Chain surveying is the branch of surveying in which linear measurements are made in the field, usually suitable for the survey of small areas with simple details and an area that is fairly flat, it derives its name from the fact that the most commonly used instrument in the chain.
It is the simplest method of detail survey in which only linear measurements are taken in the field.
In this method, the length of the lines marked on the field is measured, while the details from this line is measured by the offset.
Principle of Chain Surveying:
- The principle of chain surveying is to divide the entire area into several triangles of suitable sides.
- Care must be taken in the construction of triangles so that the chain survey process can be smooth.
- As far as possible, the triangles formed should be equilateral triangle shaped and the triangle formed should be a well-conditioned triangle.
- Triangles that are almost very equilateral are known as ill-conditioned triangles.
Instrument used in Chain Surveying:
Chain in surveying:
Various types of chains in common use as follows:
- Metric chain.
- Gunter’s chain to surveyor chain.
- Engineer’s chain.
- Revenue chain.
Tapes in surveying:
Taking measurements from tapes is very accurate as compared to chain, while tapes can be classified into 4 types, depending on the materials used in their manufacture which as follows:
- Cloth tape.
- Metallic Tape.
- Steel tape.
- Invar Tape.
Offsets in surveying:
Offsets is used to measure the lateral distance for the survey line and to find the distance of the point with respect to the survey line, the offsets are of two types as follows:
- Vertical Offset:
When lateral measurements are taken perpendicular to the chain line, it is called vertical offset.
2. Oblique offsets:
Any offset that is not perpendicular to the chain line is called oblique offsets while oblique is taking when objects are a long distance from the chain line or when it is not possible to establish a correct angle due to some difficulties.
Technical Terms Used in chain Surveying:
These are significant points fixed on the ground that indicate the initial point and end point of the survey line.
These are the basic control points of the survey, there various types as follows.
The main stations are the control points at the ends of chain lines leading to the boundaries of the survey.
Subsidiary or Tie Station:
These are stations selected on the main survey lines to run auxiliary lines designed to detect, measure and plot internal details.
The survey stations are chosen in a suitable manner so that at least the main survey stations are mutually visible and the survey lines run through the flat ground and areas as close to the boundaries as much as possible.
These stations are the main survey lines or any other survey lines known as ‘auxiliary stations’.
These stations are taken to divide the area into triangles, check the accuracy of the triangle and trace the internal details.
These stations are also a subsidiary stations on the main survey lines.
The lines connecting the stations are known as ‘tie lines’, these lines are taken to determine the internal details.
2.Main Survey Lines:
The line joining the main stations is called the main survey lines or chain lines.
The line on which the survey is designed are well-known as ‘baseline’, it is the most essential line of the survey.
Generally, the longest survey lines is considered baseline, these lines must be taken through impartially ground level, and it must be measured with great care and accuracy.
The line joining the vertex of a triangle to some fixed points on its base is known as a check line.
It is used to check the accuracy of the triangle, sometimes this line helps to detect internal details.
5. Tie Line:
The line joining the tie stations is called the tie line used to check for internal details that are away from the main lines and also to avoid long offsets.
It can also serve as a checking line.
Procedure for Chain surveying:
1. Reconnaissance survey:
It is always useful and often necessary for the survey to conduct a preliminary inspection of the area before beginning its actual detailed survey.
For the purpose of fixing survey stations and creating a general plan for a network of chain lines, this preliminary inspection of the area is generally referred to as reconnaissance or reconnoiter.
Upon reaching the ground, the surveyor must walk the entire area to examine the ground to decide the best layout of chain lines.
During reconnaissance, the surveyor must ensure that the survey stations are inseparable and must have no difficulty in chaining, and the angles of the chain triangles are not acute.
2. Marking Stations:
On completion of a successful reconnaissance, the survey stations should be marked in such a way that they can easily be discovered to modify a faulty task after some time during the progress of the survey, if necessary.
In soft ground, wooden pegs are driven, leaving a small section above the ground.
In the case of roads or hard-surface ground, nails or spikes may flow from the pavement.
3. Reference sketches:
A brief description of each survey station gives reference drawings given in the field book.
The sketch showing a least three measurements a permanent or fixed points such as a gate, pillar, light post, corner of a building, etc. is known as a reference or location sketch.
4. Running survey lines:
Upon completion of the preliminary work, the survey lines are acquired as detailed below:
The end of the bare line is made between stations, chain extends across the real alignment that places 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 placed on the ground, the surveyor moves along the chain line and takes offsets for adjacent objects to the right and to the left.
The chains record the age and offset in the field book on the chain line therefore, the process of chaining and offsetting is repeated until the end of the baseline is reached.
Other lines are also completed in a similar way, construction conditions are located in the context of survey lines as follows:
- The name or number of the chain line.
- Name or number of survey station.
- The symbol denoting the station mark.
- Direction to survey the form or end at the station.
Error in chain surveying:
The errors can be cumulative or compensating.
- Cumulative errors are in addition to those tens as the measurement continues.
- Compensating errors are those which tend to become positive sometimes and negative at other time.
The various sources of errors in the series of survey are as follows:
1. Incorrect length of chain:
The actual length of the chain used for measurement may differ from the specified length.
The chain may be shorter or longer than the prescribed length so that the chain length should check before the start of the survey and before the chaining.
2. Loose chain:
If the chain is not pulling correctly, the measured length will always be longer, so that the chain is pulled correctly during chaining.
3. Incorrect Ranging:
If the chain is kept out of line during the measurement alone, the measured length will always be longer, so the ranging should be correct before taking the measurement.
4. Temperature change:
If a chain or tape is standardized at some temperature and if temperature varies greatly during the measurement.
There will be errors in receiving due to changes in chain or tape length, this type of error must be compensated by correcting the measured length.
5. Variation in pull:
If the pull applied during the chain is different from the standard pull, the tape length will change, so that the chaining pull is applied correctly during the pull shot.
6. Errors in slope measurement:
When measuring a slope using the stepping method, the series may be straightened during the measurement resulting in an error, so the chain should be kept straight while measuring with sloping ground.
Also read: Tacheometry & Contour Interval
Advantages of Chain Surveying:
- The tools used to conduct chain surveys are simple to use.
- Chain survey is the simplest and common method used in surveying exercises.
- The devices used in series surveys can be easily replaced.
Disadvantages of Chain surveying:
- A simple chain survey cannot be done in built-up areas.
- It is time-consuming.
- Chain surveying becomes a more complex method when points are increased in between the areas being surveyed.
Also read: Plane Table Surveying, Total Station & Town Planning
Chain Surveying is used for small-scale surveys to describe the boundaries of plots and to locate the existing feature on them.
In this method the area is divided into various triangles, the edges of the different triangles are measured in a square field with a chain or tape and no angular measurements are taken.