Various instruments are used in Chain surveying.

 

A) Chain

Chains are the measuring instrument used in surveying formed by the 100 links of 4mm galvanized mild steel wire. These links are joined by 3 circular or oval wire rings. These rings provide flexibility to the chains.

Every aspect of life requires some measuring units. Measurements are used to do the work precisely and accurately. Let it be from kitchen to office, everywhere measurements are used. So as in engineering calculation or measurements holds a very greater role in construction or surveying or any other aspect.

There are various units of measurements such as meters, centimetres, feet, inches, acre, yards and the list goes on. Same as units there is the various instrument used in the measurements of any entity. One of the instruments used in measurement is chains.

                                                                   Chain

Parts of Chains used in Surveying

The chain consists of many small parts used for handling or reading the measurements.

• At the ends, the chain is provided with brass handle with swivel joint so that it can be easy to roll or unroll the chain without twisting and knots.
• At every 10^th link is provided with a tally of one teeth, 20^th link with a tally of two teeth and so on till 40^th link. This is provided for the easy reading of measurements.
• At the centre of the chain is provided with a circular talley used for easy reading.

Types of Chains used in Surveying

Depending upon the length of the chain, these are divide into following types,

1. Metric chains
2. Steel band or Band chain
3. Gunter’s chain or surveyor’s chain
4. Engineer’s chain
5. Revenue chain

1. Metric chains

Metric chains are the most commonly used chain in India. These types of chains comes in many lengths such as 5, 10, 20 and 30 meters. Most commonly used is 20m chain. Tallies are provided at every 2m of the chain for quick reading. Every link of this type of chain is 0.2m. The total length of the chain is marked on the brass handle at the ends.

2. Steel band or Band chain

These types of chain consisting of a long narrow strip of steel of uniform width of 12 to 16 mm and thickness of 0.3 to 0.6 mm. this chain is divided by brass studs at every 20cm or instead of brass studs, band chain may have graduated engraving as a centimetre.

For easy use and workability, band chains are wound on steel crosses or metal reels from which they can be easily unrolled. These steel bands are available in 20m and 30m length and the width of about 12-16mm.

3. Gunter’s chain or surveyor’s chain

Gunter chain comes in standard 66ft. This chain consists of 100links, each link being 0.66ft or 7.92inches. The length 66ft is selected because it is convenient in land measurements.

10 square Gunter’s chains = 1 Acre

10 Gunter chains = 1 Furlong

80 Gunter chains = 1 mile

4. Engineer’s chain

This chain comes in 100ft length. Its consist of 100 links each link being 1ft long. At every 10 links, a brass ring or tags are provided for indication of 10 links. Readings are taken in feet and decimal.

5. Revenue Chain

The standard size of this type of chain is 33ft. The number of links is 16, each link being 2   ft. This chain is commonly used in the cadastral survey.

*Testing and Adjustment of Chain

As the chain is a metal made, it may undergo many changes due to temperature effect or human error and etc. So for all lengths of the chain a tolerance are given,

5m chain = + or – 3mm

10m chain = + or – 3mm

20m chain = + or – 5mm

30m chain = + or – 8mm

*Chain length shorten due to

1. Bending of links.
2. Sticking of mud in the rings

*Chain length increases due to

• Opening of small rings.
• Wearing of surfaces.

*Chains maybe tested with respect to

• Steel tape
• Permanent test gauge
• Pegs are driven in the field at required distances
• Permanent test gauge made with dressed stones

*If the chain is found long, then

• Close the joins of the rings
• Reshape the elongated rings
• Remove one or two rings
• Replace worn out rings

*If the chain is found short, then

• Straighten the links
• Replace the small rings with a big one
• Insert additional rings

Flattening the circular rings

*Advantages and Disadvantages of Chains in Surveying

a) Advantages of Chains in Surveying

• Chain survey is the simplest and commonest method used in surveying exercises
• The equipment used to conduct chain survey are simple to use,
• The equipment used in chain survey can easily be replaced. For example, measuring rods can be replaced with measuring tape.
• This method does not involve a complicated mathematical calculation. I know this is the relief to those who are afraid of mathematics
• In chain survey few people are needed to conduct the survey. Normally chain survey team has three people Booker, leader and follower.

