(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*

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.

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