surveying iii (topographic and geodetic surveys) - Modern Prepper

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 theformula 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. Theeffective 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, youcan only obtain good closures in the angles of triangles or distances measured by electronic equipmentby use undue effort and expense, if at all. Therefore, the reconnaissance party should try to avoid lineslikely to give refractive trouble. Layers or currents of unequally heated air along the line of sight causehorizontal refraction. Varying air densities produce a condition similar to that of a beam of light passingthrough a series of very flat prisms. Lines passing near the base of a mountain range or bluff may beaffected by air currents flowing down side canyons and ravines (Figure 2-9). The kind of cover orvegetation, the range in temperatures between day and night, the direction and velocity of the wind, andthe humidity are important factors.EN0593 2-34