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The Maximum Bridging Distance (mbd) to be allowed in a certain project
depends, among other factors, on the standard of accuracy required in
photogrammetric control, the photo-scale, the forward overlap and the
quality of photogrammetric measurements (a function of the quality of
the photography, the stereoplotter or stereocomparator used, and the
operator), as well as on the methods used for executing and adjusting
aerotriangulations.
The idea of designing a chart which would determine the maximum
distance allowable for bridging was brought up by the author in 1956*
and has proven to be of much help in planning small scale mapping for
extensive areas. The basic idea lies in studying the propagation of
the residual errors (in planimetry and elevation) remaining after the
adjustment of aerotriangulations and confining these errors within the
allowable tolerances by restricting the bridged distances to certain
limits.
The mbd chart published in 1956* was restricted to one principal
distance. In this paper, two universal mbd charts are presented. In
addition to these charts, empirical expressions are also given for the
determination of the maximum allowable bridging distance, as well as
the expected accuracy of the deduced elevations.
In general, aerotriangulation is applied only in the case of small
scale mapping. For this reason, the 195& chart as well as the charts
given in Figs. 2 and 3 deal only with map scales ranging from 1:50 000 to
1:1 000 000. In case of such small scales, the accuracy in planimetry
is generally the decisive factor in determining the maximum distance to
* See Bibliography Entry No. 5*