Chain Survey

Chain survey/surveying is an very old method of Surveying. This article includes definition of chain survey along with all detailed information with necessary images about various aspects of chain surveying. 



Chain used for Chain Surveying

Chain survey is the simplest method of surveying. In chain survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. Here only linear measurements are made i.e. no angular measurements are made.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




Suitability of Chain Survey


Chain survey is suitable in the following cases:
  1. Area to be surveyed is comparatively small
  2. Ground is fairly level
  3. Area is open and
  4. Details to be filled up are simple and less.

Survey Station

Survey stations are of two kinds
  1. Main Stations
  2. Subsidiary or tie


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.


Chain Survey



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 center 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


Offsets 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:
  1. Perpendicular offsets
  2. Oblique offsets.
 Perpendicular offsets and  Oblique offsets.

The measurements are taken at right angle to the survey line called perpendicular or right angled offsets. For setting perpendicular offsets any one of the following methods are used:
  • Swinging
  • Using cross staffs
  • Using optical or prism square
Perpendicular Offset by Swinging:


Perpendicular Offset by Swinging

Chain is stretched along the survey line. An assistant holds the end of tape on the object. Surveyor swings the tape on chain line and selects the point on chain where offset distance is the least (Fig. 12.13) and notes chain reading as well as offset reading in a field book on a neat sketch of the object.


Perpendicular Offsets Using Cross Staffs



Figure 12.14 shows three different types of cross staffs used for setting perpendicular offsets. All cross staffs are having two perpendicular lines of sights. The cross staffs are mounted on stand. First line of sight is set along the chain line and without disturbing setting right angle line of sight is checked to locate the object. With open cross staff (Fig. 12.14 (a)) it is possible to set perpendicular only, while with french cross staff (Fig. 12.14 (b)), even 45ยบ angle can be set. Adjustable cross staff can be used to set any angle also, since there are graduations and upper drum can be rotated over lower drum.
 

Field Book


All observations and measurements taken during chain surveying are to be recorded in a standard field book. It is a oblong book of size 200 mm × 120 mm, which can be carried in the pocket. There are two forms of the book (i) single line and (ii) double line. The pages of a single book are having a red line along the length of the paper in the middle of the width. It indicates the chain line. All chain-ages are written across it. The space on either side of the line is used for sketching the object and for noting offset distances. In double line book there are two blue lines with a space of 15 to 20 mm is the middle of each book. The space between the two lines is utilised for noting the chain-ages. Figure 12.17 shows typical pages of a field books

field book




Procedure in chain survey


  1. Reconnaissance: The preliminary inspection of the area to be surveyed is called reconnaissance. The surveyor inspects the area to be surveyed, survey or prepares index sketch or key plan.
  2. Marking Station: Surveyor fixes up the required no stations at places from where maximum possible stations are possible.
  3. Some of the methods used for marking are:
    • Fixing ranging poles
    • Driving pegs
    • Marking a cross if ground is hard
    • Digging and fixing a stone.
  4. Then he selects the way for passing the main line, which should be horizontal and clean as possible and should pass approximately through the center of work.
  5. Then ranging roads are fixed on the stations.
  6. After fixing the stations, chaining could be started.
  7. Make ranging wherever necessary.
  8. Measure the change and offset.
  9. Enter in the field the book.
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