### WHAT IS LEVELLING? TECHNICAL TERMS USED IN LEVELLING.

LEVELLING

The art of determining relative altitudes of points on the surface of the earth of beneath the surface of the earth is called levelling. It is a branch of surveying and it deals with the measurement in vertical planes.

TECHNICAL TERMS USED IN LEVELLING

1. Level surface : Level surface is curved surface which is parallel to the mean spheroidal surface of the earth, such that every point on this level surface is at equal distance from the center of the earth. The surface of still water in a lake represent a level surface.

2. Level line : Any line lying on the level surface is called level line.

3. Horizontal surface : A surface tangential to the level surface at any point is known as horizontal surface. It is perpendicular to the plumb line at the tangent point.

4. Horizontal line : A line lying on the horizontal surface, is known as horizontal line. It is a straight line tangential to the level line.

5. Vertical line : A line perpendicular to the level line is called a vertical line. The plumb line at any place, is called the vertical line.

6. Vertical plane : The plane which contains vertical line at a place is called vertical plane.

7. Vertical angle : The angle between an inclined line and horizontal line at a place of vertical plane is called vertical angle.

8. Datum surface : The imaginary level surface with reference to which vertical distance of the points (above or below) are measured is called datum surface.

9. Mean sea level datum : The M.S.L datum adopted by the survey of india for determining the elevation of different points in india is that of bombay.

10. Reduced level (R.L) : The height or depth of a point above or below the assumed datum, is called reduced level (R.L). It is also known as elevation of the point. Elevations of the points below the datum surface are known as negative elevation.

11. Line of sight : The line passing through the optical center of the objective and intersection of cross hairs is known as line of sight.

12. Bench mark (B.M) : A relatively permanent and fixed referance point of known elevation above the assumed datum is called a bench mark.

13. Instrument station : The point where instrument is set up for observation is called instrument station.

14. Station : The point where levelling staff is held is called station. It is a point whose elevation is to be determined.

15. Height of instrument : The elevation of line of sight with respect to assumed datum is known as height of instrument.

16. Back sight : The first sight taken on the levelling staff held at point of known elevation is called back sight. It is also known as positive sight.

17. Fore sight : The sight taken on the levelling staff held on the point of unknown elevation to ascertain the amount by which the point is above or below the line of sight is called fore sight. It is also known as minus sight.

18. Change point : The point on which both the fore sight and back sight are taken during the operation of levelling is called a change point.