types of levelling staff in surveying-pdf

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types of levelling staff in surveying pdf
types of levelling staff in surveying

LEVELLING STAFF 

A levelling staff is a straight rectangular rod having graduations, the foot of the staff representing zero reading. The purpose of a level is to establish a horizontal line of sight. The purpose of the levelling staff is to determine the amount by which the station (i.e., foot of the staff) is above or below the line of sight. Levelling staves may be divided into two classes: 
  1. Self-reading staff in surveying, and 
  2. Target staff in surveying

A Self Reading Staff is the one which can be read directly by the instrument man through the telescope. A Target Staff by staff man. on the other hand, contains a moving target against which the reading is taken

PRINCIPLES OF SURVEYING - pdf download

(i) SELF-READING STAFF 

There are usually three forms of self-reading staff : 
  1. Solid staff ; 
  2. Folding staff; 
  3. Telescopic staff (Sop with pattern).
solid staff in surveying
solid staff in surveying

the above (a) and (b) show the patterns of a solid staff in English units while (c) and (d ) show that in metric unit. In the most common forms, the smallest division is of 0.01 ft. or 5 mm. However, some staves may have fine graduations upto 2 mm. The staff is generally made of well seasoned wood having a length of 10 feet or 3 meters. 

types of self reading staff - telescopic staff
types of self reading staff - telescopic staff

the image shows a sop with pattern staff arranged in three telescopic lengths. When fully extended, it is usually of 14 ft (or 5 m) length. The 14 ft. staff has solid top length of 4' 6" sliding into the central box of 4' 6" length. The central box, in turn, slides into lower box of 5' length. In the 5 m staff, the three corresponding lengths are usually 1.5 m, 1.5 m and 2 m. 

types of levelling staff pdf - folding staff
types of levelling staff - folding staff

It shows a folding staff usually 10 ft long having a hinge at the middle of its length. When not in use, the rod can be folded about the hinge so that it becomes convenient to carry it from one place to the other.

Since a self-reading staff is always seen through the telescope, all readings appear to be inverted. The readings are, therefore, taken from above downwards. 

The levelling staves graduated in English units generally have whole number of feet marked in red to the left side of the staff (shown by hatched lines in telescopic staff). The odd lengths of the feet are marked in black to the right-hand side. The top of these black graduations indicates the odd tenth while the bottom shows the even tenth. The hundredths of feet are indicated by alternate white and black spaces, the top of a black space indicating odd hundredths and top of a white space indicating even hundredths. Sometimes when the staff is near the instrument, the red mark of whole foot may not appear in the field of view. In that case, the staff is raised slowly until the red figure appears in the field of view, the red figure thus indicating the whole feet.

Folding Levelling Staff in Metric Units 

It shows a 4 m folding type levelling staff (IS: 1779-1961). The staff comprises two 2 m thoroughly seasoned wooden pieces with the joint assembly, Each piece of the staff is made of one longitudinal strip without any joint. The width and thickness of staff is kept 75 mm and 18 mm respectively. The folding joint of the staff is made of the detachable type with a locking device at the back. The staff is jointed together in such a way that: 

  • the staff may be folded to 2 m length. 
  • the two pieces may be detached from one another, when required, to facilitate easy handling and manipulation with one piece, and 
  • when the two portions are locked together, the two pieces become rigid and straight. 

A circular bubble, suitably cased, of 25-minute sensitivity is fitted at the back. The staff has fittings for a plummet to test and correct the back bubble. A brass is screwed on to the bottom brass cap. The staff has two folding handles with spring acting locking device or an ordinary locking device. 
Each meter is subdivided into 200 divisions, the thickness of graduations being 5 mm. it shows the details of graduations. Every decimeter length is figured with the corresponding numerals (the meter numeral is made in red and the decimeter numeral in black). The decimeter numeral is made continuous throughout the staff. 

(ii) TARGET STAFF 

target staff in levelling
target staff in levelling

Here shows a target staff having a sliding target equipped with Vernier. The rod consists of two sliding lengths, the lower one of approx. 7 ft and the upper one of 6 ft. The rod is graduated in feet, tenths and hundredths, and the Vernier of the target enables the readings to be taken upto a thousandth part of a foot. For readings below 7 ft the target is slided to the lower part while for readings above that, the target is fixed to the 7 ft mark of the upper length. For taking the reading, the level man directs the staff man to raise or lower the target till it is bisected by the line of sight. The staff holder then clamps the target and takes the reading. The upper part of the staff is graduated from top downwards. When higher readings have to be taken, the target is set at top (i.e. 7 ft mark) of the sliding length and the sliding length carrying the target is raised until the target is bisected by the line of sight. The reading is then on the back of the staff where a second Vernier enables readings to be taken to a thousandth of a foot. 

Relative Merits of Self-Reading staff and Target Staffs 

  1. With the self-reading staff, readings can be taken quicker than with the target staff.
  2. In the case of target staff, the duties of a target staff-man are as important as those of the leveller and demand the services of a trained man. In the case a self-reading staff, on the other hand, ordinary man can hold the staff concentrating more on keeping the staff in plumb. 
  3. The reading with target staff can be taken with greater fineness. However, the refinement is usually more apparent than real as the target man may not be directed accurately to make the line of sight bisect the target.
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