Figure 2-8. Similar Triangles Solution
(1) The effective elevation of the line at observation point H is now computed. In Figure 2-8,
form the similar triangles using the effective elevation of B and H. Solving for X, we arrive at the
formula shown on the left side of the figure.
(2) The effective elevation of the line at obstruction H is 284.8 + 3.5 = 288.3 meters. The
effective elevation of H is 289.7 meters, and the effective elevation of the line at H is 288.3; hence 288.3
-289.7 = -1.4 meters. The line fails to clear the obstruction at H by 1.4 meters.
g. Some lines are affected by abnormal horizontal refraction. Where such lines are involved, you
can only obtain good closures in the angles of triangles or distances measured by electronic equipment
by use undue effort and expense, if at all. Therefore, the reconnaissance party should try to avoid lines
likely to give refractive trouble. Layers or currents of unequally heated air along the line of sight cause
horizontal refraction. Varying air densities produce a condition similar to that of a beam of light passing
through a series of very flat prisms. Lines passing near the base of a mountain range or bluff may be
affected by air currents flowing down side canyons and ravines (Figure 2-9). The kind of cover or
vegetation, the range in temperatures between day and night, the direction and velocity of the wind, and
the humidity are important factors.