10 
present in the center of the project area. The solution is, therefore, 
started at this point by solving the central sub-block and determining the 
orientational and positional parameters of all its 9 constituent photo 
graphs. All around this central sub-block, and one at a time, four 
adjacent sub-blocks are formed, each including a bordering row (or column) 
of three photographs from the previously solved central sub-block. The 
known elements of such photographs are retained in the solution of the 
newly formed sub-blocks. In this manner, these elements serve as control 
for the new sub-blocks. 
The process continues from the central sub-block, working out 
ward in a radial direction, until all the photographs in the block are 
exhausted. All the new sub-blocks should overlap the previously solved 
ones by at least three photographs each. 
3,3,2 Sub-Block Best-Fit 
While the previous method of sub-block extension assumes the 
presence of enough control in the central sub-block, the method of sub 
block best-fit is more general because it does not require this condition. 
The ground control can be widely distributed within the block and it is not 
necessary to have enough control in one sub-block for its absolute solution. 
Sub-block best-fit is a method that is similar, but not identical, 
to the method of strip bridging. It is composed of the following steps: 
a) Sub-block relative orientation 
b) Sub-block assembly 
c) Sub-block adjustment 
The nature of the solution by this method does not allow error 
build-up in any direction within the block. This is contrary to the case 
of sub-block extension which, while better than strip cantilever extension, 
still possesses the property of attaching one sub-block to another, thus 
allowing error propagation to take place. Sub-block best-fit, on the other 
hand, establishes each sub-block as a rigid three-dimensional model then 
brings all sub-blocks to a best agreement among themselves and a best-fit 
to the control. For these reasons, the rest of this study will be 
entirely devoted to the method of sub-block best-fit.