Instruments used for chain surveying

The various instruments used in chain surveying are as follows. 

· ü chain

· ü arrows

· ü pegs

· ü ranging rods

· ü offset rods

· ü plumb bob

1.  Chain:

Chains are used to measure horizontal distances. Chains are formed of straight links of galvanized mild steel wire called links. The ends of each link are bent into a loop hand connected together by means of three oval rings which afford flexibility to the chain.

The chain s made of mind steel. The ends of the chin are provided with brass handles for dragging the chain on the ground. the outside of the handle is the zero point or the end point of the chain and the length of the chain measured from the outside of one handle to the outside of the other. The length of a link is the distance between the centers of the two consecutive middle rings.

The end links include the handles. Metallic tags are indicators of the chain to facilitate quick reading of fraction of a chain in surveying measurements. Metric survey chains are available in lengths of 20m and 30m. The 20m chain contains 100 links whereas 30m. Chain contains 150 links. One link of both the type of chain measure 20cm.

2. Arrows:

Arrows or making pins are made of tempered steel wire 4mm in diameter and generally 10 arrows are supplied with a chain. An arrow is inserted into the ground after the chain length is measured on the ground. Usually the length of an arrow is 40cm and one end of it is made sharp and the other end is bent into a circle for facility of carrying.

3. Pegs:

Wooden pegs are used to mark the positions of the stations terminal points of a survey line. They are made of hard timber, generally 2.5cm or 3 cm square and 15cm long, tapered at the end.

4. Ranging Rods

The ranging rods are used for making the positions of stations and for ranging the lines. They are made of ell seasoned straight grained timber teak. They circular in cross section of 3cm diameter and have a length of either 2 or 3cm, lengh being more common. They are shod at the bottom with a heavy iron points. In order to make them visible at a distance, they are pained alternatively black and white or red and white.

5. Offset

Offset rod is similar to that of ranging rod. They are should with pointed iron shoe at one end, ad provided with a notch or a hook at the other for pulling or pushing the chain through a hedges or other obstructions.

6. Plumb Bob

While chaining along sloping ground, a plump is required to transfer the points to the ground. It is also used for accurate centering of the theodolite compass, plane table etc over a station mark and for testing the vertically of ranging poles

7. Cross staff

This is the instrument used for setting out right angles to a chain line. It consists of either a frame or box with two pairs of vertical slits and is mounted on a pole shod for fixing in the ground

ü Open cross staff

ü French cross staff

ü Adjustable cross staff

Principle of Chain Surveying

The rectangle is the simplest fig that can be plotted from the lengths of its sided. Based on this, the principle of chain surveying is to divide the area to be surveyed into a network of connected triangles. Hence chain surveying is some times called chain triangulation. The exact arrangement of triangles to be adopted depends upon the shape and configuration of the ground and obstacles met with. When it contains no angle smaller than 30degree greater than 120 degree.