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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*