1. Determine first on the map the approximate region where the objective is seen.
A result which you can obtain very quickly, thanks to the points which you had found in your first reconnaissance of the terrain.
2. Investigation of direction.
This operation consists in determining the alignment of the objective. As this alignment is a straight line, you only have to know two points. One of them could be the horizontal projection of the balloon; but you must realize that this position is always changing a little, and it is hard to determine it with absolute precision. It is better to carry on the operation independent of this position, which means applying the following method:
Choose on the alignment of the center of the objective two points, one over and one short, and easily identifiable on the map. Draw with a pencil in the region of the objective the alignment thus obtained. These points should be, as far as possible, precise details of the terrain, such as a corner of woods, an angle of a house, a place where roads or trenches cross, an isolated tree, etc. When the alignment of the objective does not pass through any such points, the difficulty can be overcome by determining in what proportions it cuts a known element, such as an edge of woods or a hedge, provided this element is plainly perpendicular to the direction of observation.
This direction can be approximated to the extent of the thickness of the pencil mark. On its accuracy the final result depends. The difficulty lies in materializing the alignment—that is, the vertical line through the center of the objective—in order to lessen the chances for mistakes. Student observers should have frequent practice in this exercise.
When the point to be found is near the edge of the map it is sometimes necessary to take both reference points between the balloon and the objective; this should be avoided as much as possible, because it is apt to be less exact than when the objective is bracketed by its reference points.
Thus ([fig. 5]), two reference points A and B determine the alignment AB, O, the objective, is situated at some point between A and B. An error AA′ in the spotting of one of these points leads to a smaller error in the position of the objective OO′—that is, smaller than AA′.
Fig. 5 Fig. 6