Short Answer:

The principle of compass surveying is based on measuring the direction of survey lines using a magnetic compass, which works by pointing toward the magnetic north. The angles between the survey lines and the magnetic north are called bearings and are recorded to determine the relative positions of points.

By combining these measured bearings with the distances between points, surveyors can draw maps, locate boundaries, and calculate areas. This method is simple, fast, and especially useful in rough terrains where detailed instruments cannot be used easily.

Detailed Explanation:

Principle of compass surveying

Compass surveying is a traditional and widely used method in civil engineering for determining the directions of lines in the field. The basic principle of compass surveying relies on the use of a magnetic compass to measure bearings, which are the horizontal angles between a survey line and magnetic north. These measurements help in identifying the relative positions of different points on the ground.

This surveying technique is especially useful for small to medium-sized projects where the area is not too large, and high precision is not required. It is also used in preliminary surveys or in places where access is limited, such as forests, hills, or rural zones.

Working principle

The compass used in this type of surveying has a magnetic needle that freely rotates and always points towards magnetic north. When a surveyor aims the compass along a survey line, the angle formed between this line and the magnetic north direction is called the bearing. This angle is read directly from the compass scale and recorded in the field book.

Using these bearings and the measured distances between stations, a traverse (a chain of connected survey lines) is created. By plotting the bearings and lengths of each line on paper or with software, the layout of the land can be drawn.

Key points of the principle:

Magnetic bearing: Each survey line is referenced to magnetic north, and its direction is noted as a bearing.
Traverse method: Multiple lines are measured and connected to form either an open or closed traverse.
Field measurements: Bearings and line lengths are measured in the field using a compass and a chain or tape.
Plotting and calculation: Data collected is used to prepare a map and compute area, angles, and other properties.

Application of the principle

The principle is applied in the following way:

Select a starting point and fix the compass properly.
Sight the next station and note the fore bearing.
Measure the distance between the stations.
Move to the next point and repeat the process.
Also record the back bearing for accuracy checks.
After collecting all data, prepare the traverse and adjust errors if needed.

Compass surveying works best in locations free from magnetic disturbances, as metallic objects and electric fields can affect the needle and result in incorrect bearings.

Advantages of this principle

Quick and easy to perform.
Portable instruments that are easy to carry.
Less labor-intensive compared to advanced surveying tools.
Useful in difficult terrains where line of sight is limited.

However, the accuracy is lower than modern tools, and results must be checked for local attraction (magnetic disturbances).

Conclusion:

The principle of compass surveying is based on measuring the bearings of survey lines from magnetic north using a magnetic compass. This helps in determining the direction and position of various points on the ground. When combined with distance measurements, it provides the necessary data to draw maps and plan civil engineering works. Though simple and fast, compass surveying must be done carefully to avoid magnetic errors and ensure reliable results.