Methods Of Levelling

Step-by-step Procedure

When the instrument is set in position, levelled and adjusted, the line of collimation will be horizontal and the bubble will traverse, i.e. it will remain central while the telescope is rotated within the horizontal plane of collimation. The instrument is ready to record vertical measurements through reading the measuring staff positioned at needed stations.

When holding the staff in position at station, it should be held vertical. The survey helper is asked to stand behind at the staff and hold it among the palms. The helper will tilt the staff forward and backward so that the surveyor will record the minimum staff reading to indicate true vertical position (Figure7). In precision levelling, the measuring staff is provided with a circular bubble at the back to hold it in plumb. The staff is brought in the line of sight among two vertical lines/hairs etched on the diaphragm and the reading is recorded while horizontal hair of crosswire cuts the staff.

First of the two steps in finding the level at a station is to find the elevation or reduced level (RL) of the plane of collimation, also known as height of instrument (HI). This is achieved through taking a back sight on a bench mark once the instrument is group in position. The second step is to search the elevation or RL of all other desired stations, e.g. intermediate and change points (Figure 7).

                                      Figure  : True Vertical Position of Staff

Height of Instrument, HI = RL of the line of collimation

= RL of bench mark + Backsight on BM

RL of any desired station (B) = HI - Foresight at B

= HI - Intermediate sight at B (If B is an intermediate station).