2. The 3x3 sub-block exhibits two-dimensional symmetry about its central 
photograph. This fact is made use of in the course of sub-block tri 
angulation, as will be mentioned later on. 
3. The number of 54 unknowns (which, incidentally, could be reduced to 47, 
as will be explained later in the text) is not beyond the capacity of 
the present medium-size computer. 
Because of the advantages mentioned above, and the fact that a 
3x3 sub-block constitutes optimum strength and built-in accuracy, the 
rest of this paper will be devoted to the detailed analysis of the 
different facets of sub-block aerotriangulation, based on 3 x 3 sub-blocks 
as units. 
3. SUB-BLOCK AEROTRIANGULATION 
3.1 General 
A critical study of the various methods of strip triangulation 
yields the following remarks: 
(1) Within each strip, especially in the case of cantilever extension, 
there exists unavoidable propagation of systematic errors. In addi 
tion, even if the absence of systematic errors is assumed, model 
warping still prevails due to random errors.^2) 
(2) Model dis-continuities resulting from the commonly termed "hinge 
effects" between adjacent strips are frequently encountered owing to 
the provision for only 20% side-lap. 
The new approach of sub-block triangulation, however, avoids 
these drawbacks of strip triangulation. (For detailed discussion see 
references 3 and 4.) This is evidenced by the fact that each sub-block 
exhibits optimum internal strength as a unit. Furthermore, the methods of 
triangulation by sub-blocks, given below, will minimize error propagation, 
model warping or hinge effects. 
3•2 Mathematical Formulation 
The fundamental mathematical relationships most suitable for sub 
block triangulation are the well-known co-linearity equations of a single 
ray.^» 0 ' Figure 2 shows the basic geometric construction of one ray