surveying iii (topographic and geodetic surveys) - Modern Prepper

station is (K/2) 2 x 0.0676, or one-fourth at one end only. In the field, the method generally amounts to adetermination of an equal signal height for both stations so that the sight line clears the obstruction.Signal heights are nearly always computed in this way. Sometimes a higher signal is specified at onestation for some other line than the one under consideration. This permits a lower signal at the oppositeend of the line. The amount one signal may be reduced in height for a certain increase of the other isproportional to the distance of the two stations from the obstruction. The station nearest to theobstruction requires the least signal height to provide a clear line of sight. Figure 2-5 illustrates whereh1 and h2 are signal heights required at stations A and B respectively, to clear obstruction H.Figure 2-5. Clearance of Obstructione. Occasionally, a series of obstructions may occur along a line of sight that may not be detectablethrough inspection. A lower obstruction near the middle of the line may require higher signals to clearthan a higher obstruction near the end. The simplest way to determine a critical obstruction is tocompute the signal heights for each one in turn. The following examples illustrate typical problems:Example 1. Two stations are at water level on opposite shores of a 30-kilometer-wide bay. What isthe height (h) of equal signals required to make the line of sight graze the surface of the water?According to the formula, h = (K/2) 2 x 0.0676 = (30/2) 2 x 0.0676 = 15.2 meters. Figure 2-6 shows h asthe distance of a line joining the stations (passing below the earth's surface). Because the obstruction isequidistant from the two stations, h will also be the required height of equal signals at the two endsneeded to clear the line of sight. In actual practice, it is generally necessary to provide a certain amountof clearance over obstructions in order to reduce horizontal refraction.Figure 2-6. Signal Heights for a Line Over WaterExample 2. Figure 2-7 shows an example of two end stations not at the same elevation, with theobstruction not centered at a point midway between them. When applying the formula, first determinethe amount of curvature at the obstruction atEN0593 2-32