136 THE DESIGN OF PHOTOGRAMMETRIC PLOTTERS, DISCUSSION
action of the large pattern. Also the rapidity and
accuracy with which orientation cycles can be
performed favours the use of more cycles and
shows a more complete clearance of the Y
parallax. The stereomat can be applied to exist-
ing plotting instruments, but the speed and accu-
racy of the resulting operation is limited by the
optical and mechanical limitation of the instru-
ment chosen. Optical aperture is the fundamen-
tal factor affecting stereomat performance and
is the principal limitation governing its function
with plotters. Inertia governs the size and com-
plexity of the mechanism required to move the
carriage and floating mark assemblies. Also the
acceleration forces during fast plotting are a
function of the inertia and these functions may
disturb and reduce matching adjustments. Model
size is the most serious factor affecting inertia
since it determines the size and weight of the
carriage assembly and also the distance it moves.
Inertia varies between a third and a fifth power
of the model size. The Helava plotter seems to
be an ideal basis for the automatic stereo in-
strument. The overall construction permits large
apertures to be employed and plotting is at
photo scale. The mechanical construction can be
light and rigid and various control adjustments
were not subject to disturbing effects of machine
vibrations and stress.
Mr U. V. Herava: I would now like to call
on Mr Blachut to speak on the use of this new
type of instrument.
Mr T. J. BLacHuT: Speculation on the use of
the roll end on the scope of a piece of equipment
that is not yet built is always connected with
certain risks. The risk is still greater if there is
little experience on which to base such specula-
tion, since the instrument as it is the case is
based on new principles and presents a complete
departure from the familiar technique. How-
ever, we are discussing now this particular sub-
ject and I do not see any reason why we should
not be permitted to perform such a speculation.
First of all, I am thinking that we should
refer to a family of the instrument and not to
one single piece of equipment. As far as analyt-
ical plotters are concerned, the different orders
of plotters could be derived by a simplification
and not by a basic change in the design of the
equipment. That is quite a fundamental differ-
ence with the conventional type of plotters in
which, as you know, each plotter of a different
order is altogether an entirely different instru-
ment. That, of course, will have important
practical implications on the operational work;
it will lead to the unification which is a most
desirable feature and also it will lead to simpli-
fication and unification of the operations.
Another very important characteristic feature
of the family of instruments which are called
analytical plotters will be flexibility in intro-
ducing and using different photographic material
and in the introduction of various corrections.
At the present time it is planned to build into
the computor corrections for earth curvature,
for distortion, for atmospheric refraction, for
film shrinkage and for other corrections; in
particular, I have in mind the corrections which
we call global corrections, which will be empiri-
cal corrections, including all possible constant
errors involved in the photogrammetric opera-
tion, even if they are not symmetrical, but which
could be built in such a future plotter.
Maybe less obvious but not less important
are probable changes that may be brought about
by the fact that the relative and absolute orienta-
tion of any stereomodel can be stored within
the observation accuracy by simply setting
orientation elements on the plotter; it is some-
thing which Mr Helava mentioned in his talk.
There is not too much time to elaborate on it,
but it appears that in future it may be possible to
use directly the elements of orientation on the
instrument and even to do away with ground
control points.
[ think if you would like to express in a few
words the most important characteristic of this
type of plotter then we would agree that the
flexibility in introducing all sorts of corrections
is probably the most important and most
characteristic feature which should lead in
future to the ultimate accuracy in photogram-
metric mapping.
All in all, plotting instruments based on
numerical methods are new factors, not only
from a purely instrumental point of view but
also from the point of view of practical opera-
tions. Conventional plotters which are often
wonders of mechanical perfections have reached
the limit of their performance because of the
limitation in the physical projection system on
which they are based, and not so much because
they are not accurate. If we intend to advance
the photogrammetric techniques beyond the
limits of the tools used at present we must design
instruments not only better qualitatively but
more efficient because of their principle.
The PRESIDENT: I am afraid time is up and at
this time of the day there is no place for discus-
sion.
The meeting was continued later in another
room, under chairmanship of Mr Helava, but no
adequate record of the discussion is available
for publication.
8