BASIC RULES/PROCEDURE OF LEVELLING

  There are following rules which have been followed during levelling: -

1.      First of all the closing error has been found. These have been done by start and finish a levelling run on a TBM and closing the loop. It has been done for checking the accuracy of the instrument.

2.      After having checked the accuracy, back sight (B.S.) has been taken on levelling staff held at TBM. And height of instrument (H.I.) has been calculated.

                             H.I. = R.L. OF TBM + B.S.

3.      Then without disturbing the instrumental set up, intermediate sight (I.S.) have been taken at different points/stations (at 30m chainage interval here) until the sight is clear i.e. the intermediate points sighted between two change point (C.P.) are called as intermediate sight (I.S.). But the sight taken at last station which is to be a change point is called as fore sight (F.S.). And reduced level at every station has been calculated.

                                              R.L. = H.I. – I.S.

                                              R.L. = H.I. – F.S.

4.      The last station sighted has been made change point (C.P.) and the instrument has been set up at next to the change point and again back sight (B.S.) has been taken at the same station. The care has been provided to keep the levelling staff at the same station which is to be a change point (C.P.). Again H.I. has been calculated for this set up.

                                  H.I. = R.L. OF LAST STATION + B.S.

5.      Again (3) and (4) have been repeated until a next TBM.  After next TBM, (2), (3) and (4) have been repeated.