-13- 
The above equations are to be carefully considered in the course 
of designing for mapping projects involving bridging, to make sure that 
the expected accuracy of the method -will satisfy the specifications. 
Sometimes equations 4 and 10 will indicate that the specified 
accuracy cannot be met by aerotriangulation. This is particularly true 
in cases involving large scale mapping with very high standards of 
accuracy. In such cases, each and every model has to be terrestrially 
controlled. 
Example: 
The following example demonstrates the use of the mbd charts 
(Fig. 2 and Fig. 2-a) and the mbd formulae (equations 1 and 2). 
Given: A strip of land is to be mapped to the scale of 1:25000 
using a photographic strip composed of 9in./9in. photos. 
The horizontal accuracy requirements specify that not 
more than 10 percent of the points tested shall be in 
error by more than l/50in., measured on the publication 
scale. The vertical accuracy requirements specify a mean 
square error in elevation of +50 ft. or less. The principal 
distance of the camera is 8.25 in., and the flight height 
above ground is 27000 ft. Longitudinal overlap is 60$. 
The accuracy (mean square error) of stereoscopic measure 
ment of parallax in the image plane is +0.01 mm. For all 
practical purposes, the ground control is to be considered 
error-free. 
Wanted: Determine the maximum safe bridging distance (mbd) and the 
estimated accuracy of deduced elevations ( ). 
H