122 CONVERGENT VERSUS VERTICAL PHOTOGRAPHY, DISCUSSION 
instruments. How many cameras lying around 
the world are really calibrated regularly? It is 
one of the advantages of the big services like, for 
instance, the Geological Survey and the French 
Institut Géographique National, and there may 
be others, that they get such excellent results not 
because of the marvellous quality of their equip- 
ment but because of the careful considering of 
each link in the total chain of the procedure 
from the moment that the plane takes off until 
the moment that the photograph arrives in the 
restitution instrument. I have the impression 
that so far that has been overlooked by many 
services. 
I have said before the war many times that if 
people ask me which instrument was the best in 
your institute, the Autograph A5 or the Stereo- 
planigraph CS, I said at that time that both are 
good but much more important is the question of 
the quality of both ends, that is the photographs 
you put in it and the man who is behind it. That 
is still the point. 
We have also heard something here from Mr 
Blachut about personal errors which go above 
and beyond the precision of the equipment, and 
We are in a position at present where we have to 
talk about those sources of errors. 
Mr T. J. BLACHUT: I would like to ask Mr 
Thompson one question and also maybe give a 
very short comment. I will start with the com- 
ment. It is, in our opinion, not only the question 
of calibration of cameras which must be, of 
course, very carefully done within certain 
intervals, but also the question of processing 
the film and conditioning the film. These re- 
quirements are at least equal in importance as 
the calibration of the camera. We proved that 
if, for instance, photographs are made imme- 
diately at the beginning of the strip of film and 
so on, one cannot expect reliability and homo- 
geneity of results. That also must be observed. 
As to the question for Mr Thompson, 1 
would like to ask him if he refers to cameras of 
identical performance when stating that the 
results are 20 per cent better? Because that is, 
of course, very important. 
Mr M. M. THoMPsoN. I would like to 
emphasise what I mean by 20 per cent better. 
In 30,000 square miles of mapping you do not 
get an answer, the accuracy is so much, you get 
a probability curve, and we got our probability 
curve in the form of a cumulative frequency 
curve, the S-curve. In plotting these probability 
curves the curve for a convergent, that is a com- 
posite curve for convergent photography, is 
displaced about 20 per cent. In general when I 
say 20 per cent it is the general amount that one 
probability curve is better than the other. 
Your question has to do with the use of iden- 
tical cameras, in taking the twin photography. 
The cameras, of course, are not identical, the 
tests are not laboratory tests, our tests are 
operational tests in which we use cameras that 
have been processed and approved in the 
calibration laboratory under Mr Bean, who 
may wish to say something additional about 
this. Every camera used in this work has gone 
through our optical laboratory and been ap- 
proved, but the cameras are not identical, of 
course, so we are not comparing identical things. 
But we are pragmatic about it, we are looking 
for results which we will be getting in operation, 
and of course these are not theoretical results 
but practical results. Mr Bean may wish to add 
something on that point. 
Dr SCHERMERHORN: First I want to ask 
members of the panel if any one of them can 
answer the question from Professor Hallert. 
I would like to say a few words about this 
problem; it is one of those facts we meet in 
photogrammetry every day, viz that there is 
very little known really about the fundamental 
points and problems in photogrammetry. We 
are all measuring, but many of the problems 
raised at present by Professor Hallert still re- 
quire a clear-cut answer, based on sufficient 
statistical material. But to get this is such a 
tremendous job, which gives so little pleasure 
to the operators that hardly any of us has started 
it. It is one of the aims of Commission F of the 
OEEPE to work together on this problem. This 
question raised by Professor Hallert seems to 
be one of these unsolved problems because also 
the members of the panel have no answer, what 
[ expected. Professor Hallert himself is the man 
who can give the answer. 
Professor B. HALLERT: As a matter of fact 
we have done some experiments from a high 
tower, we have taken photographs with oblique 
cameras. We have made a perspective grid on 
the ground in order to have the points on which 
one circles around the principal point because 
then the normal equations, you know, for the 
determination of the inner orientation can be 
made in advance. In that way we can distinguish 
between some important systematic errors and 
obtain the irregular errors as a standard error 
of unit weight of the image co-ordinates. I can 
tell you that the first experiments have given 
the same magnitude for oblique cameras as for 
  
verti 
whi 
com 
moc 
floo 
to 
Thc 
rap! 
20 
pho 
in | 
met 
ord 
gen 
mo 
mo 
ph 
eve 
are 
dat 
cor 
acc 
anc 
1/1 
are 
ver 
ac 
tin 
aei 
50 
the 
vo 
Tr 
nà