24 
The location of a model point A with respect to the origin 0 is determined by the azimuths 
a and 0 and the lengths of the arms AB and BO. The arms are equal - their length R is 
known. The orthogonal coordinates can be derived from: 
X = R (sin a+ sin 0 ) 
Y = R (cos a - cos0 ) 
The principle is realised, by means of stable metalic rulers (AB=BO=R=200 mm) and 
two glass circles with angular graduations (interval in centigrades and estimation of 
10 cc ). The glass-circles are located in the pole 0 on top of each other. The upper circle 
is attached directly to the arm BO, while the lower circle is connected via a parallelogram 
to the arm AB. The scales for a and 0 are projected, together with the corresponding 
micrometers, on a screen x25 enlarged for visual readings. For this purpose, an 
optical train is provided. The observation of height is obtained by means of a dial gauge. 
This read-out of a, 0 and h can be speeded up by adding an electronic read-out of the 
values. This system allows recording on typewriter, magnetic tape, punch tape or punch 
cards, which can be fed directly into the electronic computer for the determination of 
3 orthogonal coordinates of model points. 
The fine measurement is in units of l/l000th of an interval either of the linear height 
scale (in mm) or of the a - and 0-circles (in 1 grad). 
c). Attachment for aerial triangulation. 
Bridging can be performed with the PG2 by any of the following three procedures: 
I. Extension of height control with aeropolygon method, 
H. Bridging for planimetry and height with aeropolygon method, 
III. Semi analytical method by measurement of independent models. 
For application of these methods, the following accessories to the PG2 are required. 
i) . Autocollimation system. 
The function of the system is to enable the operator to define or to restore the initial 
position of the space rods perpendicular to the XY-datum plane. It replaces the mani 
pulation with bubbles as in some other instruments. 
Figs. 17 and 18 show the design. Two autocollimating telescopes (1) are firmly attached 
to the ocular head of the instrument. The line of sight of the collimator points to a small 
mirror (2), which is attached to the central gimbal system of each rod. The mirror 
follows all angular movements of the space rods. It is obvious that only in one position 
of the space rod will the reflected collimation mark coincide with its original. In order 
to obtain this in a vertical position of the rod, the plane mirror is adjustable. 
Fig. 17 shows how the autocollimators are localed in the instrument. 
ii) . Bridging microscopes. 
The device is needed for the transfer of scale in aerial triangulation by means of cali 
brated distances on space rods. On both space rods, fiducial lines are engraved at 
equal distances from the corresponding lower cardanic joints. To the sleeves of the