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GRAPHICAL OR NUMERICAL PHOTOGRAMMETRY ?, DISCUSSION 137 
for political or other reasons. This kind of 
thing can be expressed one way by a probability 
or in another way by a sign, by a particular 
alignment. If one wished to go far enough with 
the programming I am quite sure that these 
considerations could be included, particulary 
if there are areas which are denied. These need 
merely be specified by co-ordinates and the 
eventual design would be constrained not to 
pass through those particular zones. 
Mr W. T. Pryor: I have a comment to make 
regarding the question Mr Van der Weele has 
just posed to the Group. It has been my ex- 
perience that it is often overlooked what is the 
experience and judgment of engineers in making 
decisions from looking at aerial photographs 
themselves, so consequently I believe that the 
majority of the major decisions concerning 
routes and topography, land use, political con- 
siderations, and so on, can often be made by the 
mere examination of the aerial photographs 
themselves, and the establishment of a corridor 
in which the detailed design, as outlined by 
Professor Doyle, can be made. 
It has been my experience that if we try to 
include all the routes alternatives on the basis 
of electronic methods, the cost of producing and 
assembling the data to be placed in computers 
for programming is so great, that the cost in 
terms of time at least, if not money as well, is 
too great. Therefore, the digital data processing 
methods as described are more likely to be 
applicable to the specific corridor which has 
been selected after using engineering judgment, 
after stereoscopic examination of the photo- 
graphs themselves. I have prepared a paper and 
I have included the process which goes into that 
sort of thing. I am not trying to belittle the 
process of the digital terrain model, it is a good 
method but it is a last-stage method rather than 
a first-stage method. 
Mr F. J. Dovrk: I agree entirely. 
Mr A. J. VAN DER WEELE: Time is up and we 
have to close this session. 
I should like to make a concluding remark, 
namely that although it seems from a technical 
point of view to be possible to automatise com- 
pletely the procedure for computing engineer- 
ing projects, by means of photogrammetry, there 
remains still some doubt about the economy of 
the procedure, the cost of the type of apparatus 
which will best be fitted for a certain job and the 
results which can be obtained. But I still think 
there is a task for Commission IV for the next 
Congress, to collect more data in this respect 
and give some more specific advice.