8 GIUSEPPE BIRARDI 
c) any control point, wherever located, offers its full contribution of stabi 
lity to the statics of the whole block, through a very simple conceptual and appli 
cative procedure. No particular distribution is required for the control points, and 
it is not necessary to have completely ground controlled pairs. 
Consequently this procedure, and the relative calculation program, may 
remain unchanged, whatever is the situation of the strips in the block (even that 
including longitudinal and transversal or oblique strips) and the distribution of the 
control points. Anyhow, location and density of the latter must ensure a sufficient 
stability of the whole ; to this purpose, the following points should be considered. 
8. — From a purely theoretical point of view, the ground elements which 
are necessary and sufficient to ensure the absolute orientation of the whole block 
are just those required for a single pair : i. e. two planimetrie control points, and 
three vertical control points, not on the same straight line. In fact, the internal 
constraints — considered as perfect — are sufficient to ensure the planimetrie 
and altimetric liaison of each stretch to all control points, and therefore to secure 
the absolute orientation of the whole. 
In practice, we need a much greater number of control points, especially for 
altimetry, whose internal constraints (unless particular means, like transversal 
strips, were adopted) are surely weaker than for planimetry. We deem it sufficient 
to have one or two known points for each stretch, whatever is their location inside 
the stretch itself; a stronger control would not so much improve the general accuracy. 
Anyhow, we should provide that the control points are uniformly distributed, 
as far as possible, and that a good support is ensured at the block edges, in order 
to avoid marginal cantilever areas. 
At para 14, c) we report, as an example, the calculation of a block of 15 stret 
ches, set up for the 1 : 25,000 survey of about 8 sheets (^ 800 km 2 ), based in total 
on 41 control points. The results have confirmed that the proceeding fully ensures 
the planimetrie and altimetric accuracy required for the 1 : 25,000 compilation. 
In the example of para. 14, d) we have gone even farther, calculating the same 
block with 5 control points onty (4 at the vertices, 1 at the centre) ; this has been 
done with the only purpose of checking the possibilities of this procedure at a 
limit situation as for control, below which it is impossible to descend. The results 
show that the planimetry is still fully acceptable, while the altimetry is noticeably 
worse, even if it is still suitable for less engaging purposes (for instance, for an 
expeditious 1 : 100,000 mapping) .Better results would have probably been attained, 
by imposing to the external equations a greater weight than to the internal ones ; 
this would have reduced the excessive disproportion between the number of the 
former and of the latter. Surely better elevations, at last, would have been obtained 
by introducing also transversal strips. 
The simplified procedure. 
9. — The above mentioned complexity of the rigorous calculation would 
forbid the use of the block of strips procedure to all people who have not available