surveying iii (topographic and geodetic surveys) - Modern Prepper

Beyond 300 meters, the effect of curvature and refraction must be considered and the necessarycorrections applied.4-11. Methods of Trigonometric Leveling. The difference in elevation between two stations, whosedistance apart is known, can be determined using one of the following methods:a. Reciprocal Observations. During the method of reciprocal observations, observations areperformed at both stations, either simultaneously or not simultaneously. First, determine the differencein elevation from vertical angles observed from one station to the other. The object of such reciprocalobservations is to remove the effect of uncertainty regarding the value of the coefficient of refraction.This effect will balance out when the observations are taken in both directions and are more complete ifthe observations are taken simultaneously. On triangulation work where each station is occupiedconsecutively and additional observers are not available, this method of reciprocal observations isusually used, but no attempt is made to observe in both directions simultaneously. The measurementsare sometimes made at the time of minimum refraction on different days but with less accurate results.b. Nonreciprocal Observations. During nonreciprocal observations, the difference of elevationfrom the vertical angle measured at only one of the stations is commuted. The value of the refractionmust be known. This method is available to determine the elevation of the station occupied byobservation to a point of known elevation and elevations of inaccessible and intersected points.4-12. Zenith Distances With the T-3 Theodolite. In first- and second-order triangulation, zenithdistances are obtained using a Wild T-3 theodolite or a theodolite of similar accuracy.a. The zenith is an imaginary point overhead where an extension of the plumb line intersects anassumed sphere on which the stars appear projected. The equivalent point that is directly below thezenith is called the nadir. The zenith distance is the vertical angle from the zenith, or a point directlyoverhead, to the sighted point. The zenith permits reading angles in a vertical plane without using a plusor minus. Vertical-angle measurements with the T-3 read elevation (plus) angles as values less than 900and depression (minus) angles as values greater than 900.b. Observe zenith distances between 1200 and 1600 hours whenever possible, as vertical refractionis at its minimum and more constant during these hours. Confine angle observations to this period, theduration of which varies in area. In general, good observations are possible from 1000 to 1600 hours.c. Over longer distances, the preferred method of obtaining observations is with heliotropes or othervisible objects at the stations. When it is not possible to observe zenith distances during therecommended hours, make observations at night using lights. To obtain the best results, make reciprocalobservations from two4-27 EN0593