Study of various instruments used in chain surveying and their uses
- Chain or tape
- Ranging rods
- Cross staff
- Offset rods
- Plumb bob
DESCRIPTION OF INSTRUMENTS:
1 a) Chain:
The chain is composed of 100 or 150 pieces of galvanized mild steel wire, 4mm in diameter called links .The ends of each link are bent into a loop and connected together by means of three oval rings. The ends of the chain are provided with handles for dragging the chain on the ground, each wire with a swivel joint so that the chain can be turned without twisting. The length of the chain is measured from the outside of one handle to the outside of another handle.
Following are the various types of chain in common use:
- Metric chains
- Gunter` s chain or surveyors chain
- Engineers chain
- Revenue chain
- Steel band or Band chain
Metric chains are made in lengths 20m and 30m. Tallies are fixed at
every five-meter length and brass rings are provided at every meter length except where tallies are attached
The following are the various types of tapes
- Cloth tape
- Metallic tape
- Steel tape
- Invar tape
Among the above, metallic tapes are widely used in surveying. A metallic tape is made of varnished strip of waterproof line interwoven with small brass, copper or bronze wires. These are light in weight and flexible and are made 2m, 5m 10m, 20m, 30m, and 50m.
Arrows are made of good quality hardened steel wire of 4 mm diameter.
The arrows are made 400 mm in length, are pointed at one and the other end is bent into a loop or circle
Ranging rods are used to range some intermediate points in the survey line The length of the ranging rod is either 2m or 3m. They are shod at bottom with a heavy iron point. Ranging rods are divided into equal parts 0.2m long and they are painted alternately black and white or red and white or red, white and black. When they are at considerable distance, red and white or white and yellow flags about 25 cm square should be fastened at the top.
- Cross staff: The simplest instrument used for setting out a right angle. The common forms of cross staff are:
|Open cross staff||French cross staff||Adjustable cross staff|
The offset rod is used for measuring the off set of short lengths. It is similar to a ranging rod and is usually of 3m lengths.
These are rods made from hard timber and tapered at one end, generally 25mm or 30mm square and 150mm long wooden pegs are used to mark the position of the station on.
While chaining along sloping ground, a plumb bob is required to transfer the points to the ground.
|OFFSET ROD||PEGS||PLUMB BOB|