surveying iii (topographic and geodetic surveys) - Modern Prepper

2-53. Intervisibility of Stations. To execute reconnaissance for triangulation, use either of thefollowing general methods or a combination. In the first method, which can be used in hilly ormountainous country, test the intervisibility of the stations by visiting each one. If the stations are notintervisible from the ground, clear obstructions on the line (such as trees) or adjust the tower heights toallow intervisibility from either or both ends of the line. Find the elevations of intervening high pointsby lowering a weighted tape from a helicopter or by using barometric or trigonometric leveling. In thesecond method, obtain the elevations of the stations and the intervening country from maps or othersources, and determine the intervisibility of points and the required heights of towers from the data. Inactual practice, a combination of the two methods is generally used. The reconnaissance party shouldkeep a total tower height for any one line to a minimum. Determining the instrument and signal heightsis a matter of good judgment, trial computations, and experience.a. The difference between the apparent and the true difference in elevation of two points is affectedby two factors--the curvature of the earth's surface and the refraction of light by the earth's atmosphere.Because of the earth's curvature, distant points appear to fall below a level line of sight from the point ofobservation. Vertical refraction causes the line of sight to curve downward and makes the distant pointappear higher than its true elevation.b. The effect of refraction is about one eighth as much as the curvature and opposite in sign (refer toTable 2-1). In survey work, the two are combined and the approximate resultant correction is given bythe following formula:Where h is the known quantity, the formula is:Table 2-1 gives the corresponding values of K and h.EN0593 2-30