Methods of Levelling :
The important methods of levelling used while surveying are as follows :
- Barometric Levelling.
- Trigonometric Levelling.
- Spirit Levelling.
- Differential Levelling.
- Reviprocal Levelling.
- Profile Levelling.
Barometric Levelling :
It makes use of the phenomenon that difference in elevations between two points is proportional to the difference atmospheric pressure at these points.
Trigonometric Levelling (Indirect Levelling) :
In a modified form it is called as Stadia levelling. This method is commonly used in mapping
Spirit levelling (Direct Levelling) :
In this spirit level and sighting device (telescope) are combined and vertical distances are measured by observing an graduated rods placed on the points. It is the most precise methods of levelling.
Differential Levelling :
It is a direct method in which difference in elevation of two points is determined regard less of the horizontal positions of points with reference to each other. This is also known as ‘Fly Levelling’.
Reciprocal Levelling :
The true difference in elevations is equal to the mean of two apparent differences in elevations obtained by reciprocal observations.
Where, ha & hb are staff readings at A & B. When staff is close to A and ha’, ha’ are corresponding reading readings at A and B when staff is close to B.
Reciprocal levelling is also knows as longitudional levelling
Profile Levelling (longitudinal sectioning) :
Profile levelling is the process of determining the elevations of points at short measured intervals along a fixed line such as the center line of a railway, highway, canal or sewer.
The fixed line may be may be a single straight line or it may be composed of a succession of straight lines or of a series of straight lines connected by curves. It is also known as longitudinal sectioning.
Object of Levelling
The important objects of levelling is to find the elevations of given points with respect to a given or assumed data. To establish points at a given elevation or at different elevations with respect to a given or assumed datum. To deal with angular and linear measurements in vertical plane. These are the most important objectives of levelling.
Important Terms Used in Levelling
1. Level surface : A curved surface which at each point is perpendicular to the direction of gravity at the point. The surface of a still water is a truly level surface. Any surface parallel to the mean spheroid surface of the earth is therefore a level surface.
2. Level line : Normal to the plumb line at all points.
3. Horizontal plane : Tangential to the level surface at the point.
5. Vertical line : Line normal to the level line at a point.
6. Datum : Any surface to which elevations are referred.
7. Elevation (R.L) : Vertical distance above or below an arbitrarily assumed level surface or datum.
8. Bench Mark (B.M) : Relatively permanent point of reference whose elevations with reference to some assume datum known. It is used either as a starting point for levelling or as a point upon which to close as a check.
9. Mean Sea Level : It is the average height of the sea for all the stages or tides calculated for 19 years in India.
Also Read : Contour line and Methods of Contouring