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Keywords: Levelling, dumpy level, level, levelling staff, telescope
In the previous articles we have seen that what is levelling and what are the major types of levelling employed in different circumstances. Let’s move on to the next topic in this section, which is levelling instruments and accessories.
The instruments used for levelling are:
A level furnishes a horizontal line of sight, and the levelling staff is used to determine the vertical distances of the points above or below the horizontal line of sight
Parts of a levelling instrument
The levelling instrument consists of the following parts:
i. A telescope
A telescope consists of two lenses placed inside a tube. The lens opposite to the viewing end is called the objective lens or the objective. The rays of light from the object form an image behind the objective (between the objective and the eye lens). Surveying telescopes are designed such that the image of the object is formed in a fixed plane when the instrument is focused. Diaphragm is a ring to which vertical and horizontal hairs are attached. The image of the object is made to coincide with the plane of the diaphragm.
Depending on the arrangement provided for bringing the image of the object formed by the objective in the plane of the crosshairs,there are two types of telescopes
a) External focusing telescopes
The objectives and the eyepieces are mounted on two different tubes so that one can slide inside the other. By turning the focusing screw, the objective is moved in or out relative to the crosshairs. Focusing is achieved when a clear image of the object is formed in the plane of a crosshairs.
b) Internal focusing telescopes
In internal focusing telescopes, the objective is fixed in position. An additional lens called focusing lens, is mounted on a short tube inside the telescope tube. This short tube can move between the diaphragm and the objective. By using the focusing lens, the image formed by the objective is brought in the plane of the crosshairs of the diaphragm.
ii. Eye-piece of the telescope
The eyepiece is fitted on the telescope at the near end;whereas at the far end, there is objective. The eyepiece serves to enlarge the image formed at the plane of the diaphragm. Eyepieces are of two types:
a) Inverting eyepiece
The most commonly used is the Ramsden eyepiece. The objective forms an inverted image of the object in the plane of crosshairs. The Ramsden eyepiece magnifies the image, but the image remains inverted. This type of eyepiece has two Plano convex lenses.
b) Erecting eyepiece
When the telescope is fitted with an erecting eyepiece, an erect and magnified image of the object is obtained. The erecting eyepiece consists of four Plano convex lenses.However there is a loss of brightness of image due to two extra lenses. For all precise instruments, generally the Ramsden Eyepiece is used.
iii. Level tube/bubble tube
This is an essential part of most important surveying instruments. It consists of a glass tube partially filled with a liquid and a bubbled above. The liquid should be stable and non-freezing at ordinary temperature. Also it should have low viscosity. The examples of liquid used in bubble tube are alcohol, ether and chloroform. The tube is graduated on the upper surface. The graduations areequally spaced and symmetrical about the center of the tube. When the bubble is centered in the tube, the axis of the bubble tube is horizontal. The axis of the bubble tube is perpendicular to the vertical line when the bubble is centered and thus gives the direction of the horizontal plane at that point. The vertical axis is the center line of the axis of rotation of the telescope.
Types of levels
Levelling instruments may be provided with external type telescope or internal type telescope. Old levels are of the former type while modern levels have internal focusing telescopes.
According to the construction, the modern levels are classified as
• The Why level
• The Dumpy level
• The reversible level
• The tilting level
• Automatic level
Levelling staff/levelling rod
Levelling staff is a graduated rod of rectangular cross-section.In levelling, the levelling staff is kept vertical at the point whose elevation is known or the point where elevation is to be determined. The levelling staff is sighted through the telescope of the levelling instrument that has been properly set up and adjusted. The staff reading corresponding to the cross hair is noted. They staffs are available in lengths of 3 and 4metres. Brass capsare usually fitted at the bottom of the staff to reduce wear and tear. The bottom of the staff represents zero reading and the graduations on the staff gives the distances from the bottom of the brass cap. Each meter is divided into 200 divisions. The thickness of each division is 5mm. the divisions are painted alternatively as black and white. The top of the numeral indicates the division to which it corresponds.
Types of levelling staff
• Self-reading staff
This type of leveling staff is most commonly used one. The staffman holds the self-reading staff at the point and the instrumentman takes the reading on the staff.
• Target staff
The staffman holds the staff at the point. There is a target attached to the staff, which can be moved up or down. The instrument man directs the staff man to slide the target in the staff till the line of sight bisects the target. The position of the target is fixed now. The staffman takes the reading of the staff.
Classificationof levelling staff based on construction
• Solid staff
It is available in one length of 3m with no joints or hinges in between. There is no possibility of relative movement between parts and hence readings are accurate
• Folding staff
They are available in two pieces of 2m, hinged together so that one piece can be folded over the other when not used. When taking readings, the two pieces are unfolded to form a staff of 4m. IS 1779:1961 covers the specifications for 4m levelling staff. It shall be made of well-seasoned timber of width 75mm and thickness 18mm. The folding staff is of detachable type with a locking device at the back and hence can be detached and used as a 2m staff when required. It is more accurate than the telescopic staff.
• Telescopic staff
The telescopic staff consists of three pieces with a solid upper piece and two hollow lower pieces. The upper piece of 1.25m length can be slid into the middle piece of 1.25m, which in turn can be slid into the lower piece of 1.5m. The staff can be extended to full length when being used. Thethree pieces when fully extended are kept in place by flat brass springs. They are also called as Sopwith pattern staff. It is not as accurate as the folding staff.