Levelling Instruments in Surveying – Levelling Staff, Dumpy Level etc..

Levelling is used to find the height of one level relative to another. It is used to find the elevation of one point with respect to datum. In this post, I am going to describe the levelling instruments in surveying.

Instruments used in levelling in surveying

1. Levels

A level is basically a telescope attached to an accurate leveling device, set upon a tripod so
that it can rotate horizontally through 360°. The following figure shows the level and its
components.

Instruments used in levelling in surveying

2. Tripod

The tripod consists of three legs and a head where the level instrument is mounted. The
tripod could be of aluminum or wood material. When leveling the level instrument, the
tripod head must be set approximately level beforehand by adjusting the tripod legs.

Instruments used in levelling in surveying

3. Leveling staff

The one of the main instrument used in levelling is levelling staff. The leveling staff is a box section of aluminum or wood, which will extend to 3 or 5 m in height by telescoping, hinging or addition of sections. One face has a graduated scale attached for reading with the cross-hairs of the level telescope.

Instruments used in levelling in surveying

4. Staff bubbles

These are generally a small circular bubble on an angle plate which is held against one corner
of the staff to ensure that the staff is held in a vertical position. If the staff is not held
vertical, the reading will be too large and may be significantly in error.

SETTING UP OF THE DUMPY LEVEL:

1. Release the clamp screw of the instrument
2. Hold the instrument in the right hand and fix it on the tripod by turning round only the
lower part with the left hand.
3. Screw the instrument firmly and bring all the foot screws to the center of its run.
4. Spread the tripod legs well apart and fix any two legs firmly into the ground by pressing
them with the hand.
5. Move the third leg to up or down until the main bubble is approximately in the center.
6. Then move the third leg in or out until the bubbles of the cross-level is approximately in the
center.
7. Fix the third leg firmly when the bubbles are approximately in the centers of their run.

LEVELLING UP:

1. Place the telescope parallel to a pair of foot screws.
2. Bring the bubble to the center of its run by turning the foot screws equally either both
inwards and both outwards.
3. Turn the telescope through 90º, so that it lies over the third foot screw.
4. Turn this third foot screw so that the bubble comes to the center of its run.
5. Turn the telescope through and check whether the bubble remains central.

ELIMINATION OF PARALLAX IN LEVELLING:

1. Remove the lid from the object glass.
2. Hold a sheet of white paper in front of the object glass.
3. Move the eyepiece right or left until the cross hairs are distinctly visible.
4. Direct the telescope towards the staff.
5. Turn the focusing screw until a clear and sharp image is formed in the plane of the cross
hairs.

These are the Instruments used in levelling in surveying.

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