Aquaculture Engineering (2+1)

Levelling - Terms and abbreviations

Unit 1- Surveying

Levelling

Terms and abbreviations

Station
In levelling, a station is a point where the level rod is held and not where level is set up. It is the point whose elevation is to be ascertained or the point that is to be established at a given elevation.

Height of Instrument (H.I)
For any set up of level, the height of instrument is the elevation of plane of sight (line of sight) with respect to assumed datum. It does not mean that height of telescope above the ground where the level stands.

Back Sight (B.S)
Back sight is the sight taken on a rod held at a point of known elevation, to ascertain the amount by which the line of sight is above that point and thus to obtain the height of instrument. It is also known as plus sight as the back sight reading is always added to the level of the datum to get the height of the instrument. The object of back sighting is therefore, to ascertain the height of the plane of sight.

Fore Sight (F.S)
Foresight is a sight taken on a rod held at a point of unknown elevation, to ascertain the amount by which the point is below the line of sight and thus to obtain the elevation of the station. It is known as minus sight as the foresight reading is always subtracted from the height of instrument to get the elevation of the point. The object of fore sighting is therefore to ascertain the elevation of the point.

Turning Point (T.P)
Change Point: Turning point is point on which both minus sight and plus sight are taken on a line of direct levels. The minus sight is taken on the point in one set of instrument to ascertain the elevation of the point, while the plus sight is taken on the same point in other set of the instrument to establish the new height of instrument.

Intermediate Station (I.S)
Intermediate station is a point, intermediate between two turning points on which only one sight is taken to determine the elevation of the station.