Types of Levelling Staff used in Surveying
A levelling staff is a straight rectangular rod having graduations, the foot of the staff representing zero reading. The purpose of a level in to establish a horizontal line of sight. The purpose of the levelling staff is to determine the amount by which the station is above or below the line of sight. Types of Levelling staves may be divided into two classes
- Self-reading staff
- Target staff
A self-reading staff is the one which can be read directly by the instrument man through the telescope. A target staff, on the other hand, contains a moving target against which reading is taken by staff man.
There are usually three forms of self-reading staff. They are solid staff, folding staff and telescopic staff. The figure below a and b show the pattern of solid staff in English units while c and d show that in the metric unit. The most common form of the smallest division is of0.01ft or 5mm. However, some staves may have fine graduations up to 2mm. The staff is generally made of well-seasoned wood having a length of 10 feet or 3 metres.
The figure below 9.12 shows a Sopwith pattern staff arranged in three telescopic lengths. When fully extended, it is usually of 14ft length.
Fig below 9.13 shows a folding staff usually 10ft 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 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.
Target staff is another type of levelling staff. Fig 9.14 shows a target staff having a sliding target equipped with vernier. The rod consists of two sliding lengths, the lower one of approx 7ft and the upper one of 6 ft. The rod is graduated in feet, tenths and hundreds and the vernier of the target enable the readings to be taken up to a thousand part of a foot.
For taking 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.
Relative merits of self-reading and target staffs
1. When 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 of a self-reading staff, on the other hand, an 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.
These are the types of levelling staff used in surveying.