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SIGNALISIERUNG, DISCUSSION 145 
Discussion 
Herr Dr HARRY: Ich danke dem Herrn 
Kollegen für seinen Rapport und móchte auch 
folgenden Herren danken, die in dieser Frage 
Herrn Pastorelli bei der Abfassung seines 
Berichtes beigestanden sind: Herr Dr Schirmer, 
Herr Schlumberger in Frankreich, Herr Van der 
Weele, Herr Weissman und Herr Konzett in 
Zürich. Monsieur nous avons entendu ce 
rapport. 
Mr W. T. Pryor: It has been my experience 
that photographic targets or markers, as has 
been described in this paper, are essential for 
accuracy in highway engineering. We are un- 
able to achieve the type of photogrammetric 
services that we need in highway engineering 
unless we use such markers, principally for the 
obtainment of horizontal accuracy and specific 
accuracy in elevations where bench-mark points 
are essential in the performance of the engineer- 
ing. 
Contrary, however, to the remark that the 
circle targets are better, we have found that the 
cross type targets are preferred; they give us 
better results, with the central one-third of the 
cross being of black or blood red material and 
the outer one-third of the cross being a light 
yellow or white material. We find that light 
coloured materials tend to spread and give us 
eccentric centres, whereas the black or deep red 
always holds its uniformity of position and, 
therefore, the white or the yellow is used merely 
to help us find the actual target that we have 
set for marking and the black and deep red 
constitutes the marker which is a cross. 
Mr S. G. MOLLER: Our experience is that 
the single dimension in general must be about 
45 microns measured in the negative, but I have 
seen in the papers that in other parts of Europe 
they are using single dimensions as small as 
15 microns, so I do not understand why we must 
have large dimensions in Sweden. It might be 
that we are quite near the north pole. 
Regarding the form of the signals, we have 
found that the conetic form is mostly used in 
Sweden, but it often happens that we make gross 
errors in the identification in comparison with 
stones or other things. The signal has three lines 
converging; cross lines or such signals might be 
better. 
Concerning economy, after many years of 
such work we have found that a signal is cheaper 
to arrange than to make comparable geodetic 
measurement. 
Docteur G. LE DIVELEC: Nous avons récem- 
ment fait une expérience pour signaliser des 
terrains désertiques dans la partie qui est au 
bord de la Vallée Verte du Nil. Le terrain dans 
ce cas est parfois blanc, et nous avons dû 
recourir à des signaux de couleur noire. Dans 
ce cas les dimensions des signaux qui auraient 
pu être de l’ordre de 25 cms si le terrain était 
noir et les signaux blancs, ont dû être portées 
à environ 60 cms, car le halo du terrain a réduit 
les dimensions photographiques du terrain. 
Merci. 
Mr T. J. BLACHUT: I think there is a certain 
degree of misunderstanding and, in particular, 
I cannot follow the consideration by our col- 
league from the United States. The question of 
signalisation is quite a simple one. The size of 
the target is in the first place a question of con- 
trast and I think this matter has been published 
in several publications. From an economical and 
practical point of view, of course, we try to keep 
our targets as small as possible, and that can be 
achieved by producing high contrast between the 
target and the surrounding terrain. 
A very interesting experiment has been ar- 
ranged by Professor Jackson demonstrating this 
fact. He used as a target not the flat white target 
but, so to say, small sputniks. He put a small 
highly polished ball on the bottom of boxes 
which had been painted with a highly matt black 
colour. As a result the tiny ball could be seen 
and photographed from a relatively high alti- 
tude. That is something which proves the well 
known fact that contrast is very important for 
the definition of the photographic image. 
As a result it obviously confirms what Dr Le 
Divelec mentioned; if you are working on a 
white terrain — for example, snow covered 
areas, glaciated areas, sand, mountains and so 
on — you have to use black targets and the black 
targets must be several times as large as white 
targets on black backgrounds. Therefore, I 
cannot understand the techniques used by our 
colleagues in the United States. 
Docteur H. HARRY: Il y a une difficulté. Les 
essais et les recherches ont démontré que la 
question des couleurs complémentaires joue 
vraiment un grand rôle, mais ceux qui font la 
signalisation ne peuvent pas prendre un magasin 
de signaux de différentes couleurs avec eux. Il 
faut simplement simplifier la signalisation — 
peut-être prendre avec soi deux types, un type 
pour le caractère général du terrain et l’autre 
cl 
N 
EES