Measuring in catenary
Although the measurement of base lines in catenary is virtually obsolete, it is still the most accurate method of obtaining relatively short distances over rough terrain and much better than can be obtained with GPS or even a total station.
The only difference from the procedures outlined above is that the tape is raised off the ground between two measuring marks and so the tape sags in a curve known as a catenary. Figure shows the basic set-up, with tension applied by levering back on a ranging rod held through the handle of the tape (Figure ).
Figure shows a typical measuring head with magnifier attached. In addition to the corrections already outlined, a further correction for sag in the tape is necessary. For extra precision the measuring heads may be aligned in a straight line by theodolite, the difference in height of the heads being obtained by levelling to a staff held directly on the heads.
Step measurement is the process of breaking the overall distance down into manageable short sections, each much less than a whole tape length. The tape is stretched horizontally and a plumb-bob suspended from the elevated end of the tape.
This method of measurement over sloping ground should be avoided if high accuracy is required. The main source of error lies in attempting to accurately locate the suspended end of the tape, as shown in Figure.