I.
Station:
- A point where the levelling staff is held. It is the point whose elevation is
to be ascertained or the point that is to be established at a given elevation.
II.
Height of Instrument (H.I.):
- For any set up of the level, the height of instrument is the elevation of
the line of sight with respect to the Mean Sea Level (MSL).
III.
Back Sight (B.S.):
- The sight taken on the staff held at a point of known elevation or TBM
(Temporary Bench Mark).
IV.
Fore Sight (F.S.):
- The sight taken on the staff held at any point of unknown elevation, to
ascertained the amount by which the point is below the line of sight and thus
to obtain the elevation of the station.
V.
Intermediate Sight (I.S.):
- The sights taken between two change points, on which only one sight is taken
to determine the elevation of the station.
VI.
Change Point (C.P.):
- The point on which both F.S. and B.S. are taken on a line of direct levels.
The F.S. is taken on the point in one set of the instrument to ascertain the
elevation of the point while the B.S. is taken on the same point in other set
of the instrument to establish the new height of the instrument.
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