The north end of a freely suspended magnetic needle consistently aligns itself with the Earth’s magnetic north, provided that it is free from the influence of any external forces other than the Earth’s magnetic field. This alignment forms the foundation of magnetic navigation and orientation, making the magnetic needle a crucial instrument in various fields, from geophysical surveys to traditional navigation.
However, in real-world scenarios, it is frequently observed that the magnetic needle can be deflected from its normal position due to nearby magnetic or metallic influences. When placed near magnetic rocks, iron ores, current-carrying cables, or metallic structures like iron poles, the needle experiences disturbances that cause it to deviate from the true magnetic north. These disturbances arise from localized magnetic fields generated by these objects, and the resulting effect is referred to as ‘local attraction.’
Local attraction can significantly compromise the accuracy of magnetic bearings. These bearings, which are used to determine directions relative to the Earth’s magnetic field, become unreliable when external magnetic influences are present. This poses a challenge in environments such as industrial zones, areas rich in iron ores, or places with significant human-made magnetic sources like power lines or machinery.
To ensure reliable navigation and accurate measurements, it is essential to account for local attraction at each observation station. This involves checking the magnetic needle for signs of deflection and identifying any nearby sources of interference. Once detected, corrective measures can be taken, such as physically relocating the station to a less affected area or applying mathematical adjustments to account for the distortions. By carefully managing local attraction, it becomes possible to restore the accuracy of magnetic bearings, ensuring they provide dependable readings aligned with the Earth’s true magnetic north.
Source of Local Attraction in Compass Surveying
Magnetic Minerals
1.Iron Ore Deposits: Natural deposits of magnetic minerals like iron ore can cause local attraction by pulling the needle away from magnetic north.
2.Mineralized Rocks: Rocks containing magnetic materials can affect the compass readings in certain areas.
Man-made Structures
1.Metallic Objects: Large metallic structures like bridges, fences, and metal poles in the vicinity of the survey area can interfere with the compass needle.
2.Railway Tracks: Steel railway tracks or pipelines can create significant disturbances in compass readings.
3.Vehicles: Cars, trucks, or any other metal-heavy machinery nearby can also distort the magnetic field.
Electrical Equipment
1.Power Lines: High-voltage power lines or transformers generate magnetic fields that can influence compass readings.
2.Electric Equipment: Other electrically powered equipment, such as generators or even large electric motors, can disturb the compass.
Buildings and Infrastructure
1.Steel-Framed Buildings: Buildings with a steel frame or significant metal components can cause local attraction.
2.Underground Utilities: Buried pipes, cables, and other infrastructure made of metal can impact the accuracy of the compass.
Personal Items
1.Metallic Objects on Surveyor: Items such as watches, keys, mobile phones, or belt buckles worn by the surveyor may affect the magnetic needle if too close to the compass.
Detection of Local Attraction
The fore and back bearings of a line should ideally differ by exactly 180°. If they don’t, there may be local attraction present. The discrepancy could be due to one of three causes:
- Error in Observation: This can happen due to mistakes in reading or recording the bearing.
- Local Attraction at One Station: Magnetic interference may exist at one of the stations, causing the compass needle to deviate.
- Local Attraction at Both Stations: In this case, both stations are subject to interference, causing the bearings to be inaccurate at both points.
Local attraction in compass surveying causes equal deviation in all bearings at an affected station, meaning the compass readings may be off. However, the included angles between lines remain accurate because local attraction shifts all bearings by the same amount. To detect local attraction, surveyors compare the fore and back bearings of a line; if they differ by more or less than 180°, local attraction is likely present. Corrections can be made by calculating the included angles or identifying the local attraction at each station and adjusting the bearings accordingly, starting from an unaffected station.
Method of elimination of local attraction
1.Method of Eliminating Local Attraction by Using Included Angles:
To correct local attraction using included angles, follow these steps:
Compute Included Angles: At each station in a closed traverse, calculate the included angle from the observed bearings. Since local attraction affects all bearings equally at a station, the included angles will remain unaffected and correct.
Start from an Unaffected Line: Identify the line where no local attraction is present, or where the error is minimal. Using this line as the reference, calculate the correct bearings for the next station.
Correct Successive Bearings: Using the correct included angles, proceed to adjust the bearings of successive sides of the traverse. Continue this process for each station until all bearings have been corrected.
2.Method of Eliminating Local Attraction by Applying Corrections to Bearings
To correct for local attraction by directly applying corrections to bearings, follow these steps:
Identify the Unaffected Line: Begin by identifying a station or line in the traverse where the fore and back bearings show no local attraction (i.e., the difference between them is exactly 180°). This will serve as the reference for your corrections.
Calculate Local Attraction at Each Station: For each station affected by local attraction, determine the magnitude and direction of the error. This is done by comparing the observed bearings with the known correct bearings. The difference between these values gives the local attraction at that station.
Apply Corrections to Bearings: Once the local attraction has been calculated, correct the bearings at each affected station by adding or subtracting the calculated error. Ensure that the correction is applied uniformly to all bearings at the station to maintain consistency.
Adjust Successive Bearings: After correcting the bearings at one station, continue to adjust the bearings for subsequent stations in the traverse by applying the same process—starting from the station with no local attraction and progressing along the traverse.
Verification: After all bearings have been adjusted, recheck the fore and back bearings to ensure they are now correct (i.e., they differ by 180°). If any discrepancies remain, refine the corrections as needed.
3.Practical Hints for Locating and Correcting Local Attraction
To locate and correct local attraction errors in compass surveying, follow these practical steps:
Identify a Correct Line: Begin by observing a line in the traverse where the fore and back bearings differ exactly by 180°. This indicates that the bearings for this line are free from local attraction.
Accept Correct Bearings: Use the bearings from the end stations of this correct line as your reference. These bearings are considered accurate and will be used to correct other bearings affected by local attraction.
Calculate Errors for Other Lines: For lines where the bearings are affected by local attraction, calculate the back or fore bearings using the accepted bearings from the unaffected line. Determine the error by comparing the observed bearings with these calculated correct bearings.
Apply Corrections: If the observed bearing at a station is higher than the correct value, the local attraction error at that station is positive. Apply a negative correction to the other bearings. Conversely, if the observed bearing is lower, the error is negative, and apply a positive correction to the bearings.
4.Method of Eliminating Local Attraction When None of the Lines Differ by 180° in Fore and Back Bearings
In some cases, especially in a closed traverse, you may find that none of the lines have fore and back bearings differing by exactly 180°. When local attraction affects all stations, follow these steps to correct the bearings:
Calculate Interior Angles: Determine the interior angles of the traverse and ensure their sum aligns with the expected total of 2n±4 right angles, where n is the number of sides in the traverse. This helps verify the accuracy of the traverse and the presence of local attraction.
Distribute the Error: If the calculated sum of the interior angles deviates from the expected total, distribute the discrepancy evenly across all angles. This adjustment accounts for the overall error due to local attraction.
Identify the Least Affected Line: Locate the line where the fore and back bearings differ the least from 180° (in the whole circle bearing system) or from 0° (in the quadrant bearing system). This line is the closest to being free of local attraction and will serve as the basis for further corrections.
Adjust Bearings of the Least Affected Line: Correct the bearings of this line by distributing half of the total error equally between the fore and back bearings. This helps establish a corrected reference for the remaining lines.
Calculate Correct Bearings for Other Lines: Using the corrected bearings of the least affected line, adjust the bearings of the other lines in the traverse accordingly, applying the corrections to maintain consistency throughout the traverse.
