Chain surveying is considered to be the simplest method of surveying in which measurements are taken in the field and other supplementary works like plotting calculations are carried out in office. The measurements in chain surveying are linear- angular measurements are not considered.
Moreover, it provides fairly accurate result provided that the work is conducted carefully. Chain surveying is suitable for small areas with few details. Tools and equipments required include chain, tape, ranging rod, arrows and, sometimes, a cross staff.
In this type of surveying, survey stations (main stations, tie or subsidiary stations) shall be specified carefully otherwise the outcome of the surveying process may not be accurate.
Applicability of Chain Survey
Obviously, chain surveying cannot be used in all cases. It can be used if the area under consideration meets the following conditions:
- The area shall be fairly small.
- The ground is moderately levelled.
- The area needs to be open.
- The ground has few and simple details.
Chain Survey Tools
- Cross staff
Chain Survey Stations
Survey stations are points of importance at the beginning and end of a chain line. There are two major types of stations in chain surveying:
1. Main stations
Main stations are the end of lines that determine the boundary of the surveying.
2. Tie (Subsidiary) Stations
Tie stations are points which are specified on the chain line (main survey lines) where it is required to identify interior details like buildings and fences.
Factors Affecting Survey Station Selection
- Stations should be visible from at least two or more stations.
- As far as possible, main lines should run on level ground.
- All triangle shall be defined properly (No angle less than 30º).
- Each triangle should have at least one check line.
- Survey lines should be as few as possible.
- Obstacles to ranging and chaining should be avoided.
- Sides of the larger triangles should pass as close to the boundary lines as possible.
- Trespassing and frequent crossing of the roads should be avoided.
Line Types in Chain Survey
1. Base Lines
It is the main and longest line from which all measurements to demonstrate details of the work are taken. The base line passes through the center of the field.
2. Chain Line (Main Survey) Lines
The lines that join main stations are termed as chain line or main survey lines.
3. Tie (Subsidiary) Lines
It joins two fixed points on the chain line. The advantage of tie line appears while checking surveying accuracy in locating interior details such as buildings and paths.
4. Check (Proof) Lines
It joins triangle apex to some fixed points on any two sides of a triangle. It is used to examine the accuracy of the framework. The length of check line measured on ground shall be consistent with its length on the plan.
Offsets in Chain Survey
Lateral measurements from the baseline are termed as offsets. They are used to fix locations of various objects with respect to the baseline. Commonly, offsets are established at right angle. There are two major type of offsets, namely: perpendicular offsets and oblique offsets.
Chain Survey Procedures
- Firstly, inspect the area to be surveyed and prepare key plan. This stage is termed as reconnaissance phase.
- Then, mark stations using suitable means such as fixing ranging poles, driving pegs, and digging and fixing a stone.
- After that, specify the way for passing the main line which should go through the center of the field.
- Fix ranging road on stations
- Then, the chaining can begin.
- Make ranging wherever necessary.
- Measure the change and offset and record them.