局所磁気

Local attraction refers to disturbances in compass readings caused by magnetic influences from nearby objects like metal fences, buried pipes, vehicles, buildings, power lines, or natural iron ore deposits. 

Smaller items such as wristwatches, steel tools, or belt buckles can also interfere with the compass. 

These sources create local magnetic fields that distort the Earth's magnetic field, leading to inaccurate readings.

Detecting local attraction involves comparing a survey line's forward and back bearings. The forward bearing is taken by sighting towards the next point, while the back bearing is observed by sighting back to the previous point. 

Normally, the difference between the two should equal 180°. Significant deviations suggest the influence of local attraction. 

Additional readings are taken from a reference point unaffected by local attraction to pinpoint the disturbance. 

Bearings from this reference point remain consistent, helping isolate the error to specific locations. 

Identifying and correcting local attractions ensures reliable magnetic surveys, which are crucial for maintaining navigation and land measurement accuracy.