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.
