Let us go through the examples to understand the calculation procedure of true bearings & magnetic bearings for a given magnetic declination.
Eg. 1.
The magnetic bearing of a line is N46°15'E. If the magnetic declination is 6°30' East, calculate the true bearing.
Note:
TM 👉 True meridian
MM 👉 Magnetic meridian
RB 👉 Reduced bearing
WCB 👉 Whole circle bearing
Given data:
Reduced bearing = N46°15'E
Magnetic declination = 6°30'E
True bearing =?
As magnetic declination is towards the East, the value will be +ve. as shown above.
Here,
True bearing
= [magnetic bearing + magnetic declination]
The given RB lies in the 1st quadrant. Therefore WCB = RB =46°15'
If you want to know, about the conversion of RB to WCB, Go through the article 👇
👀. How to convert reduced bearing to whole circle bearing?
True bearing
= [46°15' + 6°30']
= 52°45'
As the true bearing is less than 90°, it lies in the 1st quadrant.
Therefore the position of bearing = N 52°45' E
Eg. 2.
The magnetic bearing of line AB is 147°30'. What will be the true bearing if the magnetic declination is 4°15' W.
Given data:
Magnetic bearing = 147°30'
Magnetic declination = 4°15' W
True bearing =?
As the magnetic declination is towards the West, the value will be -ve.
The given magnetic bearing is in the WCB system.
True bearing
= [magnetic bearing - magnetic declination]
= [ 147°30' - 4°15']
= 143°15'
Eg. 3.
The true bearing of a line AB is 217°30'. If the magnetic declination is 5°E, calculate the magnetic bearing.
Given data:
Magnetic declination = 5°E
True bearing = 217°30'
Magnetic bearing =?
As magnetic declination is towards the East, the value will be +ve.
True bearing = [magnetic bearing + magnetic declination]
Therefore,
Magnetic bearing
= [ true bearing - magnetic declination]
= [217°30' - 5°]
= 212°30'
To understand A to Z of surveying, click here.
Thank you for going through these calculation steps❤. Have a good day 😄.
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