JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year MINE SURVEYING Previous Question Paper (March - 2021 - R18)

To identify local attraction, we compare the difference between the Fore Bearing (FB) and Back Bearing (BB) for each line. The theoretical difference should be 180°.

Since the difference for line AB is exactly 180°, stations A and B are free from local attraction. Consequently, all other stations (C and D) are affected by local attraction.

Correction calculation:

1. Station B is unaffected. Correct BB of BC is FB of BC + 180° = 68° 15' + 180° = 248° 15'.
2. Observed BB of BC = 246° 00'. Correction at C = 248° 15' - 246° 00' = +2° 15'.
3. Correct FB of CD = Observed FB of CD + 2° 15' = 310° 30' + 2° 15' = 312° 45'.
4. Correct BB of CD = 312° 45' - 180° = 132° 45'.
5. Observed BB of CD = 135° 15'. Correction at D = 132° 45' - 135° 15' = -2° 30'.
6. Correct FB of DA = Observed FB of DA - 2° 30' = 200° 15' - 2° 30' = 197° 45'.
7. Correct BB of DA = 197° 45' - 180° = 17° 45'.

Corrected Bearings:

Local attraction is the phenomenon where the magnetic needle of a compass is deflected from its true magnetic meridian due to the presence of external magnetic influences. These influences include iron ore deposits, steel structures, electric cables, railway lines, or even personal items like keys and spectacles carried by the surveyor.

The presence of local attraction causes the observed magnetic bearings to be incorrect. To detect and rectify this effect, the following procedure is followed:

• Detection: In a closed traverse, the fore bearing (FB) and back bearing (BB) of each line are measured. The theoretical difference between the FB and BB of a line must be exactly 180°. If the difference is not 180°, local attraction is present at one or both stations.
• Identification: If the difference between the FB and BB of a line is exactly 180°, both stations are free from local attraction. If the difference is not 180°, the error is distributed between the two stations. By checking all lines in the traverse, stations that are free from local attraction are identified.
• Rectification (Method 1 - Calculation): Starting from a station known to be free from local attraction, calculate the correct bearings of all subsequent lines by applying the local attraction correction determined at each station.
• Rectification (Method 2 - Included Angles): Calculate the included angles at each station using the observed bearings. Since included angles are independent of local attraction (as the error affects both the FB and BB of lines at a station equally), they remain constant. Use these correct included angles to recompute the correct bearings starting from a line whose bearing is known to be unaffected.

By comparing the observed bearings with the computed correct bearings, the local attraction error at each affected station is determined and applied as a correction to all bearings measured at that station.