LEVELLING
DESCRIPTION OF THE INSTRUMENTS: -
The level: -
The instrument which is used for measuring related elevations is known as a level and consists
of the following parts.
1. A telescope to provide a line of sight.
2. A level tube to make the line sight horizontal.
3. A leveling head to bring the bubble of the level tube at the center of its run.
4. A tripod head to support the above three parts of the level.
Types of level: -
The dumpy level: -
1. This consists of a telescope rigidly fixed to its support.
2. It can neither be rotated about it longitudinal axis nor it can be removed from its support.
The wye level: -
1. The telescope is supported in Y supports and is not rigidly fixed to the supports.
2. The telescope can be removed from the supports reversed end to end and can be revolved
about its longitudinal axis.
The reversible level: -
1. The telescope can be rotated about its longitudinal axis in the sockets and also can be
withdrawn from its sockets and replaced end for end.
The tilting level: -
1. The telescope can be tilted within few degree in vertical plane by a tilting screw.
2. This designed for precise work.
Temporary adjustments of the dumpy level: -
a) Setting up the level: -
1. This includes fixing the instrument on the tripod and levelling the instrument
approximately by leg adjustment.
2. To do this release the clamp hold the instrument in the right hand and fix it on a tripod
by turning rounded the levelling head with left hand.
3. The tripod legs are adjusted so that the telescope is at a convenient height and is leveled
approximately.
b) Levelling up: -
1. This is done with the help of three-foot screws and by using plate levels.
2. The object of levelling is to make its vertical axis truly vertical.
3. First loosen the clamp screw and turn the instrument until the longitudinal axis of the
bobble tube is parallel to a joining any two levelling screws (say A and B).
4. Holding these two foot screws with the thumb and first finger of each hand turn them
uniformly so that the thumbs move either towards each other on away from each other until
the bubble comes to the center of the tube.
5. Rotate the upper plate through 90° until the axis of the plate level coincides a line joining
the third foot screw C and the midpoint of the first two screws A and B.
6. Hold the third with the thumb and find finger of the right hand and turn it until the plate
bubble is central.
7. Rotate the upper plate through 90° to its original position and repeat step 4 till the bubble is
central.
8. Rotate again through 90 and repeat step 6.
9. Repeat steps 4 and 6 till bubble remains central in both the position.
10. Rotate the instrument through 180° and in this position the bubble should remain central if
the instrument is in adjustment.
Elimination of parallax: -
1. Parallax is a condition arising when the image formed by the objective is not in the plane
of the cross hairs.
2. To get accurate sighting those should be eliminated and this is done.
3. By focusing the eyepiece for distinct vision of the cross hairs and
4. By focusing the objective to bring the image of the object in the plane of cross hairs.
Focusing the eyepiece: -
1. Direct the telescope either towards the sky or hold a sheet of white paper in front of the
objective.
2. Move the eyepiece in or out till the cross hairs appear distinct.
Focusing the objective: -
1. Direct the telescope towards the leveling staff.
2. Turn the focusing screw till the image appears clear and sharp.
3. The image formed must be in the plane of cross hairs.
DEFINITIONS
Important terms: -
The following are the important terms used during levelling.
1. Level surface: -
A level surface is any surface parallel to the mean spheroidal surface of the earth.
2. A level line: -
It is a line lying in a level surface and normal to the plumb line at all points.
3. A horizontal plane: -
A horizontal plane through a point is a plane tangential to the surface at that point.
4. A horizontal line: -
It is a line lying in the horizontal plane.
5. Vertical line: -
Vertical line any point is a line normal to the level surface through that point.
6. Vertical plane: -
A plane is a plane containing a vertical line.
7. Datum surface: -
It is any arbitrary assumed level surface form which vertical distances are measured.
8. Elevation: -
Elevation of a point is its vertical distance above or below the datum also known as
reduced level (R.L)
9. Bench mark: -
It's fixed reference point of known elevation.
10. Line of collimation: -
It is the line joining the intersection of the cross hairs to the optical center of the object
glass and its continuation also known as line of sight.
11. Axis of telescope: -
It is the line joining optical center of the object glass to the center of the eyepiece.
12. Vertical axis: -
It is the center line of the axis of rotation.
13. Back sight (B.S): -
It is a staff reading taken on a point of known elevation (i.e.) on Bench Mark or change
point, and is the first reading taken after the level is set up and leveled.
14. Fore sight (F.S): -
It is a last staff reading on a point whose elevation is to be determined as on a change
point.
15. Intermediate sight: -
It is any other intermediate staff reading taken on a point of unknown elevation from the
same set of the level.
16. Change point: -
It is a point denoting the shifting of the instrument. It is a point on which the back and
foresights are taken.
17. Station: -
It is a point whose elevation is to be determined or a point which is to be established at a
given elevation.
18. Height of instrument: -
It is the elevation of the plane of collimation when the instrument is correctly levelled.
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