Chain Surveying

· In chain surveying only linear distances on the field are measured.

· These distances are used to define the boundary of field and mark simple details.

Principle :

It is to form a network of triangles by using the distances measured.

Better accuracy will be obtained if the triangles thus formed are nearly equilateral in shape.

Classification of surveying:

· Chain Surveying

· Compass Surveying

· Theodolite surveying

· Plane Surveying

· Techeometric Surveying

Accessories used in Chain Surveying:

The different accessories used in chain surveying are

(a) Metre Chain

(b) Chain Pins (arrows)

(c) Measuring Tape

(d) Ranging rod/Offset rod.

Metric surveying chain:

· A surveying chain is a device used to measure distance between two points on the ground.

· Metric chains are available in lengths of 5 m, 10m, 20m and 30 m.

· 20m – 30 m chain is normally used for the field of surveying.

· A surveying chain contains brass handles with brass eyebolt and collar, galvanized mild steel links and wire rings.

· In the case of 20 m and 30 m chains, brass tallies are provided at every 5 m length and indicating brass wire rings are attached at every metre length except where tallies are provided.

· The distance between the outside faces of handles of a fully stretched out chain is the length of the chain.

· The length of the chain, like 20m is engraved on the handles.

· While measuring the long distance, the chain will have to be used a number of times.

· Arrows are driven at the end of every chain length.

· For holding the arrows in position, grooves are cut in the outside face of the handles.

· The radius of the groove is the same as that the arrows.

· For convenient handling of the chain, the handle joint is made flexible so that it is possible to swivel to handle round the eye bolt.

Chain Pins:

· Chain pins or arrows are used with the chain for marking each chain length on the ground.

· The arrow is driven into the ground at the end of each chain length is measured.

· Chain pins the arrow should be made of good quality hardened and tempered steel wire of minimum tensile strength of 70 kg/mm2 .

· The overall length is 400 mm and thickness is 4mm.

· The wire should be black enamelled.

· The arrow has a circular eye at the one end is pointed at the other end .

Pegs:

· Wooden pegs of 15cm length and 3 cm square in section are used to establish the station points or the end points of a line on the ground.

· They are tapered one end and are driven into the ground by using a wooden hammer.

· About 4 cm is left projecting above the ground.

Measuring Tape:

There are different types of tapes are used. They are

(a) Cloth or linen type

(b) Metallic Tape

(c) Steel Tape

(d) Invar Tube.

Metallic tape and steel tapes are most commonly used.

· Metallic Tape is made of varnished waterproof linen.

· It is reinforced with fine brass copper or bronze wires.

· Tapes are available in lengths of 10, 15, 20, 30 or 50 metres.

· In metallic tapes every metre is divided into 100 divisions (cms).

· In steel tapes, the centimetre division are also subdivided.

Ranging Rod:

· It is also known as ranging pole or picket.

· Ranging rod is used for ranging or aligning long lines on the ground in field surveying.

· Ranging is a straight line means fixing a series of pegs or other marks such that they all lie on a straight line.

· Ranging rods are used marking points on the ground so that the positions of the points are distinctly visible from some distant way.

· The length of ranging rod may be 2 m and 3 m and its diameter is 30 mm.

· Ranging rod made of steel tube has an internal diameter of 32 mm.

· The ranging rods are made of well-seasoned, straight grained timber of circular cross section.

· Ranging rods should be straight and free from warps.

· The deviation in straightness should not exceed 5mm in a 2 m length.

· The ranging rod is painted in red and white in alternate band lengths of 200 mm each.

· The bottom end of the rod is fitted with a pointed, hollow, cast iron shoe or steel shoe of 15 cm length

Advantages and disadvantages of chain surveying:

Advantages:

· It is simple

· It does not require any costly equipment

· It is adopted for preparing plans for small area

Disadvantages:

· It cannot be used for large areas

· It cannot be used in thick bushy areas with ups and downs.

· Chain surveying is not always accurate.