LOCATING ON THE MAP AN OBJECT SEEN ON THE GROUND.
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
On the other hand, let both the reference points “a” and “b” ([fig. 6]) be situated short of the objective O, “a” being nearer the objective and “b” nearer to the balloon. An error aa′ in spotting “a” leads to an error OO′ in the objective greater than aa′. Notice that this error diminishes as “A” approaches O, thus “a” being as a₁, the error a₁a′₁ equal to aa′ leads to an error OO′₁, in the objective, less than OO′. We would thus obtain an analogous result if we would move the point “b” farther away.
Therefore, when you are obliged to take the two reference points between you and the objective, choose one as near the objective as possible and the other as near as possible to the balloon.
3. Investigation of range.
Identify details of the terrain situated over and short of the objective on the alignment. Narrow this bracket down step by step; situate the objective on the map according to its relative distance from the two nearest identifiable reference points, taking into account the deformations due to the laws of perspective and the relief of the ground.
If you have a vertical photograph of the region, trace the alignment on this photograph and make the investigation in range by the same means.
The dangers against which I warned you before in connection with the investigation of range apply in this case also, so it is unnecessary to repeat them.
When the two last identifiable reference points are some distance from each other, the situation of the objective has a possible error, of which you know the size according to the distance between the two reference points; it might be interesting to remember this in case different information is obtained on this objective from that obtained in the balloon.
This error can be considerably diminished if you use a vertical photograph; the investigation can then be carried on by the same method as on the map, with greater precision. In the case of a battery, particularly, it is for the observer to find the position of each piece.
In case, on account of dead ground or of a mask before the battery, the observer sees the flashes or the smoke without seeing the battery itself, he should mark the exact alignment in which the flashes or smoke are seen, and determine the bracket in range—that is, the reference points nearest the objective which are clearly over and short. This document compared with other information can facilitate the identification of the battery.
Chapter III.
OBSERVATION OF FIRE.
The observation of fire is essentially the following operation, repeated for each shot or salvo: Locating on the ground the position of one point, which is the point of burst, and announcing its situation in reference to another point, which is the target.
But it has been demonstrated that it is impossible, without using the map, to determine the error in range of one point relatively to another point not on the same alignment.
The operation must consist in:
1. Spotting on the map the point of burst.
2. Reading its position in reference to the target.
The observation of the burst—that is, the spotting of the point of impact—is the same whether the observation is direct or lateral.