What is chain survey

What is chain surveying? Principle, procedure, offset, Instruments, errors

What is Chain Surveying?

Chain surveying is one of the methods of land surveying. It is the type of surveying in which only linear measurements are made, no angular measurements are taken.

It is the simplest and accurate method of land surveying, In this, linear distances are measured on the field. This is a very easy method of surveying.

The Principle of Chain Surveying

The Principle of chain surveying is to divide the whole area into a number of triangles of suitable sides. Great care has to b taken in the formation of triangles os that the process of chain surveying becomes smooth. As far as possible, the triangles formed should resemble the shape of an equilateral triangle. The triangle formed should be a well-conditioned triangle. The triangles which are very nearly equilateral are known as ell conditioned triangles.

Types of Chain surveying

Various types of chains in common use

• Metric chain
• Gunter’s chain to surveyor chain
• Engineer’s chain
• Revenue chain

The instrument used in Chain Surveying

• Chain
• Taps
• Arrows
• Pegs
• Ranging rods
• Offsets rods
• Plumb Bob

Tapes in chain surveying

Measurement take from tapes is much accurate than the measurement taken from the chain. Tapes can be classified into 4 types, depending upon the materials used in the manufacturing of them.

• Cloth tape
• Metallic Tape
• Steel tape
• Invar Tape.

Offsets In chain surveying

Offsets the lateral distance measured for the survey line, to locate the position of a point with respect to the survey line. So the offsets are two types.

• Perpendicular offsets
• Oblique Offsets

Oblique Offsets

Any offset not perpendicular to the chain line is said to be oblique. Oblique is taking when the objects are at a long distance from the chain line or when it is not possible to set up a right angle due to some difficulties.

Perpendicular offsets

When the lateral measurements are taken perpendicular to the chain line, they are known as perpendicular offsets.

Suitability of chain surveying

• The area of surveying is completely small.
• The ground is fairly small.
• The area is open

The procedure of Chain surveying

• Reconnaissance survey
• Making of stations
• Preparation of reference sketches
• Running of a survey line
• Locate the position of the building with reference to the survey line

Reconnaissance survey

It is always useful and often absolutely necessary for the survey to make a preliminary inspection of the area before commencing his actual detailed survey, for the purpose of fixing the survey stations and forming a general plan for the network of the chain lines. Such preliminary inspection of the area is generally known as reconnaissance or reconnoiter.

On arriving at the ground the surveyor should, therefore, walk over the entire area to examine the ground to decide upon the best layout of the chain lines.

During reconnaissance, the surveyor should ensure that the survey stations are intervisible, there is no difficulty in chaining and the angles of the chain triangle are not acute.

Marking Stations

On completion of successful reconnaissance, the survey stations should mark in such a way that they easily discover during the progress of the survey even after some time, if necessary, to revise a faulty work.

In soft ground, wooden pegs are driven, leaving a small portion projected above the ground.

In the case of roads or hard surface ground, nails or spikes may be driven flush with the pavement.

Reference sketches

A brief description of each survey station gives and reference sketches drawn in the field book.

The sketch, showing a least three measurements to permanent and definite points such as the gates, pillars, light posts, corners of the building, etc. know as a reference or location sketch.

Running survey lines

So on completion of preliminary work, the survey lines ar earn as detailed below

• The ranging is done between the end stations of the bare line.
• The chain stretched in the true alignment keeping one end of the chain at the starting station.
• As the arrow is fixed at the other end of the chain and the chain is kept laying on the ground.
• The surveyor walks along the chain line and takes offsets to the adjacent objects on the right side and left side as the case may.
• The chainages on the chain line and the offsets record in the field book.
• So the process of chaining and offsetting is repeated until the end of the baseline is reached.
• Other lines are similarly completed.

Located the position of building with reference to survey lines

• The name or number of the chain line.
• Name or number of the survey station.
• The symbol denoting the station mark.
• The Direction of surveying off form or ending at the station.