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CONVERGENT VERSUS VERTICAL PHOTOGRAPHY, DISCUSSION 121 
It seems to me that in these questions we 
have the real centre of the problem today, and 
no doubt Professor Schermerhorn agrees with 
me in this case, according to the statement in the 
publication. 
Dr SCHERMERHORN: I must ask the members 
of the panel, and first of all my American 
friends, to answer my question. 
Mr M. M. THoMPson: I will try. The ques- 
tion on the accuracy of the model involves in 
our estimation two phases, the theoretical accu- 
racy and the practical accuracy. The theoretical 
accuracy has been discussed frequently at 
great length by Von Gruber in his famous 
collected essays in the 1930s. It has been dis- 
cussed further by Professor Hallert in his pub- 
lications emanating from Ohio State University. 
We are interested not only in the theoretical but 
the .practical, we have taken the mathematics 
as derived by Von Gruber and carried further 
by Hallert in his publications, but what actually 
happens in practice? In 1954 Mr Bean pub- 
lished in ,, Photogrammetric Engineering” a 
brief article which was called “Use of the 
E.R.55", but it had to do with its use with 
convergent photography. It described the results 
of a test over Los Angelos, California, where we 
had a great deal of control, all street intersec- 
tions over a large area; flight height, 8,000 feet, 
mean square error, vertical error was 8/10 foot, 
which gives you H/10,000 as the vertical 
accuracy. 
We have done other testing of that nature, 
but that is an example. We did not go too far 
with mere testing, we went ahead with actual 
compilation of topographic maps. 1 would not 
be talking about three or four test compilations 
in the laboratory, I would be talking about 
30,000 or more square miles of actual topog- 
raphic maps compilation. For the vertical 
accuracy, of course, we have a testing pro- 
gramme which we are required by our regula- 
tions to conduct. I did not have the material 
available at the time I sent my memorandum in, 
but on 37 projects tested for vertical accuracy 
— these are projects not models, each project 
may consist of a great number of models — we 
found that the vertical accuracy with conver- 
gent photography was of the order of 20 per 
cent better than with vertical. 
I would like to add at this point that I tried 
to make it plain in my memorandum to Dr 
Schermerhorn that we do not think convergent 
photography is the answer to all problems. Very 
early in our application we came to a conclusion 
which we had rather suspected before, that in 
terrain of very high relief it loses its advantages, 
but we have found that it has great advantages 
in areas of moderate and low relief. 
I wish there had been time or space in Dr 
Schermerhorn's report to mention the matter 
of costs. We are very much interested in costs, 
and thus the reason that we went to convergent 
photography was purely a matter of economy. 
We want to map as much of an area as we can 
at as little cost as possible. The figures that we 
have concerning 30,000 sq miles about equally 
divided between convergent and vertical photog- 
raphy so that our costs with convergent photog- 
raphy are some 20 per cent better. Both the 
accuracy and the cost seem to come to a 20 per 
cent advantage. This applies to a ten-foot con- 
tour interval at a scale of 1 : 24,000 which is one 
of our principal mapping sets of parameters. 
I might go on and mention the point of cali- 
bration which Dr Schermerhorn mentions. It is 
certainly true that that is one of the great prob- 
lems, and perhaps the difficulties that we have 
had with calibration problems in convergent 
should make us all realise that we have been 
neglecting this problem quite a lot, along with 
vertical. We have had this bad film and bad 
calibration from the start, and this is just some- 
thing which brings it to a head. I believe that 
I have tried to answer all your points. 
Dr ScHERMERHORN: I thank Mr Thompson 
for his statement which is really an answer to 
my question. The value of it is that it is based, as 
he says, on practical work and that means it is 
of statistical value. That is the trouble with all 
of what we call here “essais controlés”, com- 
paring such a result with all the trouble we have 
had with Monti di Revôira. Then I say let us 
look first at what has been done by the services 
and look at their results. I am very grateful that 
Mr Thompson has now given the figures, which 
[ lacked before. 
I would like to stress one point again with 
Mr Thompson. There is the need for, let us say, 
calibration and regular calibration of the 
cameras and testing the material we put in our 
instruments. We are at present in a situation, 
and have been so for a long time, where the 
restitution instrument, and now still more the 
comparators, are in a state to do several times 
better than that of the average photographic 
material we put in the instruments. I mentioned 
only a few bad figures which are naturally 
extreme values, but what is the sense of talking 
about microns if we do not even know what is 
the quality of the photographs we put in the