Chapter II.SPOTTING OF POINTS.

Generalities.—In all spotting operations, whether working from the map to the terrain or vice versa, the difficulty is due to the fact that the situation of the point has to be found on a two-dimension surface.

The best method of work will be, then, that which suppresses as quickly as possible one of these dimensions and to conduct the research on a straight line.

Any point can be placed on the terrain or on the map if you know the following elements:

1. Its “direction” or alignment.

2. Its situation on this alignment—that is, its “range.”

In oblique vision, a digression in direction is always much more apparent than a digression of the same size in range.Thus the direction of a point can be identified with more facility and precision than its range.For these reasons, the following methods consider two distinct phases in all spotting operations:

1. Investigation of direction.

2. Investigation of range.

The investigation in direction always comes first, as it is easier, and its result makes the investigation for range easier.

If it is a question of a very visible point (cross-roads, an isolated house, a corner of woods, etc.), the spotting can be done almost immediately, it was found in the general reconnaissance of the terrain, which was discussed in chapter 1.

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.

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.

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 absolutelyavoided—

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. 1Fig. 2

Example(fig. 1).—Suppose there are two trees, A and B, A being nearer the balloon and higher than B. It can happen that, in oblique vision (fig. 2), B having its image B´ and A its image A´, the depression of the image B´ is more than that of A´. In this case, the observer will be tempted to believe that the tree B is nearer him than the tree A.

2.All oblique alignment in investigating the range must be absolutely avoided.

Oblique alignment means a line connecting two points on the map and not passing through the horizontal projection of the balloon.

You might be tempted to use an alignment to find the range of an objective after having determined the direction. The process would consist in finding on the map two points so placed that the straight line between them passes through the objective, visualizing this line on the terrain, and placing the objective at the intersection of this visualized line and the direct alignment. This result, which would be accurate if the ground were absolutely flat, is made erroneous by theunevenness of the terrain. On account of this, the oblique alignment does not pass, in oblique vision, through the same points as its horizontal projection on the map.

Fig. 3

Example(fig. 3).—On the map C is the objective, A and B two points so situated that the line AB passes through C, and EF the direct alignment, or the line balloon objective. The line AB coincided on the terrain, with the trace of the vertical plane passingthrough A and B. In oblique vision (fig. 4) it is different. The line A′C′B′ is a curve which follows the irregularities of the ground, and the point C′ is not on the oblique alignment A′B′.

Fig. 4

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. 5Fig. 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 thetwo 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.

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