-9- 
where N is the number of models bridged. In this equation f-L and f 
should be in inches or millimeters, while Z and B should be in feet or 
meters, to get in feet or meters, respectively. 
It must be underlined that while the charts in Figures 2 and 3 
have some limitations in the ranges of the parameters, the three 
formulae shown above are free from such restrictions. As an example; 
if the longitudinal overlap is other than 60$ or 7^1», other charts 
have to be compiled or deduced. The formulae, however, can handle any 
overlap since the aerial base B appears as such in the equations. 
Two examples will be given later in this chapter to demonstrate 
the use of the mbd charts and formulae. 
Even though the mentioned formulae have been developed for a 
certain combination (Bachmann*s Method of relative orientation, the 
aeropolygon method of triangulation, and the Cross-Bases Method of 
ground control and adjustment), they could be carefully generalized 
to cover other combinations of photogrammetric systems. Experience 
has shown that there is no significant difference in the accuracy of 
photogrammetric work executed according to the different methods known 
today. Should a drastically new method of relative orientation, of 
triangulation, of control, or of adjustment be devised, it then would 
be wise to examine the propagation of the errors remaining after the 
adjustment before adopting the mbd charts or the formulae. 
Ideally, each combination of methods should be analyzed to get 
its own mbd formula. At present, however, no such "personalized'* 
formulae exist, and the above mentioned equations could be used in 
different cases. Studies are underway at the University of Illinois 
to develop mbd formulae covering a wide variety of photogrammetric 
systems (including analytical aerotriangulation).