Examkin

Question

Difficulty: Very hardElementary Surveying Instruments and Fieldwork

During a prismatic compass survey along a open traverse line P−Q−R−SP-Q-R-S, a student records the following forward bearings (FB) and back bearings (BB):

- Line PQPQ: FB = 045∘045^\circ, BB = 225∘225^\circ
- Line QRQR: FB = 130∘130^\circ, BB = 308∘308^\circ
- Line RSRS: FB = 220∘220^\circ, BB = 042∘042^\circ

Based on these field observations, which of the survey stations is affected by local attraction?

  1. A
    Station P
  2. B
    Station Q
  3. Station RAnswer
  4. D
    Station S

Answer

Station R is affected by local attraction.
Station R is the correct answer because it is the intersection station for both Line QR and Line RS. In magnetic compass surveying, when the difference between forward bearing and back bearing of a line is exactly 180∘180^\circ, both stations forming that line are free from local magnetic disturbance (as seen with Line PQ). Since Line QR (308∘−130∘=178∘308^\circ - 130^\circ = 178^\circ) and Line RS (220∘−042∘=178∘220^\circ - 042^\circ = 178^\circ) both display a 2∘2^\circ error, the common station to both lines—Station R—must be the sole source of local attraction.

Step-by-Step Solution

1
Calculate the difference between Forward Bearing (FB) and Back Bearing (BB) for Line PQ.
∣225∘−045∘∣=180∘|225^\circ - 045^\circ| = 180^\circ
A difference of exactly 180∘180^\circ indicates that both Station P and Station Q are free from local magnetic attraction.
2
Calculate the difference between Forward Bearing (FB) and Back Bearing (BB) for Line QR.
∣308∘−130∘∣=178∘|308^\circ - 130^\circ| = 178^\circ
The difference is 178∘178^\circ, showing a 2∘2^\circ discrepancy. Either Station Q or Station R is affected by local attraction.
3
Calculate the difference between Forward Bearing (FB) and Back Bearing (BB) for Line RS.
∣220∘−042∘∣=178∘|220^\circ - 042^\circ| = 178^\circ
The difference is 178∘178^\circ, showing a 2∘2^\circ discrepancy. Either Station R or Station S is affected by local attraction.
4
Synthesize the results to isolate the affected station.
Station R is common to both erroneous lines (QR and RS), while Stations P and Q are verified correct from Line PQ.
Because Station R is present in both lines that exhibit the 2∘2^\circ magnetic error, Station R is the station experiencing local attraction.

Key Concept

Local Attraction Detection in Prismatic Compass Surveying
Estimated Time:2m 0s
Rate this question

Topics

Elementary Surveying Instruments and Fieldwork

More questions from this topic