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paper we hope we have succeeded in giving some aspects of this 
vast subject. 
Naturally, attention must also be paid to the many pro- 
blems which still have to be investigated, and these are some 
which we hope to be able to take up in the future: 
- research must be done towards defining a system of control- 
points, a system that would be useful for photographs of vario 
us kinds with a high degree of precision and stability; and this 
is not an easy problem to resolve, as other researchers have 
already said; the solutions that we have experimented with will 
still undergo changes that we hope will constitute improvements; 
- more research in depth is necessary on the problem of deter 
mining the distortion curves and subsequently correcting them 
three-dimensionally with a type of non-linear interpolation 
(here, too, the proposals made by different researchers vary). 
- the use of converging exposures creates a whole series of pro 
blems of considerable interest, involving also the aspect of 
the preparation of the computer programs that can be used, 
and the precisions that can be obtained etc, 
- we think that the use of analytical plotters can also open 
up new prospects. 
The present limitations and difficulties relating to the 
instrumentation and to the methods used by us, (all of which 
must certainly be improved) should not stop us from looking 
far ahead, though with the realisation that if we want to do 
precision photogrammetry there will always be many problems, 
and our approach, the devices and the methods used, must all 
be equally subtle. 
The authors wish to thank Mr. G. Malan for his invaluable 
co-operation during the period of these experiments.