criticisms, many of us speak nearly impromptu, and often express nothing but well-worn prejudices. However, 
that is not to say either that discussions are not worth holding or that everyone agrees with me about recording 
or publishing them. Nevertheless, I think we should now decide whether, if the intention, as I suspect, will be 
to hold a symposium during the next quadriennium, the discussions that will inevitably take place should be 
recorded. 
What can I say about the progress of the subject since the last Congress? Things tend to change slowly, and 
it is not perhaps easy when one is close to events to sense trends, but I have the general impression that there 
are signs of a crystallization due I think to the improvement and development of computers. For many years 
now there has been controversy about the correct methods of computing aerial triangulation, of which there 
appear to be three: the use of polynomials; the use of formed picture pairs as building blocks; and the use of 
independent perspective bundles. I hope that our discussions at this Congress will point the way to the most 
profitable direction in which to move. However, let me say that one point is clear. Much time and trouble has 
been expended in the past, not least by myself, in devising methods that show some economy in computation. 
I now have the feeling that this is no longer a worthwhile goal. Within reason, all computation is possible, with 
one exception, to which I shall refer below; and for serious aerial triangulation the goal is clearly to arrive at 
the method of computation that will give the most accurate results from given data. This is not a simple 
problem to solve in practice, for the main reason that real data differs so very much in its reliability. For this 
reason the Commission set up the Working Group to investigate the results of different methods of calculating 
from the same data. Our session on Wednesday is devoted to hearing the Report of this Working Group, and 
we shall have some time to discuss it, and, if necessary, I shall try to find further time for this most important 
subject. It would be wrong of me to give my views here, but even if we do not find we can arrive at a final 
conclusion, I hope that the Report and the discussion will serve to give us firm guide lines for further work. 
You will notice that we are still interested in the use of Auxiliary Data. It is obviously useful, but equally 
obviously its use gains little ground. Let us not be despondent: the same phenomenon can be observed in the 
progress of ground triangulation. When I first became acquainted with geodesy, auxiliary data was very little 
used; but it is becoming more and more popular, and the reasons are in some ways similar to those that obtain 
in the air. The Theodolite, like the air camera, is a simple straightforward instrument, and there has been a 
great deal of reluctance to complicate the acquisition of data with unreliable and awkward apparatus; it is more 
than enough that the aeroplane will fly, and that the photographer has taken off the lens cap on a fine day. But 
if installation is made easier, and the apparatus becomes more reliable, then auxiliary data must come more 
and more into its own. 
I have written elsewhere on the tendency of photogrammetric plotting equipment to become more and more 
expensive and complicated: not, I think, because this is necessary or inevitable, but because no effort is directed 
into the opposite direction. Much the same tendency is to be observed in aerial triangulation. Highly 
sophisticated methods are being developed, and are being used by a few organisations, but it is only too obvious 
that many practitioners are being by-passed. I may be told that this is precisely what happens in ground 
triangulation: only the largest organisations carry out geodetic triangulations. But the case is a little different. 
A geodetic triangulation occurs relatively infrequently (we have had two only in Great Britain in a century and 
a half) and covers a large area. The surveyor of topographical maps or large scale plans does not expect to carry 
out his own geodetic triangulation. But an aerial triangulation is a massive operation closely linked to the 
mapping itself, and the two processes are seldom divorced. If methods of aerial triangulation are not within 
the competence of the practitioner they will not be used, and this is precisely what is happening in small 
organisations throughout the world, although with some notable exceptions. Aerial triangulation can always 
be avoided if sufficient control is available on the ground; and, as the density of this control is reduced, the 
aerial triangulation methods become more and more elaborate and massive. Is there not a place for something 
intermediate between no aerial triangulation and the full orchestra? I know of workers who are considering, 
and have considered, the instrumental side of this problem: how plotting equipment may be used to acquire 
the data, or how data so acquired may be transformed to a suitable form for calculation. But this is only one 
side of the problem, and not perhaps the most pressing side. The average, self-styled photogrammetrist does 
not like arithmetic. I know of some to whom the division of the sum of four numbers by four is a hurdle to 
be avoided if possible. It is for this kind of man that something must be done, if aerial triangulation is not to 
remain an esoteric exercise for those of us who can count beyond 10. It is surely possible to develop methods 
that are less efficient in a statistical sense than the best methods, but which will give acceptable results with 
considerably less effort by a slight increase in the control density? The large organisations are concerned to 
reduce ground control to the lowest possible density, but is this necessarily the right line for all of us? I am sure 
there is here scope for those of our members whose ingenuity is perhaps not being fully exploited by the 
computer.