134 GRAPHICAL OR NUMERICAL PHOTOGRAMMETRY?, DISCUSSION
of the results separately achievable from graph-
ical and numerical photogrammetry because he
has made those points very clearly in his paper.
However, it would be inappropriate if I did not
mention the fact that in engineering especially
numerical and graphical photogrammetry go
hand in hand, and so it could not be a case of
graphical or numerical. It is really a case of
graphical and numerical photogrammetry being
utilised throughout the separate stages of en-
gineering.
We can think about numerical and graphical
photogrammetry as separate subjects. We must
also recognise the fact that they are dependent
strictly on the recognition and interpretation of
the features which shall be measured, or the
features that we desire to measure, and therefore
it revolves around the ability of individuals and
the information which they desire to get from
the photographs, a choice of one versus another,
strictly from a utility point of view.
When graphical photogrammetry is utilised,
most of the results can be obtained stereoscopic-
ally to a better degree of accuracy and fuller
scope. However, sometimes non-stereoscopic
photogrammetry can be applied. It can be
applied more fully in the numerical than in the
graphical. The graphical results of photogram-
metry are always required in engineering regard-
ing traverses and profiles and cross-sections and
the plotting of cadastral data for rights of way.
The graphical may have come, of course, from
the numerical, and in many cases the reverse
is true. We first get the graphical and then we
convert it to the numerical.
The degree of accuracy required in each one
of the uses of these two types of photogramme-
try will depend on the stage at which we are
working. We have reconnaissance stages in
which the accuracy can be low because we do
not require precision in reconnaissance stages.
We are more interested in differences rather
than specific accuracies of points and detail. As
we get further along in the steps of our engi-
neering the accuracy increases and the scope
of detail decreases, and we get to the point
finally where we need to have precision to the
nearest fraction of a metre and sometimes centi-
metre. I do not believe that we need to go to
millimetres.
My time is up so I will just conclude by
saying that the graphical data and the numerical
data in engineering are actually inseparable and
that we utilise them at all stages of engineering.
It is not strictly a matter of direct selection of
one as against the other. It is strictly a matter of
correlation and utilisation as the work proceeds.
Mr Van der Weele has introduced a very
excellent subject.
Mr C. O. TERNRYD: Under point IV in his
invited paper Mr Van der Weele deals with the
problem of graphical and numerical photogram-
metry in civil engineering, and after some dis-
cussion he makes the statement that a combina-
tion may be the most suitable. He discusses the
problem of using photogrammetry in the de-
tailed phase of civil engineering work. The
reason for that is that there are so many differ-
ent means used for the preliminary planning
that no general procedure exists.
In our opinion and from our experience there
is a very great advantage in using photogram-
metry in civil engineering needs also in the pre-
liminary planning phase when it is a question of
finding the best preliminary alignment of a road
or a water power station. Therefore, the ques-
tion of graphical or numerical photogrammetry
also should include this phase for giving a com-
plete discussion of the question.
The two greatest problems of using photo-
grammetry in civil engineering are, first, to
evaluate the procedure which includes as well as
the preliminary phase the detailed phase, and
second — which is more important — to educate
the engineers to using the picture according to
that procedure. The procedure evaluated should
be built up so that the demand on a large survey
in the preliminary phase can be combined with
the demand of high accuracy in the results of
the design phase.
[ should like to give some short comments on
the preliminary planning phase as comments on
the design phase will be given by Dr Zarzycki. In
this phase, the preliminary planning phase, a
very great part of the problem can be solved by
performing the planning directly in the photo-
grammetric model itself without the necessary
mapping. This type of photogrammetry is some-
thing between graphical and numerical. The
main question is then to find the photogramme-
tric instrument that at the same time gives a
good survey over wide areas and a sufficient
accuracy in the method of numerical data. The
advantages of using the entire model lies therein,
that it gives a much greater amount of informa-
tion than a very detailed map can give you.
When planning roads, for instance, the model
gives the information as well as the topography
of the landscape as well as the geology and the
drainage of it. My paper presented to this Con-
ference has been distributed and it gives the
method of using the Kelsh plotter for the preli-
minary planning phase. That procedure gives
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