study on various type of chain used in chain survey and its component and study of error in surveying

  principle of chain surveying: Chain surveying is the type of surveying in

which only linear measurements are made in the field. The main principle of

chain surveying or chain triangulation is to provide a framework consist of

number of well-conditioned triangles or nearly equilateral triangles. It is used

to find the area of the field. 

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.

5. Survey Station

6. Survey stations are of two kinds

7. Main Stations

8. 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 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 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. 

Well - conditioned triangle: A well conditioned triangle is a triangle in which

no angle is less than 30 degrees. One of the way to survey the area is to divide

the entire area is smaller triangles and then take the measurement of sides of

the triangles.

A well conditioned triangle will have its edges far enough and vertex will be

clearly identified. Ill-conditioned triangle can have ambiguity in vertex position

and so, it can lead to wrong measurements. Compare two triangles below: 

 

 

ERRORS IN CHAINING: 

Errors in chaining may be classified as:

1. Personal errors

2. Compensating errors, and

3. Cumulative errors

Personal errors: Personal errors like wrong reading, wrong recordings, reading

from wrong end or chain and miscounting of the chains are serious errors. It is 

not easy to detect unless they are too big. Hence, care should be taken to avoid

such errors.

Compensating errors: These errors can be positive or negative. Hence, they

are likely to get compensated when a large number of readings are taken. The

following are the examples of such errors: 1) Incorrect marking of the end of

chain

2) Fractional parts of the chain may not be correct when the chain is corrected

by adding or removing a ring.

3) Graduation in the tape may not be exactly of same length throughout

4) In the method of stepping for measuring sloping ground, method of

plumbing may be crude.

Cumulative errors: These are the errors which occur always in the same

direction. Hence, as more number of chain lengths is required while measuring

a line they go on accumulating. Hence, even if each one of such errors are

small they are considerable when longer lengths are measured.

Examples of such errors are: 1. Bad ranging ( +ve) 

2. Bad straightening

3. Non – horizontality

4. Sag in the chain

5. Erroneous length of chain

6. Temperature variation

7. Variation in pull. 

First four errors are always + ve since they make measured length more than

actual. Last three errors may be +ve or –ve.