How do you correct for local attraction in compass surveys?

Short Answer:

To correct for local attraction in compass surveys, you first identify the stations affected by checking if the difference between fore bearing and back bearing is not exactly 180°. Once affected stations are known, corrections are made by adjusting the bearings based on unaffected stations and ensuring consistency throughout the traverse.

These corrections help remove the influence of magnetic disturbances and make the recorded bearings accurate. Without correction, the survey data may lead to wrong plotting or misalignment of property lines and construction layouts.

Detailed Explanation:

Correcting for local attraction in compass surveys

In compass surveying, local attraction causes the magnetic needle to point in the wrong direction due to nearby magnetic objects or disturbances. If not corrected, this results in incorrect bearings, leading to faulty survey results. Therefore, correcting for local attraction is an important step in ensuring the accuracy and reliability of a compass survey.

Surveyors follow a step-by-step method to detect and correct these errors, especially when doing closed traverses where each line has a fore bearing and back bearing.

  1. Detection of local attraction

The first step is to detect whether local attraction is present. This is done by comparing the fore bearing (FB) and back bearing (BB) of each line. Under ideal conditions, the difference between FB and BB should be exactly 180°.

  • If FB – BB = ±180°, no local attraction exists at either station.
  • If the difference is not 180°, one or both of the stations is affected by local attraction.

For example:

  • If FB of AB = 70°, BB of BA = 250° → Difference = 180° → No local attraction
  • If FB of BC = 120°, BB of CB = 290° → Difference = 170° → Local attraction exists

By analyzing the entire traverse this way, you can identify unaffected stations and use them as references.

  1. Choosing a reference station

Choose a station that is not affected by local attraction (i.e., where FB and BB differ by 180°) as the reference or base station. This station’s readings are assumed correct and are used to compute corrections for other stations.

  1. Calculating corrected bearings

Once the unaffected station is fixed:

  • Use its correct bearing values to compute the correct bearing of the next connected line.
  • Apply the mathematical rule:
    BB = FB ± 180°
  • Adjust the incorrect bearings based on the reference and apply corrections step by step around the traverse.

In this way, one incorrect bearing is corrected using the known correct bearing of the other end.

  1. Final check

After applying corrections at all affected stations:

  • Recheck the adjusted bearings.
  • Make sure all FB–BB differences are 180°.
  • Ensure that the traverse closes correctly (i.e., the final plotted point matches the starting point in a closed traverse).

This confirms that all local attraction errors have been removed.

  1. Precautionary steps

To reduce the chance of local attraction:

  • Do not keep metallic objects near the compass.
  • Avoid surveying near electric poles, iron fences, or underground cables.
  • Use instruments like gyrocompasses or digital compasses in disturbed areas.

However, if unavoidable, always follow correction methods after field observations.

Conclusion:

Correcting for local attraction in compass surveys involves identifying stations with incorrect bearing differences and adjusting them based on unaffected stations. This ensures that magnetic disturbances do not affect the final survey results. Careful checking of fore and back bearings and step-by-step correction help maintain accuracy and reliability in all compass-based surveying work.