Traditional survey - chain survey -02

  Chain Survey

Chain survey is the simplest method of surveying. In this survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. This is most suitable adapted to small plane areas with very few details. If carefully done, it gives quite accurate results. The necessary requirements for field work are chain, tape, ranging rod, arrows and some time cross staff.



Principles of Chain Survey
A triangle is a simple figure which can be plotted from the lengths of three sides even if the angles are not known. In chain survey, the area to be surveyed is divided into a framework consisting of triangles.
Selection of survey station and Survey lines
I. The main station should be mutually invisible so that ranging can be done easily.
II. Survey lines should be as few as possible so that framework of triangles can be plotted easily.
III. Survey lines should pass through a level open ground as far as possible so that chaining can be done easily.
IV. The frame should have at least one long baseline that runs through the middle of the area.
V. The main survey lines should form well conditioned triangles with no angles less than 30 degrees or more than 120 degrees.
VI. The survey lines should be selected such that there are no obstacles in chaining and ranging.
VII. The survey lines will be close to the details to avoid long offsets.
VIII. If required, number of subsidiary stations should be selected and subsidiary lines run to avoid long offsets.
IX. Each triangle should have a check line to detect mistakes in measurements and plotting.
X. While selecting main stations and survey lines, the basic principles of surveying of working from whole to the part should be followed.

Main Stations
Main stations are the end of the lines, which command the boundaries of the survey, and the lines joining the main stations re called the main survey line or the chain lines.

Subsidiary or the tie stations
Subsidiary or the tie stations are the point selected on the main survey lines, where it is necessary to locate the interior detail such as fences, hedges, building etc.

Tie or subsidiary lines
A tie line joints two fixed points on the main survey lines. It helps to checking the accuracy of surveying and to locate the interior details. The position of each tie line should be close to some features, such as paths, building etc.



Base Lines
It is main and longest line, which passes approximately through the centre of the field. All the other measurements to show the details of the work are taken with respect of this line.

Check Line
A check line also termed as a proof line is a line joining the apex of a triangle to some fixed points on any two sides of a triangle. A check line is measured to check the accuracy of the framework. The length of a check line, as measured on the ground should agree with its length on the plan.

Offsets
These are the lateral measurements from the base line to fix the positions of the different objects of the work with respect to base line. These are generally set at right angle offsets. It can also be drawn with the help of a tape. There are two kinds of offsets:
• The measurements are taken at right angle to the survey line called perpendicular or right angled offsets.
• The measurements which are not made at right angles to the survey line are called oblique offsets or tie line offsets.

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Traditional survey - chain survey-01

Chain

The chain is a instrument and unit, using for surveying and measure distances. The chains used in surveying are generally of the following types.

I. Gunter’s chain

II. Engineer’s chain

III. Metric chain

IV. Steel Band Chain

V. Revenue Chain

         

I. Gunter’s chain

surveyor’s chain, also called Gunter’s chain, measuring device and arbitrary measurement unit still widely used for surveying in English-speaking countries. Invented by the English mathematician Edmund Gunter in the early 17th century, Gunter’s chain is exactly 22 yards (about 20 m) long and divided into 100 links. In the device, each link is a solid bar. Measurement of the public land systems of the United States and Canada is based on Gunter’s chain. An area of 10 square chains is equal to one acre.

II. Engineer’s chain

This steel engineers’ chain would have been used by a surveyor measuring out lands and plot sizes.  It is different from a surveyor’s chain as it is larger in length however it would have been used for the same purpose, measuring out horizontal distances.  The chain measures 100 ft and contains 100 links, each measuring 12 inches.  The links each consist of a thin round steel rod joined with an oval ring on each end.  Tags mark the chain at every ten links.  It was made by W. & L.E. Gurley of Troy, New York.  Brothers William Gurley and Lewis formed the company in 1852; they were both graduates of Rensselaer Polytechnic Institute in New York.

III. Metric chain

A metric chain is generally divided into 100 or 150 links. The links are composed of pieces of galvanized mild steel wire 4mm in diameter. The ends of each link are bent into loops and connected together by means of three oval shaped rings which afford the flexibility to the chain. The joints of the links are usually pen but in good quality chains, these are welded so that true length of the chain does not alter due to stretching. The ends of the chain are provided with brass handles with swivel joints so that the chain can be turned round without twisting. The outside of the handle is the zero point or the end point of the chain. The length of a chain is 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 also include the length of the handle. Metallic tags of different patterns are fixed at various important points of c chain.

IV. Steel Band Chain

It consists of a ribbon of steel with bras handle at each end. It is 20 or 30 long and 16 mm wide. It is wound on an open steel cross or on the metal reel in a closed case. The graduations are etched as meters decimeters, centimeters on one side and 0.2 m links on the other. Brass tallies are fixed at every 5 m length of the band.

V. Revenue Chain

It is commonly used for measuring fields in cadastral survey. It is 33 ft long and divided into 16 links. Each link is 2.0625 ft long.

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