If, on the contrary, the point under consideration is difficult to find (a piece of trench in a confused and cut-up region, a battery emplacement, etc.), we must have recourse to a precise method.
1. RESEARCH IN DIRECTION.
Join on the map the projection of the balloon and the center of the objective. Identify this direction on the terrain by finding on the alignment a prominent point. This line can be drawn in the basket. It is a good thing to draw the alignment on a vertical photograph of the objective also, in order to have a greater number of reference points than the map could give.
2. INVESTIGATION OF RANGE.
Identify on the map (or photo) two points, one situated over and one short of the objective. Narrow down this bracket step by step until the object is recognized.
As this investigation of the range is the more difficult, observers must be warned against certain methods which are to be absolutely avoided—
1. Never identify the range of a point by comparing it with that of a near-by point situated on a different alignment.
If these two points are not at exactly the same height, the deformations due to oblique vision can falsify their apparent relative range. The point farthest away can even seem nearer, and the nearest point farther away.
Fig. 1 Fig. 2