12 GIUSEPPE BIRARDI 
c) The case shown in fig. i should be of particular practical interest for 
small aero-photogrammetric survey agencies. The block consists of two stretches 
only, transversally adjacent, each having two axis points determined on the ground, 
with a total of 4 known points (which might be reduced to 3 if one point A falls 
in the area of transversal overlap). It should be noted that any block of strips 
may, in the calculation phase, be subdivided into elements of this type, main 
taining the essential advantage of a block formulation (very few control points 
located wherever one likes) and loosing very little in accuracy (due to the missing 
link with the adjacent elements). 
In this case, system 4) takes the following simplified form : 
5) Dlt - D*t + (Z{-= 
with as many equations as is the number of 7\ points, in the two y}, y 2 unknowns. 
If we assume : 
D l = a k 
Di = \ 
71 
7 2 
the resolving system will be : 
6) 
[b b] [al] — [a b] [b l] 
[1a a] [1b b] — [a b] 2 
2 [a b] [a l] — [a a] [6 l] 
X = [a a) [b b] — [a b] 2 
which results easy to calculate even by simple mechanical computers. We give 
below (para. 14, a) a complete example, made out for the 1 : 25,000 survey of a 
block of two stretches, possessing a total of 4 trigonometric points as control. 
Should such block be considered separately, it would be advisable to take the 
axis of each stretch as close as possible to the outside edge of the stretch itself, 
in order to avoid, or minimize, the cantilever areas. Anyway, the considerations 
made under para. 12 should always be kept in mind.