Introduction:
No doubt in chain surveying obviously change or tape is required but apart from chain or tape some additional instruments used in chain surveying such that Arrows, Ranging rods, Offset rods, Pegs, Plumb bob, Clinometer, cross-staff etc…
Table of Contents
These are 7 instruments used in chain surveying or simply changing which is listed below:
Arrow:
The arrow is the instrument used in chain surveying to marks the end position of the chain or tape on the ground surface Arrow is made of good quality hard tempered steel wire of 4 mm diameter. Generally, at least 10 arrows are supplied with a single chain. The length of the Arrows varies between 25 cm to 50 cm.
As per IS-1842: The overall length of the Arrow is taken as 40 cm and 5 cm diameter loop. One end of a row is sharp and the other end of the Arrow is bent into a loop of 5 cm diameter as shown in the figure.
Advantage of Arrow:
Arrows are used to record the number of chain lengths or tape lengths which is already laid during measuring the survey line.
Ranging rod:
Ranging rods is the instruments used in chain surveying which are used to locate intermediate points such that this point lies on the straight-lining joining the two end stations.
Ranging rods are generally used for ranging of two points on the ground. A ranging rod is required in cases where the length of the chain and tape is small as compared to the distance between the two survey stations.
Ranging rates are made from well seasoned timber with 30 mm of nominal diameter.
As per IS 2288 ranging rods are available in 2 m long length and 3 m long length. 2 m long length ranging rods are available with 10 bands of 200 mm each and 3 m long length ranging rods with 15 bands each of 200 mm length. These bands are painted black and white alternatively to make them clear and visible for long distances.
When the distance is very large then flags of red and white colour or sometimes yellow and white colour of size 250 mm x 500 mm are used which are attached at the top of the ranging rod as shown in the figure.
The bottom of the ranging rod is fitted with a hollow cast iron or steel sheet so that its end does not wear easily.
The shapes of the ranging rod are circular or octagonal in cross-section and diameter of 30 mm generally.
Ranging poles:
Ranging pols are instruments used in chain surveying it is similar to ranging rods but ranging pols are longer and greater in diameter than ranging rods and it is used in case of very long survey lines. The length of ranging poles varies from 4 m to 8 m and the diameter from 60 mm to 100 mm. Provided greater stability as compared to ranging roads
Offset rods:
Offset rods are similar to ranging rods but it is provided with short slots passing through the section of the rod just like in cross-staff. These slots are made at a right angle to each other.
Offset rod is instruments used in chain surveying which used to measure the offset line which is at the right angle to the survey line.
The length of the offset rod is usually 3m and is provided with a steel shoe at one end of the offset rod.
With the help of offset red offset lines are aligned at a right angle to the survey line by looking through two slots. A typical diagram of an offset rod. As shown in figure.
Pegs:
Pegs instruments used in chain surveying which is used to mark the survey station and end point of survey lines on the ground these pegs are also used to mark the intersection of lines and other such points which are more or less permanent.
Pegs are generally made of wood with a square section and tapper at one end. The size pages are 25mm X 25mm X 150mm long and 40mm X 40mm X 400mm long. The pegs are driven into the ground using a millet with about 40 mm length project one above the ground A nail is driven into the peg to mark the exact location of the station.
For very hard stratum, instead of wooden pegs steel dowels are used of about 10 mm diameter. As shown in figure
Plumb Bob:
A freely suspended plumb bob always aligns itself in the direction of gravity and points towards the centre of the earth. It indicates whether a line is truly vertical or not.
Plumb Bob is instruments used in chain surveying which used to placing the tape directly over a point when a tape or a chain is suspended over it. These are also used to transfer the point on the tape to the ground (generally in case of sloping ground). In surveying plumb bob is mainly used to centring instruments like magnetic compass, theodolite, plane table etc.
Plumb Bob is made up of bronze or brass in the shape of an inverted cone, a hook is provided at the top of the plumb Bob to attach a string. The weight of the Plumb Bob varies from 2 Newton to 5 Newton and the length of Plumb Bob is about 50 mm. As shown in figure.
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Cross staff:
The cross-staff is the instrument used in the chain survey for setting out the offsets to the chain line from the given point. The cross-staff is also used for setting out a right angle on the ground. For very high accuracy demanding work, a theodolite is used to lay of right angle to the chain line. As shown in figure
Clinometer:
The clinometer is the instrument used in chain surveys for measuring the slope of the ground. It consists of a graduated protector with a pinhole at the eye vane. A plum Bob is suspended from the centre of the protector. When the ground surface is horizontal then the plum bob marks zero reading and when the ground surface is sloping the plumb Bob Mark the corresponding reading and the gives the slope of the ground. Typical diagram of clinometer is shown in figure Clinometer is on in figure.
