BASIC PRINCIPLE OF LEVELLING
- When the levelling instrument has been properly levelled then the bubble tube and the line of sight are truly horizontal and the vertical axis of the instrument is truly vertical.
- From this setting of the levelling instrument, when the telescope of the instrument is rotated, the line of sight remains in the horizontal plane only.
- Because distances involved in levelling are very small compared to the radius of earth, there is not much difference between the level line and the horizontal line.
- In field, the levelling instrument is set up at a certain suitable location and staff at a location of known elevation.
- Reading is taken on the staff at a location of known elevation so as to determine the level of line of sight. This staff reading is known as back sight (BS).
- Now the staff is shifted to a location of unknown elevation and staff reading is again taken. This staff reading is known as fore sight (FS).
- Thus the elevation of the point is determined from the level of line of sight and the foresight reading at the location of unknown elevation.
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