Figure 15-26.—Field notes for base line measurement.Base Line Measurementtensiometer (spring balance). When the proper tensionThe accuracy of all directions and distances in asystem depends directly upon the accuracy withwhich the length of the base line is measured.Therefore, base line measurement is vitally important.A transit must be used to give precise alignment whilemeasuring a base line. For third-order triangulationmeasurement with a steel tape, you are required toincorporate all the tape corrections described in theEA3 TRAMAN. For measurement over rough terrain,end supports for the tape must be provided by postsdriven in the ground or by portable tripods. Thesesupports are usually called chaining bucks. The slopebetween bucks is determined by measuring thedifference in elevation between the tops of the buckswith a level and rod.On the top of each buck, a sheet of copper or zincis tacked down, which provides a surface on which tapelengths can be marked Bucks are setup along the baseline at intervals of one-tape length. The tape, withthermometers fastened at each end, is stretched betweenthe supports and brought to standard tension by ais indicated, the position of the forward end is markedon the metal strip with a marking awl or some otherneedle-pointed marker. At the same time, thethermometer readings are taken.If stakes, driven at tape-length intervals, are used astape supporters, the end of the tape may begin to lieslightly off the metal marking strip on the buck after afew tape intervals have been laid off. To take care of thissituation, the head chairman carries a finely divided (to0.001 ft) pocket scale. With this scale he measures thedistance that the tape must be set back or set forwardto bring the end again on the marking strip. The set backor set forward is entered in the field notes and deductedfrom or added to the tape length for that particularinterval.Figure 15-26 shows field notes for a base linemeasurement. In this case the tape was supported onstakes, driven at full-tape, 100-foot intervals. With theexception of the interval between stakes 5 and 6, theamounts to the standard tape length (with the tapesupported at both ends, and with standard tension15-33
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