CONVERGENT VERSUS VERTICAL PHOTOGRAPHY, AUTHOR'S PRESENTATION 119 
results, however, proved to be hardly any better. This gives the impression that also 
here the inner orientation could be the cause and that the camera is guilty! 
The conclusion of all considerations is that at present a fully justified comparison I 
of convergent and vertical photography is impossible. As long as we have such a great | 
difference in precision between the various experiments the geometrical proportions are I 
of a very limited value. The exception is perhaps the use of the twincamera in aerial | 
survey for highway construction, since there a larger scale of photography has less | 
influence on the economy. 
Nevertheless it must be considered an important problem to find out which are the 
real qualities of convergent photography in order to determine in which cases its appli- 
cation will have advantages above vertical photography. I agree, however, fuly with a 
statement, which Hallert made in a letter to me on April 14: “Special tests of convergent 
aerial photographs should be performed for determinations of systematic errors and 
estimations of the residual irregular errors. 
Measurements of y-parallaxes in connections with restitution should regularly be 
performed (in particular the residual parallaxes after finishing the relative and 
absolute orientation). The theoretical connections between the errors of the fundamental 
operations and the final results should be derived for different assumptions”. 
It is evident that all this is missing. Perhaps there will be a chance to fill this gap 
by the proper use of the new material taken in Germany for the commissions B and C 
of the O.E.E.P.E., among this material are a great many convergent models as well as 
vertical photographs of different types. The sense of this paper is then no other than 
to show how poor the position of research in photogrammetry still is at present notwith- 
standing all the theory of errors published during the last 20 years. 
Author’s Presentation of the Paper 
in the Meeting held on Thursday, 8th September, 1360 
  
Mr President, Ladies and Gentlemen, first 1 
have two remarks. I regret very much that not- 
withstanding the fact that the first two issues 
of “Photogrammetria” were ready on 16th 
August, and have been sent from Amsterdam, 
they will perhaps arrive today in London. They 
were kept in Holland until 2nd September some- 
where, and after that time, between the Hook 
of Holland and this building, I do not know 
where. However, it has been promised that they 
will be arriving today for those who want to 
have them here. 
The second remark is that I will try to make 
this a proper discussion and not a series of 
lectures prepared at home. This means that I 
will make a few short remarks and then ask the 
public to raise questions, not to make speeches 
but only to raise certain questions which must 
be answered by the members of the panel. Then 
perhaps there might be a short discussion about 
the same questions amongst the members of the 
panel who perhaps also can reply to some of 
my questions because there are quite a lot that 
remain. 
The conclusion of my story is that at the 
moment when I finished this report there was 
not enough sufficient reliable data available to 
evaluate the merits of each of the two systems. 
If you look at the geometrical position, that 
viewpoint is without any doubt in favour of 
convergence. You need less pairs and the 
coverage is larger, but that is only one viewpoint. 
Another viewpoint is the precision which can be 
obtained. A second consideration is the time 
necessary, and the last consideration is the over- 
all cost of the equipment, because we should 
remain with our feet on the ground in all photo- 
grammetric problems. 
What I have said already from the geomet- 
rical point of view is not so difficult. Bruck- 
lacher has delivered a paper to me which I 
have included, in its main points, in my small 
report. There you find the advantages from the 
geometrical point of view. But I went to pieces 
entirely at the moment I tried to evaluate the 
precision. I regretted very much that, from let 
us say the most important colleagues, at present 
advocating convergent photography, the US. 
Geological Survey, I did not get any more com- 
ments on this aspect, as it was such a good 
method. That is what I knew already. We have 
been using it in Delft since 1932, but I might