TECHNICAL TERMS USED IN LEVELLING | SURVEYING | LCETED

 

TECHNICAL TERMS USED IN LEVELLING

1. Level surface: A level surface is any surface parallel to the mean spherical surface of the earth. It is a curved surface, which at each point is perpendicular to the direction of gravity at that point. Every point on a level surface is equidistant from the centre of the earth (below Figure).

 

Level surface

Figure showing level line, datum, mean sea level and ground surface

2. A level line: A level line is a line lying on a level surface. It is normal to the plumb line at all points (below Figure).

 

3. A horizontal plane: A horizontal plane through a point is a plane tangential to the level surface at that point.

 

4. A horizontal line: A horizontal line is any line lying in the horizontal plane.

 

5. A vertical line: A vertical line at any point is a line normal to the level surface through that point.

 

6. Elevation: The elevation of a particular point is the vertical distance above or below a reference surface. Usually, the mean sea level is used as a reference.

 

7. Mean sea level (MSL): Mean sea level is obtained from the average height of the sea’s surface for all the stages of the tide, over a period of 18.6 years.

8. Datum: Datum is any arbitrarily assumed level surface to which elevations are referred.

 

9. Reduced level (RL): The reduced level of a place is its elevation or vertical distance above or below the datum or any fixed point.

 

10. Line of sight: It is the imaginary line joining the intersection of the crosshairs of the diaphragm to the optical centre of the object glass and its continuation.

 

11. Backsight (BS): A backsight is the first staff reading taken after setting up the instrument at any position. This will always be reading on a point of known elevation. It ascertains the amount by which the line of sight is above or below the elevation of the point. Backsight enables the surveyor to obtain the height of the instrument.

 

12. Foresight (FS): A foresight is the last staff reading taken before shifting the instrument. This will always be reading on a point whose elevation is to be determined. This reading indicates the shifting of the instrument. It is also generally known as minus sight as the fore sight-reading is always subtracted from the height of the instrument (except when the staff is held inverted) to obtain the elevation.

 

13. Intermediate sight (IS): An intermediate sight is any staff reading, taken on a point of unknown elevation, after the back sight and before the foresight. This is necessary when more than two staff readings are to be taken from the same position of the instrument. It may be noted that for one set of a level there will be only a backsight and foresight but there can be any number of intermediate sights.

 

14. Changepoint (CP): This is an intermediate staff position and it is used for the purpose of shifting of the instrument. Both backsight and foresight are taken from this intermediate staff position. Great care is necessary for taking readings at the change point since an error in reading affects every succeeding point of observation (elevation). Any firm point, which can be easily found, may serve as a change point.

 

15. Benchmark (BM): A benchmark is a fixed point of known elevation and the elevation of this point will be noted on it with respect to a datum. The reduced level of the benchmark is used to determine the reduced levels of other points. The benchmark is usually taken as the plinth of an important building, top of parapets of bridges, etc. It will be marked by an identifying mark like a cross mark or a circle with a dot at its centre.

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