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* For complete details, refer to Bibliography Entry 5 or 9. 
-20- 
2. FUNDAMENTAL ELEMENTS OF TEE CROSS-BASES METHOD* 
2.1 Ground Control: 
Instead of requiring the position coordinates of numerous ground 
control points, the ground control in the case of the Cross-Bases Method 
consists of a minimum of two independent base lines, one at the 
beginning and one at the end of the bridged distance (see Fig. 6). 
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Figure 6 
For each of these baselines, which are approximately transverse to the 
strip axis, the length, azimuth and slope are to be determined. It is 
also necessary to terrestrially determine the elevations of three points 
at the beginning and three points at the end of each bridged distance 
in such a manner as to permit the determination of the longitudinal 
tilt ( ) at both ends of the bridged distance. 
2.2 AdJ u s tment: 
The error analysis and adjustment of the machine coordinates and 
elevations of the triangulated points are undertaken, in the case of 
the Cross-Bases Method, via four QUASI-OBSERVATIONS: the scale (s), 
the swing (k), the lateral tilt ( ^ ), and the longitudinal tilt ( (J) ) 
of the different models in the strip. Comparing the terrestrially 
determined values for the quasi-observations (given a subscript T in 
the following equations) to the values deduced from the machine