Short Answer:

Local attraction in compass surveying refers to the disturbance or deflection of the compass needle caused by nearby magnetic influences. These influences may come from metallic objects, electric poles, underground cables, or natural minerals near the survey location.

Due to local attraction, the compass does not point correctly toward magnetic north, resulting in wrong bearings. Detecting and correcting local attraction is very important in compass surveying to ensure accurate direction measurements and avoid plotting errors in the final survey map.

Detailed Explanation:

Local attraction in compass surveying

In compass surveying, accurate direction measurement depends on the compass needle pointing exactly toward the magnetic north. However, sometimes the needle gets disturbed due to nearby magnetic objects or fields. This disturbance is known as local attraction. It causes the compass to give incorrect readings, which leads to wrong bearings and errors in the survey results.

Local attraction is a common issue in the field, especially in areas with human-made structures or natural magnetic materials. A surveyor must always check for local attraction at every station to ensure the reliability of the survey data.

Causes of local attraction

Local attraction is caused by any nearby source that creates a magnetic field and interferes with the Earth’s natural magnetic field. Common causes include:

Iron or steel objects (e.g., fences, gates, pipes, survey rods)
Electric poles or transmission lines
Railway tracks or vehicles
Underground cables or metal pipelines
Natural mineral deposits (e.g., iron ore)
Mobile phones, magnetic tools, or survey equipment kept near the compass

These sources affect the movement of the magnetic needle and make it point away from the actual magnetic north.

Effects of local attraction

If not identified and corrected, local attraction can lead to:

Wrong bearings
Inaccurate plotting of directions
Incorrect closure of traverses
Distorted maps or layouts
Legal boundary disputes

Therefore, understanding and handling local attraction is crucial for successful compass surveys.

Detection of local attraction

Local attraction is usually detected by comparing fore bearing (FB) and back bearing (BB) of the same survey line. Under normal conditions (without local attraction), the difference between FB and BB should be exactly 180°.

If the difference is not 180°, then one or both stations may be affected by local attraction. For example:

FB of line AB = 60°, BB of line BA = 245° → difference = 185° → local attraction exists

By checking this for all survey lines, the affected stations can be identified.

Correction of local attraction

Once local attraction is detected, surveyors apply corrections to the bearings:

Identify unaffected stations (where the FB and BB differ by 180°)
Adjust the bearings at affected stations using the known corrected values from unaffected points
Apply the corrected bearings in drawing the map or calculating angles

Some precautions to avoid local attraction include:

Keeping metallic objects away from the compass
Avoiding surveys near known magnetic sources
Using non-metallic tripods or supports
Using digital or gyro compasses in high-risk areas

Conclusion:

Local attraction in compass surveying is the deflection of the magnetic needle due to nearby magnetic sources. It causes errors in direction measurement and must be detected and corrected for accurate survey results. Surveyors can identify local attraction by comparing fore and back bearings and apply necessary adjustments. Proper care and precaution help in avoiding its effects and ensuring reliable compass-based surveying.