Local Attraction in Compass Survey
In compass surveying, accurate measurement of bearings is essential for correct plotting of survey lines. However, sometimes the magnetic needle does not point towards the true magnetic north due to external influences. This effect is known as Local Attraction. In ITI Draughtsman Civil, understanding local attraction is important for detecting and correcting errors in compass survey.
1. Definition of Local Attraction
Local attraction is the deviation of the magnetic needle from its normal position due to the presence of nearby magnetic materials such as iron objects, electric currents, or magnetic rocks. This causes incorrect bearing readings.
2. Causes of Local Attraction
- Presence of iron or steel objects (rails, fences, tools).
- Electric currents or overhead power lines.
- Magnetic rocks or minerals in the ground.
- Nearby vehicles or machinery.
3. Detection of Local Attraction
Local attraction can be detected by comparing the fore bearing (FB) and back bearing (BB) of a line.
- In the absence of local attraction: BB = FB ± 180°
- If this condition is not satisfied, local attraction is present.
By checking bearings of different lines, the station affected by local attraction can be identified.
4. Correction of Local Attraction
Once local attraction is detected, corrections must be applied to obtain accurate bearings.
Methods of Correction
- Identify stations free from local attraction.
- Use correct bearings from unaffected stations as reference.
- Apply corrections to affected stations.
- Recalculate all bearings accordingly.
5. Effects of Local Attraction
- Incorrect measurement of bearings.
- Errors in plotting of survey lines.
- Distortion of traverse shape.
- Inaccurate calculation of area.
6. Precautions to Avoid Local Attraction
- Keep the compass away from metallic objects.
- Avoid surveying near electric lines or machinery.
- Check bearings at different stations.
- Use non-magnetic tools when possible.
7. Importance of Local Attraction
Understanding local attraction helps in improving the accuracy of compass survey. It allows surveyors to detect and correct errors, ensuring reliable results.
Role of Draughtsman Civil
A Draughtsman Civil uses corrected survey data to prepare accurate plans and drawings. Knowledge of local attraction helps in interpreting field data properly and avoiding errors in design.
Conclusion
Local attraction is a common source of error in compass surveying caused by nearby magnetic influences. It affects the direction of the magnetic needle and leads to incorrect readings.
For ITI Draughtsman Civil students, understanding its causes, detection, and correction is essential for accurate surveying and construction work.