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.
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′.
Fig. 4