Fore Bearing (F.B)
i) Fore bearing is the bearing of the line in the forward direction of surveying.
ii) The formula used to calculate the fore bearing of the progressive line F.B = B.B ± 180° { + sign when B.B less than 180° and – sign when B.B more than 180°}.
Back Bearing (B.B)
i) Back bearing is the bearing of the line in the opposite direction of surveying.
ii) The formula used to calculate the back bearing of line B.B = F.B ± 180° { + sign when F.B less than 180° and – sign when F.B more than 180° }.
*If the difference of F.B and B.B is 180° then consider no error in measurements.
*If the difference of F.B and B.B is not 180° then consider error means affected by local attraction.
Read Carefully:
Bearing represents a measurement of angles w.r.t clockwise (examples: S 77° W or N 89°45́ E) or anticlockwise ( S 69° E and N 55° W) direction or angles w.r.t clockwise direction in surveying, where angle simply represent a degree in geometry.
Example:-
Fore bearing of line AB = 260° (given above) and included angles are also given.
Back bearing of line AB ( means BA) = 260°- 180°= 80°
Fore bearing of line BC = Bearing of line BA + angle B
= 80° + 62° = 142°
Back bearing of BC= 142° +180° =322°
Fore bearing of line CD= Bearing of line CB +angleC
=322° +60° =382°
Therefore, Fore Bearing of line CD=382° -360° = 22°
Back bearing of line CD =22° +180° =202°
Fore bearing of line DA=Bearing of line DC+ angleD
=202° +98° =300°
Back Bearing of line DA= 300° -180° =120°
Question asked in comments:
if B.B or F.B is 180° then how we calculate:
Well friends, as we sight through compass from A to B point, we already came to know what is fore bearing.
As the line is straight then Back Bearing at B point it is 0°. Here, if we use + sign then B.B will become 360°. Here two traverse were shown i.e. one at L.H.S(ABC) and other at R.H.S(ABD). If not understood please consult your teachers.
Here we got a new question: Please comment for any query.
Question: The bearing of AB = N40W, bearing of BC = S70°E, then the value of ∠ABC is 30° (anticlockwise)(-ve).
we know,
Included angle = Bearing of next line(BC) – Bearing of preceding line(BA)
= 110° – (320° – 180° )
=-30°
= 30° (anticlockwise)(-ve)
If this answer seems to be wrong then comment answer with explanation.
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How do you get the fore bearing when the line AB bearing is N29°16’E
Fore Bearing of AB is 29°16′ and
Back bearing of AB is 29°16′ + 180° = 209°16′
How do you get the distance when the line AB bearing is N29°16’E
As we have founded the fore bearing and back bearing, now just measure the distance i.e. AB between them with the help of chain or tape. But the traverse should be completed.
How to calculate if total included angle is Greater that 360 and deferent between the bearing is already 180
How do I calculate reverse bearing given total bearing is greater than 360° ?