2 
operational practice best suited to contain and reduce the extension 
of unavoidable errors. 
Experience has anyhow shown that the accuracy of the aero- 
triangulation may be increased, within certain definite limits, by suitably 
limiting the lenght of strips and increasing the control possibilities [2]. 
The practice most frequently employed in applying aerotriangu- 
lation to blocks of strips is that which, originating from the classical 
preparation on the ground control of all stereograms, divides the various 
strips alternatively into main and secondary ones. The former undergo 
bridging, the latter do not [3]. 
This practice obviously requires a special care in the preparation of 
the planimetric control of the strips which have to be bridged. This 
preparation must in fact result very accurate (4 or 5 control points 
in each of the end stereograms and a few others along the strip) and 
should then allow to determine, in the most convenient and suitable 
manner, also the minor control points indispensable for stereoplotting 
of the intermediate filling strips. 
It is clear that such a preparation may sometimes cause a note 
worthy increase of field operations, particularly when the specialized 
staff is greatly burdened by levelling operations. 
A solution attempting to minimize the cost of this field preparation 
may in that case be suggested after more carefully evaluating the 
possibilities offered by an integral appliance of aerotriangulation to all 
strips of the block under consideration. 
In this respect it should be noted, however that although this 
solution on the one side reduces the cost of field operations, on the 
other hand it increases the cost of aerotriangulation, practically doubling 
the number of strips to be dealt with. 
Applying aerotriangulation to a whole blocks of strips further 
implies a more extensive elaboration of the observations made during 
bridging operations. The final success of this method is in fact a function 
of a good connection between the various strips realizing their con 
tinuity. 
Considered in its strict formulation, the problem of the blocks of 
strips may often entail much larger burdens than the simple field 
preparation of the main strips considered so far. This does not apply, 
of course, to any approximate solution which, trying to harmonize