Short Answer:

Compass surveying is a method used to measure angles and directions of survey lines using a magnetic compass. It helps in determining the relative positions of points by observing bearings or directions from one point to another.

This type of surveying is simple, quick, and mostly used in rough terrains or where high accuracy is not needed. It is useful for small projects like road planning, forest surveys, or preliminary work before detailed surveys. Compass surveying depends on magnetic north, so errors may occur due to nearby magnetic materials or local attraction.

Detailed Explanation:

Compass surveying

Compass surveying is one of the basic and oldest types of surveying methods used to measure horizontal angles and determine directions with the help of a magnetic compass. It is commonly used in civil engineering for quick and easy surveys of land, especially in areas where detailed tools like total stations or GPS are not practical or available.

This method is based on observing the bearings of survey lines and recording the angle between them. A bearing is the direction of one point relative to another, measured clockwise from the magnetic north. The compass, being lightweight and portable, allows surveyors to move easily across different terrains and record bearings of lines without needing complicated setups.

1. Equipment used

The main instrument in compass surveying is the prismatic compass or surveyor’s compass. It is mounted on a tripod and aligned using a plumb bob. A tape or chain is used for measuring distances between points, while a field book is used to record all observations.

Modern compasses may also include a mirror, sighting slit, and graduated circle for more accurate reading.

2. Method of working

The surveyor begins at a known point, aligns the compass with the line to be measured, and records the bearing (angle from magnetic north). Then the length of the line is measured using a tape or chain. This process is repeated for all lines forming a closed loop or traversing the area.

The data collected includes:

• Fore bearings and back bearings (forward and reverse direction)
• Lengths of each line
• Angle between successive lines

This information is then used to prepare a traverse map or sketch of the area.

3. Types of compass surveys

There are two types of compass surveys:

• Open traverse: The survey does not return to the starting point. Useful for road, pipeline, or boundary line surveys.
• Closed traverse: The last line ends at the starting point or a known position. Used for areas like plots or building sites.

Closed traverses are preferred because they help in error checking and adjustment.

4. Errors and limitations

Compass surveying is simple but has some limitations:

• Magnetic variation: The compass points to magnetic north, not true north. This variation must be corrected.
• Local attraction: Nearby magnetic materials (metal fences, electric lines) can disturb the compass needle and cause wrong readings.
• Low accuracy: Compared to modern instruments, compass surveys are less accurate, especially for large areas or detailed work.
• Visibility issues: Requires clear sight between points, which may be difficult in forests or hilly areas.

To reduce errors, surveyors often check bearings in both directions and avoid working near magnetic objects.

5. Applications

Compass surveying is useful for:

• Preliminary route or road surveys
• Forest and mountain area surveys
• Minor construction layout
• Military or exploration work
• Quick reference mapping in remote locations

It provides basic direction and layout information needed for further detailed work.

Conclusion:

Compass surveying is a quick and easy method of measuring directions and angles using a magnetic compass. It is most useful for small-scale or preliminary surveys where high accuracy is not critical. Though it has some limitations due to magnetic errors, it remains a useful technique in civil engineering, especially in rough or remote areas.