Levelling is a branch of surveying, the object of which is to
- Find the elevation of a given point with respect to the given or assumed datum.
- Establish a point at a given elevation with respect to the given or assumed datum.
It is defined as a curved surface which at each point is perpendicular to the direction of gravity at the point. For example, the surface of still water is truly level surface.
It is a line lying in a level surface.
It is a line normal to the level line at a point.
It is any surface to which elevations refereed. The mean sea level affords a convenient datum world wide.
It is a plane tangential to the level surface at that point.
It is a point on or near the surface of earth is its vertical distance above or below the assumed level surface.
Mean sea level:-
It is the average height of the sea for all stages of tides.
It is a relatively permanent point of reference whose elevation with respect to some assumed datum is known.These are fixed point or mark determined with reference to the datum line. They served as reference point for finding the RL of new point or for conducting levelling operation in projects involving Roads, Railways etc.These are of 4 types.
1 GTS Bench Mark:-
This bench mark are established by the survey of India at a large intervals all over the country. The values of RL, the relative position and the no of bench mark are given in a catalog published by a department.
2 Permanent Bench Mark:-
These are fixed points or marks establish by different Govt. Departments like P.W.D, Railways, Irrigation etc.The R.L of these points are determine with reference GTS benchmark and are kept on permanent point like the plinth of a building, parapet of bridge, culvert etc.
3. Temporary Benchmark:-
When the bench mark are established temporarily at the end of day work is known as temporary benchmark.They are mostly made of roots of trees, parapet of nearby culvert etc.
4. Arbitrary Benchmark:-
When the R.L of some fixed points are assumed are known as arbitrary bench mark
Reduced Level ( RL ):-
The vertical distance of a point above or below the datum line is known as reduced level at that point. Reduced level may be +ve of -ve according to the point above or below the datum.
Line of Collimation:-
It is an imaginary line passing through the intersection of the cross hairs of the diaphragm and the optical center of the object glass and its continuation is known as line of sight.
Axis of Telescope:-
This axis is an imaginary line passing through the optical center of the object glass and optical center of the eyepiece.
Axis of Bubble Tube:-
It is an imaginary line tangential to the longitudinal curve of the bubble tube acts its middle point.
This is the first staff reading taken in any setup of the instruments after the levelling has been perfectly done. This reading is taken at a point of known R.L
It is the last staff reading at any set up of the instrument and indicates the shifting of the instrument later.
Intermediate sight reading:-
It is any other staff reading between the back sight and foresight in the same setup of the instrument.
3 Height of the instrument:-
when the levelling instrument is properly leveled, then RL of line of collimation is known as the height of instrument.This is obtained by back sight reading to the RL of Bench Mark or change point on which the staff reading was taken.
The operation of setting the eyepiece and the object glass at a proper distance apart for clear vision of the object is known as focusing. This is done by turning the focusing screw clockwise or anticlockwise.
The apparent movement of the image related to cross-hair is known as parallax. This occur due to imperfect focusing when the image does not fall in the plane of diaphragm.
Methods of Levelling:-
1 Barometric Levelling:-
This levelling makes use of phenomenon that difference in elevation between two points is directly proportional to the difference in atmospheric pressure at that same points.But at a given point, the atmospheric pressure does not remain constant in the whole day, even in the hours due to change in temperature. So this method becomes inaccurate and is less used in surveying.
2. Indirect Levelling:-
It is the process of levelling in which the elevations of point are computed from the vertical angles and by measuring horizontal distances in the field.
3. Direct levelling:-
in this method of levelling, the vertical distance w.r.t horizontal line is used to determine the relative difference in elevation between two adjacent points by using spirit levelling instrument. This method of determining elevations is widely used by engineer.
Types of Level
Followings are the different types of levels
- Dumpy Level
- Wye Level
- Cooke’s reversible Level
- Cushing’s Level
- Auto Level
1 Dumpy Level:-
The telescope of the dumpy level is rigidly fixed to its supports. It can not be removed from its supports nor can be rotated about its longitudinal axis. This instrument is stable and retains its permanent adjustment for a long period of time. This instrument is commonly used for levelling purposes.
2 The Wye Level:-
In this levelling instrument, the telescope is held in 2 ‘y’ supports. It can be removed from the supports and reversed from one end of the telescope to the other end.The ‘y’ supports consists of 2 curved clips which may be raised. Thus the telescope can be rotated about its longitudinal axis.
3. Cookis Reversible Level:-
This is the combination of the dumpy level and ‘y’ level. It is supported by 2 rigid sockets. The telescope can be rotated about its longitudinal axis, withdrawn from socket and replaced from one end of the telescope to the other end.
5 Cushing Level:-
In this instrument, the telescope can not be removed from the socket and the eyepiece and the object glass are removable and can be interchanged from one end of the telescope to the other end.
5 The modern tilting Level:-
The Telescope can be tilted about its horizontal axis with the help of a tilting screw. In this instrument the line of collimation is made horizontal for each observations by means of tilting screw.
6 Auto Level:-
This is also known as the self- aligning level.This instrument is leveled automatically with in a certain till range by means of a compensating device.