What Is Levelling Staff | Types | Holding A Staff For Taking A Reading | Surveying

 

LEVELLING STAFF

The reading given by the line of sight on a levelling staff held vertically is the height of the line of collimation above the point on which the staff is held (Below Figure). The levelling staff may be divided into the following classes.

1. Self-reading staff

2. Target staff

 

SELF-READING STAFF

A staff on which readings are directly read by the observer through the telescope is known as self-reading staff. Self-reading staffs are of three types as discussed below.

Solid staff

These are usually 3 m long. Due to the absence of a hinge or socket on these staffs, greater accuracy in reading is achievable, but on the other hand, it is inconvenient to carry them in the field. The use of a solid staff is generally restricted to precise levelling work.

 

 

 

 

 

Folding or hinged staff

A folding staff is made of well-seasoned timber. It is 4 m long and consists of two portions, each being 2 m long and hinged together. The width and thickness of the staff is kept at 75 mm and 18 mm, respectively. The foot of the staff is provided with a brass cap to avoid wear and tear due to usage. Sometimes, a plummet is also provided to test the verticality of the staff. Each metre length is subdivided into decimetres and each decimetre is further divided into 20 equal divisions of 5 mm width. Decimetre numerals 1–9 for each metre length are marked in black and metre numerals in red.

The graduations are marked inverted so that they appear erect when viewed through the telescope. In modern levelling staff, the graduations are marked erect. The staff may be folded together so that one 2 m piece is capable of folding on the other when not in use and two pieces are detachable from one another so that one half may be used while working in plain areas.

 

Telescopic or Sopwith type staff

It is made up of three pieces. The top piece is solid 1.25 m long, whereas the central piece of 1.25 m length and the lower piece of 1.5 m length are hollow. The top portion slides into the central portion telescopically. When fully extended, the total length of the staff is 4 m. The upper two pieces are held by brass spring catches. The smallest division of this type of levelling staff is also 5 mm. The metre numerals, which are shown on the left, are marked in red. The decimetre numerals 1–9 are shown on the right and marked in black. The decimetre number 10 of each metre length is omitted and the letter M is marked to indicate the end of the metre length. Graduation is marked erect and when viewed through the telescope it appears inverted. While using a telescopic staff it may be ensured that the three parts are fully extended in length when using the full length, i.e., 4 m

 

TARGET STAFF

The target staff consists of two rods, one sliding over the other. The two rods are held together by means of brass clamps. Raising the sliding can extend the length of the staff. The face of each rod is graduated in feet, tens and hundredth of a foot, while the back of a sliding rod is similarly graduated, but from the top downwards. The staff is provided with a movable target equipped with a vernier. With the help of this vernier, one can read up to 0.001 ft. The sliding rod carrying the target is bisected by the line of sight.

 

Holding a staff for taking a reading

Care should be taken to hold the levelling staff truly vertical while the reading is being taken. To hold the staff in a vertical position, the person holding the staff should stand behind the staff with his heels together, having the heel of the staff between his toes and holding it in his hand at the height of his face. When the level of the required point is very much higher than the height of the instrument, an inverted staff reading is to be taken.

Relative merits of self-reading and target staff

a. With the self-reading staff, the readings can be taken quicker than with the target staff.

b. In the case of target staff, the services of trained personnel are necessary.

c. The reading with the target staff can be taken with greater fineness. But if the staff man does not direct accurately to make the line of sight bisect the target, it gives more apparent readings.

d. The surveyor himself takes the reading on a self-reading staff. But in the case of target staff, the staff man is responsible for noting down the readings.

e. It is tedious to adjust the target such that the line of sight bisects it accurately.

 

Fundamental axes of a levelling instrument

a. Line of collimation or principal line of sight: It is the imaginary straight line that joins the optical centre of the object-glass with the point of the intersection of crosshairs of the diaphragm.

b. Axis of bubble tube: It is an imaginary line tangential to the longitudinal curve of the tube at its middle point. It is also known as the bubble line. It is horizontal when the bubble is centred.

c. Axis of the telescope: It is a line joining the optical centre of the object-glass to the centre of the eyepiece.

d. The vertical axis: It is the imaginary line passing through the centre line of the axis of rotation.

e. The height of the instrument (HI): The height of the instrument is the elevation of the plane of collimation or plane of sight when the instrument is correctly levelled. When a level in adjustment is accurately levelled, the line of collimation will revolve in a horizontal plane known as the plane of collimation or plane of sight.

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