Chain Surveying | Definition, Details, Procedure – 2020 Guide – Chain survey/surveying is a very old method of Surveying.
This article includes the definition of chain survey along with all detailed information with necessary images about various aspects of chain surveying, Chain Surveying -Procedure, principle , Instrument, Objectives , Types of Chain Surveying. Surveying chain image is given Below. We Will also discuss types of chain surveying in civil engineering.
Definition of Chain Surveying – Chain Surveying is that type of surveying in which only linear measurement are made in the field. Chain surveying is suitable for surveying of small extent on open ground to secure data for exact description of the boundaries of a piece of land or to take simple details.
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 suitably adapted to small plane areas with very few details. If carefully done, it gives quite accurate results.
The necessary requirements for field work are
- Cross staff
- Chain Triangulation
- Survey Stations
- Survey Lines
- Locating Ground Features: Offsets
- Field Book
- Field Work
- Instruments for Setting Out Right Angles
- Basic Problems in Chaining
- Obstacles in Chaining
- Cross Staff Survey
- Plotting a Chain Survey
The principle of chain surveying or chain triangulation as it is sometime called is to provide a framework consisting of a number of connected triangles.
Suitability of Chain Survey
Chain survey is suitable in the following cases:
- The area to be surveyed is comparatively small
- The ground is fairly level
- The area is open and
- Details to be filled up are simple and less.
What is station in chain surveying?
Survey stations are of two kinds-:
- Main Stations
- Subsidiary or tie
Main Stations (Survey stations)
Main stations are the end of the lines, which command the boundaries of the survey, and the lines joining the main stations recalled the main survey line or the chain lines.
Subsidiary or the tie stations
Subsidiary or the tie stations are the points 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 in Chain Surveying
A tie line joints two fixed points on the main survey lines. It helps to check 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 in Chain Surveying
It is the 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 to this line.
Check Line in Chain Surveying
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 checking line, as measured on the ground should agree with its length on the plan.
Offsets in Chain Surveying
What is offset in chain surveying ?? Offsets are the lateral measurements from the baseline to fix the positions of the different objects of the work with respect to the baseline. 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:
- Perpendicular offsets
- 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 is used:
- Using cross staffs
- Using optical or prism square
Perpendicular Offset by Swinging:
The chain is stretched along the survey line. An assistant holds the end of the tape on the object. Surveyor swings the tape on chain line and selects the point on the 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 a stand. The 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)), an even 45º angle can be set. Adjustable cross staff can be used to set any angle also since there are graduations and upperdrum can be rotated over a lower drum.
Field Book in Chain Surveying
All observations and measurements taken during chain surveying are to be recorded in a standard field book. It is an 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 in the middle of each book. The space between the two lines is utilized for noting the chain-ages. Figure 12.17 shows typical pages of field books.
Survey Stations:- The fixed Points
Survey Lines:- Base Lines, Check Lines, Tie Lines
Survey line and Baseline:- The line joining the main survey stations are called main survey line. The biggest of the main survey line is called the baseline and the various survey stations are plotted with reference to these. If the area to be surveyed has more than three straight boundaries, the field measurements must be so arranged that they can be plotted by laying down the triangles.
Check Lines(Proof Lines):– Check lines are the lines which are run in the field to check the accuracy of the work. The length of the check line measured in the field must agree with its length on the plan.A check line may be laid by joining the apex of the triangle to any point on the opposite side or by joining two points on any two sides of a triangle. Each triangle must have a checking line.
Tie Lines:- A Tie line is a line which joins subsidiary or tie stations on the mainline. The main object of running a tie line is to take the details of nearby objects but it also serves the purpose of check lines. The accuracy in the location of the objects depends upon the accuracy in laying the tie line.
Conditions to be fulfilled by Survey lines or Survey stations:-
- Survey stations must be mutually visible
- Survey lines must be as few as possible. So that the framework can be plotted easily.
- The framework must have one, or 2 baselines.
- The main lines should form well-conditioned triangles.
- Each triangle for the caution of the framework must be provided with sufficient check lines.
Procedure in chain survey
- Reconnaissance: The preliminary inspection of the area to be surveyed is called reconnaissance. The surveyor inspects the area to be surveyed, surveyor prepares index sketch or key plan.
- Marking Station: Surveyor fixes up the required no stations at places from where maximum possible stations are possible.
Some of the methods used for marking are:
- Fixing ranging poles
- Driving pegs
- Marking a cross if the ground is hard
- Digging and fixing a stone.
- 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.
- Then ranging roads are fixed on the stations.
- After fixing the stations, chaining could be started.
- Make ranging wherever necessary.
- Measure the change and offset.
- Enter in the field the book.
What is the basic principle of chain surveying?
The principle of chain surveying is triangulation. This means that the area to survey is spilled into a number of small triangles which should be well-conditioned.
In chain surveying, the side of the triangles are measured directly from the field by chain or tape, and no angular measurements are used. Here, the check lines and tie lines control the accuracy of the given work.
It is noted that plotting triangles requires no angular measurements to be made if the three sides are known.
Chain surveying used in this situation
- More or less level of the ground surface.
- A small area is to be surveyed.
- A small-scale map is to be prepared.
- The formation of well-conditioned triangles is easy.
Chain surveying is unsuitable when
- The area is crowded with many details.
- The area consists of too many undulations.
- The area is very large.
- The formation of well-conditioned triangles becomes difficult because of obstacles.
Obstacles in Chain Surveying
- Possible to chain around the obstructed
- Impossible to a chain around the obstacle
Obstructed in Chaining & Ranging
Chain Surveying Related QUESTION AND ANSWER
1 A trangle is said to be well-conditioned when its angles should lie between
= 300 and 1200
2 Perpendicular offsets my be taken by setting the right angle in the ratio
3 Oblique offset consider
4 Number of offset consider
= straight, parallel, zigzag boundary
5 Limiting length of offset
6 The size of the field book is
= 20cm * 12cm
7 Diffarent type of cross staff
= 3 type
8 What the principle of chain surveying?
9 Chain survey is recommended when the are is
= simple and fairly level
10 In chain survey, the area is divided into
11 The sketch prepared during reconnaissance survey is known as the
= index sketch
12 The working principle of the optical square is based on
= double reflection
13 The field records of the chain survey is entered in a/an
= field book
14 The chainman who drags the chain is called the
15 The ptrliminary inspection of the area to be surveyed is known as
= reconnaissance survey
16 The limiting length of offset depends upon the
= scale of plotting
17 The main survey stations are located on the ground by
= reference sketches
18 For taking an oblique offset which makes an angle of 450 with the chain line, the instrument used is the
= French cross-staff
19 If a wooded area obstructs the chain line then it is crossed by the
= random line
20 A flag is provided on a ranging rod when the ranging distance exceeds about
21 A cross-staff is used for
= setting perpendicular lines to survey line
22 The position of a point can be fixed more accurately by
= perpendicular offset
23 The main object of running a tie line is
= to take details of a nearby object
24 Which of the following angles can be set out with the help of French cross-staff?
= either 450 or 900
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