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