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Unit 4 : Chain Surveying (Offsets)

Survey Stations: 

Survey stations are the points at the beginning and end of a chain line. These stations can be categorized as follows:

Main Stations:

Main stations are located along the boundary of an area and serve as controlling points.The line connecting main stations is referred to as the main survey line.Main stations are denoted by letters (e.g., A, B, C, D).They should cover the entire area to be surveyed.



Subsidiary Stations:

Subsidiary stations coincide with the main survey line or any other survey line.
These stations play a crucial role in subdividing the main triangulated area into smaller triangles.
Subsidiary stations are essential for checking the accuracy of a triangle and locating interior details.
They are denoted by a circle and letters (e.g., S1, S2, S3, etc.).

Tie Stations:

Tie stations are also subsidiary stations taken along the main survey line.
The lines connecting tie stations are known as tie lines.
Tie lines are used in chain traversing when triangulation is not feasible.
Sometimes tie lines help locate interior details.
Tie stations are denoted by a circle with a center dot and letters (e.g., T1, T2, T3, etc.).

Base Line:

The base line is the fundamental line upon which the survey framework is built.
It is the most critical line in the survey.
Generally, the longest survey line is designated as the base line.ey ensure accurate alignment and measurement.

 Check Line:

These lines are run to verify the accuracy of the framework of a triangle.
Used for quality control and ensuring precise measurements.

Offset:

Lateral measurements taken from an object to the chain line.
Two types of offsets:

Perpendicular Offset

Oblique Offset

Perpendicular Offset:

1. By setting a perpendicular offets with swinging. 


Measured perpendicular to the chain line.
Fix a perpendicular using a tape from the object to the chain line.
Read the tape to get the minimum measurement.

2. By setting a right angle in the ratio 3:4:5 apart


 
3. By setting a right angle with the help of builders square or try square

4. By setting a right angle by optical square

 

Oblique Offset:

Taken at an angle other than 90 degrees to the chain line.
Useful for locating objects relative to the chaining.Oblique offsets are taken when the offsets are long and perpedicular offsets cannot be set.

Required Number of Offsets

The number of offsets are taken as per the nature of ground.

1. When the boundary of ground is approximately parallel to chain line then offsets are taken at equal interval.

 


2. If the boundary is straight, perpendicular offsets are taken at both ends of it.


 

3. When the boundary line is zig-zag, perpendicular offsets are taken at every point of bend to represent the shape of a boundary accurately in such cases.

 

4. When a road crosses the chain line perpendicularly, note the chainage of the intersection point.


 

5. When a road crosses a chain line obliquely, note the chainage of intersection points ‘a’ and ‘b’. Take at least one offset on both sides of the intersection points. More offsets may be taken depending on the nature of the road.


 

6. When a building is large and zig-zag in shape, intersecting obliquely with the chain lines, follow these steps:

Fix corners using perpendiculars and offsets.
Draw a separate full plan of the building on a separate paper, including all dimensions.
Attach this plan to the field book at the appropriate place.

 

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