The ultimate and most rigorous solution to the general problem of 
analytical aerotriangulation would result from the simultaneous solution 
of all photographs in the block covering a project area in one massive 
operation. This approach, however, is not practical from the standpoint of 
the amount of computations involved and the limitations imposed by present- 
day computers. For example, a medium size block of 200 photographs would 
require the forming and solution of 1200 simultaneous equations, which is 
not an easy task to achieve at the present time. The other extreme in 
analytical photogrammetry would be to limit oneself to the solution of a 
single photograph, which involves the reduction of a system of only six 
equations. This is also impractical from the standpoint of requiring 
enough ground control in each photograph. 
Between the aforementioned two limiting approaches lies a com 
promising solution. Square units assimilating the basic unit (the single 
photograph) may be formed by a multiple of overlapping photographs. The 
size of such units — hereafter referred to as sub-blocks — depends 
primarily on the capacity of the available computer. 
For absolute solution, a 2x2 sub-block involves 24 unknowns; 
a 3x3 sub-block, 54 unknowns; a 4x4 sub-block, 96 unknowns, etc. 
Of these various sizes of sub-blocks, the 3x3 sub-block offers some 
advantages : 
1. In reference (1) the author intro 
duced the concept of using stereo 
triplets rather than stereo-pairs in 
strip triangulation and pointed out 
its various advantages. It is 
obvious from figure 1 that the 
3x3 sub-block is mainly composed 
of two perpendicular triplets. 
(Hence the name two-directional 
triplets.) Therefore, with an area 
of triple overlap in both directions, 
extension of control could be more 
effectively carried out by making 
the best use of existing control. 
FIGURE 1