IO 
GIUSEPPE BIKARD1 
instrumental and calculation operations have been correctly performed. It will any 
how be advisable to have multiple axis points (i. e. that we have determined on 
the ground, for each of them, two or three photographic references) so as to dimi 
nish the effects of collimation errors, and to have a check againts gross errors; 
c) that the calculation of altimetry, based on two axis points only, should 
give as well equally reliable results, except the effect of the /} rotation. The 
above suggestions are valid also in this case, and it will be advisable to use, as 
axis points for elevation, the barycentres of the respective multiple reference points ; 
d) that the yj rotations should be sufficiently small, so as not to cause any 
secondary effect into the elevations. This will always take place, provided that the 
absolute orientation of the first pair of each strip is correctly carried out, and the 
total errors of the bridging are not abnormally great ; 
e) that in the calculation of the mean values of the coordinates for points 
Tn • , the correlations and weights are negligible. By means of this operation we 
destroy the rigidity of the model and — as far as planimetry is concerned — we 
take into consideration the liaison to the known points of the adjacent stretch 
only, instead of all those of the wTiole block ; hence the mean values thus obtained 
are surely not the most probable ones, though providing a value which is generally 
more reliable than those coming from each origin. 
ii. — Operations a), b), f) of para. 9 may be performed according to the 
methods described in the above mentioned Note ( J ), to which we refer (see also 
Annex 1). 
For operations c) and d) —- calculation of 7 rotations and successive correc 
tion of elevations — we should proceed as follows : 
a) for each point T x J- common to several stretches, and for any point A' 1 
of which elevation is known, we should calculate the planimetric distances D [ 
from the axis of the stretch, or stretches, they belong to, by means of simple for 
mulas of analytic geometry. I. e. : if we call X Y, X x Y x , X 2 Y 2 respectively the 
coordinates of the point under consideration and of the axis points, and S 12 the 
distance of the axis points, we have : 
X Y 1 
AT IT 1 
X 2 Y t 1 
D 
which may be written out : 
2) D = P Y — Q X + R 
wherein the P Q R coefficients, valid for each stretch, are given by :