lying altitude too 
g machines seems 
s at much larger 
are not generally 
ithout reduction. 
egative scales for 
es 1s found about 
relation scale of 
r maps and plans 
more strict than 
r 1/4; it descends 
| very large tole- 
Remarks 
Plastic image 
1:10,000 
180 X 180 
F — 125 mm 
1:25,000 
:100,000 
2,000—1:4,000 
4,000 
125,000 
(351) 
As to the relation Base/Flight altitude, the ratio is determined by the gene- 
rally adopted 60% overlap. For square negatives and normal angle cameras the 
ratio varies between '/ and !/»s; for wide angle cameras the ratio B/H varies 
between 0.5 and 0. 
Establishment of the Control. 
1. Exclusively on the ground. 
a. Marking before measurement and photography. 
Certain services in the USA use this method partially; Some of the control 
points are measured on the ground before the photography and the re- 
mainder after the photography. The IGM of Florence utilize lime for pre- 
marking the control points but only for surveys of very high precision. 
In Switzerland for the Cadastral Survey all the triangulation stations 
are marked before photography thus diminishing the number of points to 
be specially measured. For the 1 : 25,000 and 1 : 50,000 scale national maps 
only a part of the triangulation available is marked in advance. 
b. Without marking before photography. 
This method is used in France by the IGN for the 1 : 20,000 scale and by 
the Cadaster for plans at 1 : 2,000. The Netherlands uses ground measure- 
ments for controlling its metropolitan maps. Also Canada where aero 
triangulation is not used except where sufficient ground control cannot be 
obtained because of terrain conditions. Ground control without premark- 
ing is the method most frequently used in the United States. 
2. By air triangulation or radial triangulation. 
One finds here variations of several methods, as some pairs are entire- 
ly controlled by preceeding models, or where only a superabundance of 
control is required, all the points being determined on the ground from a 
minimum number of (triangulation) points. 
The Netherlands uses air triangulation or radial triangulation depending 
on circumstances for tropical terrain. 
In the United States the Multiplex and the Stereoplanigraph are used to 
give planimetric positions of control points of which the height is determined 
on the ground, that is, the employment of air triangulation only in X and Y. 
In certain cases they use transverse strips of photographs adjusted to a standard 
curve to establish planimetric control at the ends of strips to be oriented. 
These transverse strips are spaced from 20 to 35 kilometers apart. In France 
aerial traverses (Special air triangulation) determines the position of an average 
of four or five models between those controlled on the ground for the 1: 
:50,000 scale maps of North Africa. Radial triangulation (Slotted templet 
method) is used for 1:100,000 scale maps of the territories, with heights deter- 
mined barometrically. 
Sweden employs aerial triangulation to break down the geodetic triangu- 
lation sufficiently for plotting. 
Italy employs air triangulation in territories using the solar periscope. In 
the mainland it is not needed except for short distances (8 to 10 km) to bridge 
a wooded zone without points visible on the ground.