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.
LOCATING ON THE GROUND AN OBJECT SEEN ON THE MAPS.
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.
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.
[Illustration: _Fig. 1_
_Fig. 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 the
unevenness 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.
[Illustration: _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 passing through 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′.
[Illustration: _Fig. 4_]
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′.
[Illustration: _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.