Local Attraction

Local attraction refers to disturbances in compass readings caused by magnetic influences from nearby objects such as metal fences, buried pipes, vehicles, buildings, power lines, or natural iron ore deposits. Small items like wristwatches, steel tools, or belt buckles can also interfere with the compass by creating local magnetic fields that distort the Earth's natural magnetic field. These distortions lead to inaccurate readings, posing navigation and land surveying challenges.

Local attraction is detected by comparing forward and back bearings along a survey line. A forward bearing is taken by sighting towards the next point, while the back bearing is observed by sighting back to the starting point. Normally, the difference between these bearings should equal 180°. Any significant deviation from this standard indicates the influence of local attraction. To pinpoint the source of the disturbance, surveyors take additional readings from a reference point known to be unaffected by local attraction. Bearings from this neutral location remain consistent, helping to isolate and identify areas where errors occur.

Detecting and correcting local attractions is vital for ensuring the accuracy of magnetic surveys. Errors caused by unaddressed magnetic disturbances can lead to significant issues in navigation and land measurement. Surveyors ensure precise readings by systematically identifying and compensating for these distortions, which are critical for creating reliable maps and maintaining navigational safety.