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 ., shows the basic set-up , with tension applied by levering back on a ranging rod held through the handle of the tape .

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 .