for control both 
en geodetic con: 
e geodetic points 
10d has been ex- 
jon. 
lling wooded or 
ngulation is em- 
mmetric Service 
: 5,000 and one 
han five models 
ground. 
for 1 :1,000. 
1d sure plotting 
adjustment by 
irate horizontal 
use the number 
the number of 
control of the 
two horizontal 
spacial triangu- 
may be readily 
metry; it is not 
hat are furnish- 
its stations are 
ise in Switzer- 
juare kilometer 
ntal points per 
r. Occasionally 
1gles, direct or 
ical points per 
is 2 horizontal 
ital point well 
WO contiguous 
sually triangu- 
r determining 
maps at large 
ll scales which 
(353) 
are generally concerned with vast territories not yet mapped and very often 
without triangulation, the most rapid, least onerous and less precise methods 
are used, for example horizontal control by a radial triangulation adjusted to a 
geodetic or astronomical grid with heights from differential barometric mea- 
surements. This is the case for the French 1 : 100,000 maps of the territories of 
the French Union and for the Netherlands planimetric maps of tropical coun- 
tries. 
The graphic method used by the Italian IGM for obtaining horizontal 
points for plotting at 1 : 25,000 must also be noted. 
This report covers only the determination of control points on the 
ground. Commission III covers their determination by air triangulation. 
As to the precision sought in ground control, the triangulation and tra- 
verse procedures have been used for many years, and the instruments available 
are so precise that the largest errors to be feared in their measurements are only 
a small fraction of the acceptable errors in the maps. Moreover, in the data 
given by the different countries, the methods have been adapted to the scale of 
the map so that this condition if found everywhere. On the contrary, with the 
present techniques, control points obtained by air triangulation give the maxi- 
mum errors admissible in the tolerances and even exceed them in unfavorable 
cases, mostly in altitude measurements. 
Methods of Plotting. 
1). By independent models. 
Bridging is systematically employed with all instruments with which it is 
practicable, because of the considerable advantages of quite close absolute orien- 
tation and scale obtained from successive models. Orientation by independent 
models is employed only for the first model of a strip; the orientation of the 
following models is made by successive approximations with small corrections 
to scale and absolute orientation made from the control points. 
A special method is applied in Finland: orientation of each model by in- 
troducing the tilt values from images of the horizon on the negatives, then 
correction of the relative orientation, and afterward, absolute orientation is 
precisely completed on vertical ground control points. 
2). By successive connections of models of the same strip. 
Because of its advantages this method is very frequently employed except 
for the first models or when the plotting machines do not permit. 
The Italian report indicates that this process is not used systematically in 
Italy where the IGM employs it only for bridging wooded areas or those 
without control points. At the EIRA either method is employed. In Canada, 
independent models if control is complete for each model; bridging when not. 
It is necessary to remember that at least in certain methods, the connec- 
tions of models is used for aerial triangulation. In France this method is no 
longer used for aerial triangulation rather aerial triangulation by constant alti- 
tude and constant bases (see communication by IGN in Commission IIT). 
Relations between Scale of Plotting and Scale of Map. 
The scale of plotting is here considered as the scale of the stereoscopic 
image which may be different than the drafting scale.