The method of determining relative altitudes of points on the surface of the earth or beneath the surface of the earth is called levelling.
The instrument used for levelling is provided with either external type telescope or internal type telescope. In modern surveying instruments, internal focusing telescopes are becoming obsolete.
Learn about the Surveying
According to the construction, the levels may be classified as:
The dumpy level: It consists of a telescope rigidly fixed to its support which can neither be rotated about its longitudinal axis, nor it can be removed from its supports. A long bubble tube is attached to the top of the telescope.
The wye level: It consists of a telescope held in two vertical wye supports which may be raised to enable the telescope to be rotated about its longitudinal axis. And it is also removed and turned end to end.
The reversible level: It consists of a telescope that is supported by rigid sockets into which it is introduced from either end and fastened by screws. Once the telescope is fastened with the screws, it acts as a dumpy level.
The tilting level: It consists of a telescope attached with a level tube which can
be tilted within few degrees in a vertical plane by a lilting screw.
Terms used in levelling survey
Level surface: It is a surface parallel to the mean spheroidal surface of the earth, e.g. the surface of still water in the lake. A level surface may be regarded as a curved surface, every point on which is equidistant from the center of earth. It is normal to the plumb line at all points. Any line lying in a level surface is called a level line.
Horizontal surface: A horizontal surface through any point is a surface tangential to the level surface at that point. It is perpendicular to the plumb line. Any line lying in the horizontal surface is called a horizontal line and it is tangential to the level line.
Vertical surface: A vertical surface through any point is a surface normal to the level surface at that point. Any line lying in the vertical surface is called vertical line & is normal to the level line.
Datum surface or line: It is an arbitrary surface or line from which the elevation of points are measured and compared
Elevation of a point: It is a vertical distance above or below the datum also known as reduced level (RL)·
Bench mark (B.M.): It is a fixed reference point of known elevation.
Backsight (B.S) or, Back sight-reading: It is a staff reading taken on a known elevation (or R.L.) as on a benchmark or a change point. It is the fins staff reading taken after the level is set up and leveled.
Foresight (F.S.) or fore sight-reading: It is a staff reading taken on a point whose elevation is to be determined as on a change point.
Intermediate sight L.S.) or Intermediate sight-reading: All sights taken between the backsight and foresight are intermediate sights.
Changepoint (C.Р.): It is point denoting the shifting of the level.
Station: It is a point whose elevation is to be determined.
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