b) Disadvantages of Chains in Surveying

• Simple chain survey cannot be conducted in built-up areas and large areas.
• Simple chain survey is subject to several chances of errors of accumulation which may cause by the problem of the chain. The chain linkage may fail to stretch up properly and result in inaccurate data. Also clogging of a chain may read to an error in reading.
• It is time-consuming
• It may not be conducted in areas with steep slopes or waterlogged areas. Chain survey is usually conducted in dry areas with gentle slopes. It becomes more complicated when a survey is conducted in areas that are too wet.

Chain survey becomes more complicated method when there are raised points (obstacles) in between areas to be surveyed.

2) Tapes are grouped into four categories depending upon the material used for its construction

o    Cloth or Linnen tape

o    Woven metallic tape

o    Metric steel tape

o    Inver tape

o    Synthetic tape

o    Cloth or linen tape

It is made of a varnished strip of woven linen 12 to 15 mm wide. It is available in lengths of 10m, and 30m and 30m it is little used in surveying, but can be used for making subsidiary measurements such as offsets of a building.

a) Woven metallic tape

It is made of cloth strip woven with fine brass wires. It is also 16mm wide and available in length of 10, 15, 30 and 60m. The brass wires durable prevent the tape from twisting and stretching. It is more durable than cloth tape. Hence, it is used for general purposes.

b) Metric steel tap

It is made of steel or strain – less steel, may be provided with a vinyl coating. It is also 6mm to 10mm wide and available in length of 10, 15, 30 and 60m. it is used to measure the distance accurately.

c) Inver tape

It is made of an alloy of steel and nickel. It is also 6mm wide and available in length 15, 30, and 100m. It has great accuracy. Hence, it is used for work of the highest precision such as measurement of the baseline in the triangulation survey and in city work.

d) Synthetic tape

It is made of glass – fibre having P.V.C coating. It maintains its length. It is strong and durable. So it is used for measuring length with a good degree of precision. It is available in 5m, 10m,20m and 30m length.

 

C) Arrows

Arrows or chain pin is a rod of iron or steel, of diameter 4mm. it is 300 to 400mm in length. The arrow has a loop of diameter 50mm at one end whereas the other end is pointed for a length of 15mm. they are used for recording the chain length measured.

 

D) Pages

It is made of hardwood and is 2.5cm square in cross-section and 150mm long. It is tapered
At another end to facilitate easy driving. It is used to mark the position of the survey station or the endpoints of survey line. The pegs are driven into the ground using a mallet or wooden hammer such that its length of about 40 mm project above the surface of the ground.

E) Ranging rods

These are made of timber and steel. They are circular, octagonal in shape and of diameter 2 to 3cm. they are provided with a tapering edge and shade with cross shoe 150mm long at the bottom end facilitate easy driving. There is an order to make them visible from a distance, they are pointed alternately black and white or red and white. Ranging is necessary before starting the measurement of a line whose length is more than the chain length so that the measurement is done in a straight line.

F) Offset rods

An offset rod is a similar ranging rod and has a length of 3. They are round wooden rod shoed with a pointed iron shoe at the one end and provided with a notch or a hook at the other. The hook facilities pulling and chain through hedges and other obstructions. 

H) lath

The laths used by plasters for plastering the walls may be used for ranging in a level or open ground with an obstruction such as hedges, walls, or when crossing a depression. They are very light in weight and can be carried from place to place easily. The lower end can be easily sharpened to a point and cut to any desired length when required.

I) Whites

Whites are the pieces of sharpened thick sticks cut from the nearest place in the field. One end of the stick is sharpened and the other end is split. White papers are inserted in the split to improve visibility. Whites are also used for the same purpose as laths.

J) Plumb Bob

The pump is a ball made of brass or bronze of the shape of a pear. It has a fine steel point. There is a hook at the top for attaching a string of nylon. Its length is about 50mm. the plumb bob is used for measuring distance on the sloping ground. used in the centring of various instruments such as a magnetic compass, plane table, dumpy level or theodolite etc.