Compass Surveying and Plane Table Surveying

Why it matters

Compass surveying and plane table surveying are fundamental techniques in civil engineering used for land measurement and mapping. These methods are crucial for creating accurate maps and plans, which are essential for construction projects, land development, and resource management.

Key ideas

• Compass Surveying: This method uses a magnetic compass to determine the direction of survey lines. It is suitable for small areas with moderate accuracy requirements.

  • Prismatic Compass: A type of compass used in this method, which allows for direct reading of bearings.
  • Bearings: The direction of a line with respect to a reference meridian, usually measured in degrees.
  • Local Attraction: Magnetic interference that can affect compass readings, requiring correction.

• Plane Table Surveying: A graphical method of surveying where field observations and plotting are done simultaneously.

  • Plane Table: A drawing board mounted on a tripod, used for plotting the survey directly in the field.
  • Alidade: A sighting device used with the plane table to draw lines of sight on the map.
  • Radiation Method: A technique where lines are drawn from a central point to various points in the field.
  • Intersection Method: Involves plotting points by sighting them from two different stations.

Bearing corrections

Specify whole-circle bearings clockwise from north. Let c be the signed correction added to an observed compass bearing to remove local attraction. Then corrected magnetic bearing MB = observed bearing + c. True bearing TB = MB + δ, where magnetic declination δ is east-positive and west-negative; reduce results to 0–360°. Local attraction and declination are different effects.

Worked example

Observed bearing = 120°, known local-attraction correction c = −5°, and east declination δ = +2°. MB = 120−5 = 115°. TB = 115+2 = 117°. Without declination, the corrected magnetic bearing alone cannot generally determine true bearing.

Common mistakes

• Ignoring Local Attraction: Failing to account for local magnetic interference can lead to inaccurate results.
• Incorrect Plotting: In plane table surveying, errors in plotting can occur if the table is not properly leveled or oriented.

For GATE CE

Questions on this topic often involve calculating bearings, correcting for local attraction, and understanding the methods of plane table surveying. Practice problems that require you to apply these concepts in practical scenarios.

Quick check

1. What is the main instrument used in compass surveying?
2. How does local attraction affect compass readings?
3. Name one method used in plane table surveying.

Answers: 1. Prismatic Compass 2. It causes deviation in the bearing. 3. Radiation Method