily include, at the extremities of 
each strip, the points necessary to define the 
lateral tilt, which would be an oppressive 
requirement, whereas an essentially rigid 
block can be supported by a small number of 
points distributed in random fashion. The 
adoption of a lateral overlap greater than 50 
percent provides this rigidity; if also the 
camera stations were alined laterally per 
pendicular to the direction of the strips, one 
would obtain an ideal arrangement (this 
correspondence can be assured easily enough 
by adopting a longitudinal overlap of 90 per 
cent and by choosing afterwards the photos 
which are suitable). But such an arrangement 
for the taking of the photographs has not been 
used very much, except for very limited 
tests, because of the excess of work which it 
imposes, notably in the instrumental phases 
as a consequence of the increase in the num 
ber of photographs to be treated. This in 
crease in work can, however, be maintained at 
an acceptable level by the use of super-wide- 
angle cameras, and it would be largely com 
pensated by the near-elimination of field work 
and by the gain in accuracy due to the redun 
dancy of the system. 
A less laborious solution for assuring the 
internal rigidity of the block is to use cross 
strips, long known and successfully tested in 
the research of the ISP on the Massif Central 
test area. It is truly surprising that such a 
simple and efficient technique is not used 
more often, 
It may be well to note that a judiciously 
conceived plan for taking photographs will 
effectively reduce the propagation of certain 
systematic errors: the taking of photographs 
in parallel strips with alternately reversed 
headings minimizes the effect of horizontal 
curvature and the effect of progressive varia 
tion in scale; if there are transverse strips, 
systematic torsion can also be eliminated. 
But no flight scheme will eliminate the ver 
tical curvature; this must be taken care of by 
elevation measurements on the ground or by 
the use of statoscopes. 
Instrumental Procedures 
From the earliest days, there has been a 
general tendency toward simplification of 
instrumental procedures, achieved at the cost 
of an increase in the volume of calculations. 
The earliest methods required the use of 
universal plotting instruments, with which 
the models of each strip were formed succes 
sively, the scale and the absolute orientation 
being effectively transmitted from model to 
model. But as early as 1948 there appeared at 
IGN the method called constant altitude in 
which no attempt was made to transmit 
longitudinal tilt, the Bz component being 
maintained equal to zero. Very soon the 
models were formed with the By component 
equal to zero and with a constant Bx compo 
nent in order to maintain as constant as 
possible the effect of the instrumental errors. 
Still later, the transmission of transverse tilt 
was also abandoned, ending in the method 
called independent models. 
The transmission of absolute orientation 
from model n— 1, n to model n, re + 1 can be 
done in two ways: 
(1) . By measuring the parameters defining the 
orientation of the camera n in each of the two 
models. These parameters can, for example, 
be the angles of inclination of the principal rays, 
referred to the reference system of the instru 
ment. If the instrument does not have graduated 
circles or levels for making this measurement, 
the orientation can be defined by the parameters 
governing two particular perspective rays, 
corresponding, for example, to two marks on the 
edge of the camera frame or to points marked on 
the photographs. It is sufficient to align these 
points monocularly, at an arbitrary distance z 
and to record the x and y instrumental coordi 
nates as if they were ordinary control points. 
(2) . The second procedure does not require 
any special measurements: the transmission of 
the orientation can, in effect, be calculated from 
the spatial coordinates x, y, z of at least two 
points in the two models, referred to the com 
mon perspective center. 
The two methods are not in all respects 
equally rigorous. The first involves only 
directions, and is not affected by errors of 
identification: the transmission of orientation 
is, in effect, based entirely on the reduction of 
the transverse parallax. The second, on the 
contrary, is concerned with points, intersec 
tions of two corresponding rays; but these 
rays will not intersect exactly because there is 
always some residual parallax, so that the 
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