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LUS-
RADIAL TRIANGULATION, AUTHOR'S PRESENTATION 75
schliesslich der Beobachtungsfehler, wird durch die Tafel illustriert. Neue Re
methoden und die Möglichkeit von Anwendung der I.T.C.-Jer
thode werden signalisiert. Auf dem Gebiet der mechanischen
genannt die Verwendung von astronomisch orientierten, nicht lokalisierten,
Punktegruppen und die Anwendung der Bildschlitzmethode
Schliesslich erwartet man eine Anwendung der rezent entwie
lisierungsmittel in der Radialtriangulation.
chnungs-
ie Blockausgleichungsme-
Radialtriangulation wird
örtlichen
auf Konvergentaufnahmen.
kelten sehr genauen Stabi-
Author’s Presentation of the Paper
I propose just to give a short introduction as
we are already behind‘ time, and first of all I
would like to point out that, as most of you may
know, radial triangulation has a certain limita-
tion, in that it gives only the planimetry as con-
trasted with spatial triangulation which gives
planimetry and height. This means that the
application of radial triangulation is restricted
to special cases which, by the way, occur rather
frequently.
Among these special cases I will mention a
few. First of all, the case when only the plani-
metry of flat, or rather flat, country is required,
as for instance for a cadastral or statistical map.
These may be maps of rather flat terrain, as I
said, but it is equally possible that also maps of
more undulating regions, or even hilly country,
may be made and can be made by radial trian-
gulation.
The second case is, when for showing the
shape of terrain, it is sufficient to use form lines
instead of contour lines, and these form lines
could be put into the map by using very simple
methods, such as table stereoscope, and the
like.
The next case is rather an important one,
and a frequently occurring one, and one which is
more or less the opposite of the second case:
that is, when the requirements for height ac-
curacy are so high that it is impossible or un-
economical to determine these heights photo-
grammetrically, so that they have to be deter-
mined by classic ground survey. This may be the
case for special maps for irrigation and drainage
purposes.
On the other hand, apart from these natural
limitations of the radial triangulation method,
there are certain advantages of the method. In
the first place, the equipment which is necessary
to carry out radial triangulation is inexpensive
as compared with the equipment for spatial
triangulation. This is quite evident in the case of
mechanical radial triangulation, but it also
applies in a more restricted sense to the in-
Strumental, or what is called numerical, radial
triangulation.
The second advantage — a little one but I
should mention it for the sake of completeness —
is that there is no need to compensate for radial
distortion.
A third advantage, which is rather important,
I think, is that it has a greater speed of opera-
tion, no relative orientation having to be carried
out. To give you an idea of the speed of opera-
tion, the observation or rather the measurement
of one pair of photographs in the radial trian-
gulator — I am referring to numerical radial
triangulation — takes only twenty minutes,
whereas relative orientation in spatial triangula-
tion normally takes at least one hour.
Then there is the less high requirement of
skill and experience of the operators, for the
only thing they have to do is just to measure
by applying the principle of stereoscopy, no
relative orientation being carried out.
The last advantage is that the computations
in radial triangulation are very simple, so that
if necessary they could be carried out just by a
table computor, although it is preferable to use
an electronic computor in order to speed up the
work. ;
There are two types of numerical radial
triangulation: the principal point triangulation
and nadir point triangulation. The principal
point triangulation is mostly used because of the
ease of determining the location of the principal
point of the photograph which is then used as a
centre for the radial directions to be measured.
It has a disadvantage that these directions
measured from this point are affected by sys-
tematic errors which are a function not only
of the inclination of the photograph but also of
the ground elevation. The nadir point can be
found if the inclination of the camera is known,
for instance with the horizon camera or a gyro-
scope. In the meeting just before this, we heard
that it is now possible within an accuracy of
about two to three minutes of an arc only. When
measuring this nadir point the direction can be
easily reduced for this remaining inclination,
by a simple formula which does not contain
ground elevation. This does not mean, of course,
that these corrected directions are errorless, for
the inclinations from which the corrections are