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.