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LACUNES DE LA RESTITUTION, DISCUSSION 181
graphers to the stereo model and the ease of
plotting the detail was varied. I vividly remem-
ber one discussion between an office photo-
grammetrist and a field topographer. “This
looks very good but how do you draw contours
in this dense vegetation?” asked the field man
sarcastically. “It is very simple, we guess at
them in the areas of dense vegetation” said the
photogrammetrist.
Several decades later our equipment has be-
come rather more sophisticated, but we still face
the problem of how much guessing should a
photogrammetrist do in stereoplotting. We
should not forget that it is the human operator
who puts the art into photogrammetry, for it is
he who interprets the photographs and translates
the data into useful and meaningful cartog-
raphic products. It is the map — the end pro-
duct — which justifies the existence of photo-
grammetry. The value of photographic maps —
regardless of how they are compiled — is eval-
uated in terms of how completely it meets the
requirements of the user. As Dr Le Divelec
notes in paragraph 1.7 of his paper: perfect
accuracy is not a possible human achievement.
Consequently, we must accept the fact that the
degree of accuracy and completeness we can
expect to be portrayed will be limited by the
nature of the photogrammetric techniques and
the ability of the human operator. I am firmly
of the opinion that the role of the human
operator is the most important link in photo-
grammetric procedures. I propose that it is the
most important factor we should consider when
we discuss the ways to overcome “omissions,
errors and photogrammetric plots. I cannot
accept the philosophy that voluntary gaps
should be accepted as part of a less troublesome
solution, and that the plotting of a feature which
is hard to discern leads to mistakes and should
be stopped.
We know from the mathematical theory of
statistical decisions and the probability of
making a correct decision, whether it be under
certainty or uncertainty, is not a simple opera-
tion, but neither is it so complex that it cannot be
solved by the stereo operator when necessary.
His development in this respect can be calibrat-
ed only if he is expected and required to make
decisions. If we reduce his task to a mere plot-
ting of sure features and allow him to stop the
plotting in areas which are difficult to interpret,
| am afraid that we shall develop a generation of
photogrammetric compilers who will be hob-
bling on the crutches of the pre- and post-editing
rather than inheriting the intellectual power to
solve the photogrammetric plotting problems.
Perhaps my feeling on the place of the human
operator and what should be expected from him
is influenced by my close association with the
field of photogrammetric mapping to support
the photogrammetric charting of constructions.
If we accept the limitation of aerial photography
and limit our photogrammetric plotting to fea-
tures which are definite, then it can contribute
very little support to surveying. We expect our
compilers to make decisions, such as where is the
location of the high water line; where it is possi-
ble we supply him with photographs taken at high
water stages and simplify his task considerably.
Where are the shoal areas which are dangerous
to navigation, such as sunken rocks and so
forth? What is the classification of the foreshore
and backshore areas? These are not features
which one can definitely see on a photograph or
as a definite image. To get these answers by
field pre-editing is a time-consuming task and
not resorted to except in the most complex areas.
Photo-interpreter techniques are the most useful
tools. We attempt to give the stereo compiler all
the information which is available for him to
make his decision and accept the fact that there
may be gaps in the plot. In this connection, we
feel that the aerial negative is the most important
source of information, and we concentrate upon
the photographic processing techniques which
enable him to extract the maximum information
content from the aerial negatives. For example,
prints and diapositives and plotting plates are
prepared with automatic electronic equipment
under strict photographic laboratory control.
In the final analysis we must consider the
total effort, that is the cost to prepare the map;
the efficiency is not increased if the effort is
shifted from the plotter to the field topographer.
Hindsight is wonderful but pre-checking is not
efficient unless we assume that the operator will
miss or mis-identify or omit certain features.
Photo-interpretation is extremely efficient.
We should collect and train stereo-plotters with
the ability and capacity to become good photo-
topographic interpreters. Let us not subscribe to
the concept that the topographic maps can be
efficiently prepared by semi-skilled stereo-
operators. A good-quality map in addition to
accuracy needs topographic expression. It is a
skilled art which must be cultivated.
In conclusion, I might mention that photo-
grammetry is one of the many tools which can
be used on the construction of a map. Its use is
always justified when it is the most efficient tool
to be used. As in everything else in the world,
we must always accept a risk. The amount of
risk which can be tolerated is a matter of per-