4 
oxzy and terrestrial OXZY, the relations between the plane coordinates 
of the two systems result as being: 
( 1 ) X — k ( x. cos0+ y. sen 0) -f a 
Y = k(—x. sen© + y. cos0) + b 
where k is the scale ratio ground-instrument, 0 the angle formed by 
the two OX and ox directions and a and b two constants. 
The transformation of the coordinates from one reference system 
to the other becomes possible when one knows the unknown values k, 
0, a and b, which may be determined by having, in the initial stereo 
gram, at least two known planimetrie points. If a larger number of 
known points is available, this of course allows better to determine the 
said values, if employing the method of least squares in the calcula 
tion to determine them [ 5 ]. 
After finding out the numerical coefficients intervening in the (1), 
one can proceded with the transformation of the various x h y { co 
ordinates, in the OXY reference system, and with the consequent 
calculation of the discrepancies A X, and A Y, to be found on the 
other known points (those relating to the end stereogram and those 
along the strip) by means of the comparison between the known 
topographic coordinates and X*, Y* coordinates transformed by 
means of (1). 
The experience acquired in studying several instrumental bridgings 
of gridplates and actual photographs, carried out with the technique 
on the progressive altimetrical settlement of the models on the known 
quoted points, has often evidenced that the two k and 0 parameters 
acting in (1) are subject to an almost continuous variation during the 
actual carrying out of the bridging [3]. The discrepancies A X; and 
A Yj, can be considered essentially the function of this parameters, 
which at last result as depending from the x, y planimetrie position 
in the strip of points considered. 
Unfortunately it is not tGO easy to determine in each case, by 
using the only control elements at the end of the strip, the actual 
variation law of these parameters. If this were possible, analogical