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Signals of classes b) and c) (cardboard plates of 20 X 20 or 30 X 30 cm respectively) 
are generally considered as the most favourable signal forms for medium scales of up to 
1 : 12500. Signals of class b) (30X30 cm) are still measurable in scales of up to 1 : 15600. 
The operator considers signals of class a) (cardboard plates of 40 X 40 em) in a 
scale of 1:12500 and larger, too large for a precise measuring because of their halo. 
The question as to wich actual precision results from such signals has not yet been 
examined. For small photo-scales these signals are considered as very favourable. 
The measuring precision of the signals in the large photo-scales (up to 1 : 6000) is 
influenced by the deformation of the signals owing to image movement (time of exposure 
1/250th second). 
In general the measuring of heights can be carried out better and with more pre- 
cision on the terrain surface than on the signal. 
The Centres indicate 1,3 to 3 minutes for the time necessary for the pointing of a 
signalized point in two cycles (forward and backward measurement). The duration of 
the pointing depends on the instrument — especially on the fact whether or not the in- 
strument has automatic registration of coordinates — on the operator and on whether 
the restitutions by a Centre have been carried out by one or by several operators. 
Furthermore the time spent by the operator in searching points which are difficult to 
identify, has had much influence. Moreover, the average times for pointing differ accor- 
ding to the strips: for the small scales more time is needed, because the identification is 
more difficult due to the small size of the signals; for the large scales the longer distan- 
ces between the points, as well as the fact that the signals are too large — so that the 
pointing becomes more difficult — have a retarding influence. The average time of 
observation of a single point is generally more for the forward measurement than for 
the backward measurement, because a certain time is needed for the search of those points 
which are difficult to find or to identify, whereas the points to be measured are already 
known when carrying out the backward measurement. 
The control points to be used for the restitution have been indicated to the Centres. The 
tendency existed to limit as much as possible the total number of control points which, 
with their coordinates and terrestrial heights had to be communicated to the Centres; 
otherwise the Centres would have had too many possibilities of comparison due to the 
multitude of control points resulting from the large amount of strips on different scales. 
Therefore one has renounced to select the control points always in their optimum posi- 
tion with regard to the surface of the model: moreover, one has renounced to choose the 
control points in such a way that for the adjacent models the same control points at the 
edges of the models could be used. The situation of the control points, which is partly 
unfavourable, must be taken into account when judging the precision of the results. 
Commission C of the OEEPE hopes to be able to publish at the SIP Congress in 
Stockholm the first results regarding the planimetric and altimetric precision which have 
been deduced from the comparison of transformed photogrammetric coordinates with 
terrestrial coordinates. 
Following the resolution formed by Commission C on May 11 and 12, 1955 a new 
area of 600 X 600 m in size and with about 300 signalized control-points has been 
covered by aerial photographs on October 26, 1955. This area is situated in Austria 
(Vorarlberg) in the region of the experimental field of Commission B. The photographic 
flight was simultaneously performed with Wild-Aviotar RC5 and Zeiss-Topar RMK 18/18 
in photoscales 1 : 5000 and 1 : 10 000. 
In that area the Austrian Bundesamt fiir Eich- und Vermessungswesen has arranged 
the control-points in a particularly regular way, and moreover it has laid out a great 
variety of signals compressed into a very narrow space with a view to perform a special 
study. This experimental area, therefore, will offer the possibility of investigating the 
ratio of the plotting accuracy to the location of points within a model, and of studying 
the most suitable method of signalization. 
Up to the present, we have not yet begun plotting the new aerial photographs.