Magnetic Declination

[odu_engineer]

Can anyone help explain this problem: Magnetic bearing of an old survey line N7 25W is now N2 30E. What is declination?

My understanding is that magnetic bearing is measured from magnetic north (MN) and not true north. How would you know where the magnetic north is? MN can be in the NE or NW quadrant.

Thanks

 

[ptatohed]

odu, is this the complete question? Are there A - D answer choices? It seems that there is not enough information to determine the magnetic declination (if that's what's being asked for). Magnetic Declimnation is the angular difference between Magnetic (compass) North and True North. You can't find the mag dec from two different magnetic bearings. Unless I am not understanding the question correctly, it seems to me we are given two magnetic bearings - an old magnetic bearing (let's say 1950) and a current magetic bearing (let's say 2014), for the same survey line. But, without knowing true north, we can't calculate the mag dec (which varies over time) for either 1950 or 2014. The question doesn't even ask which mag dec we're looking for (the old or the current)?

Maybe the question is saying that N 2o 30' E is the true bearing of the line? And maybe they are asking for the old mag dec from 1950? If that's the case, then the mag dec would be -9o 55'.

 

[odu_engineer]

Thank you for the reply. I have attached the problem from workbook.

 

[odu_engineer]

File attached.

 

[civilized_naah]

The key word there is the "change" in magnetic declination. Based on the data, it seems like magnetic north has swung WEST of north by angle 9 deg 55 min.

 

[Peele1]

If it has gone from West to East, then why is the change WEST?

 

[ptatohed]

The key word there is the "change" in magnetic declination. Based on the data, it seems like magnetic north has swung WEST of north by angle 9 deg 55 min.

Yes, now that I see the whole problem statement, I agree the key word is 'change'. So what is your answer c_n? It seems like you are leaning toward answer D (N 9o 55' W) (although I would contend that the bearing swung only 7o 25' west of north. )? But the problem is asking for the change, and I assume they mean the change from old to new, agreed? So, wouldn't that be a change of 9o 55' to the east (from the old mag bearing of N7W to the new of N2E). Answer B?

 

[odu_engineer]

The answer in the solution is stated as A)

 

[civilized_naah]

By convention the declination is positive when magnetic north is east of true north, and negative when it is to the west.So, at first the line was N 7d25m W and later was N 2d30m E. In other words the magnetic north was N 7d25m E and later was N2d30m W with respect to the line (which is fixed). Therefore, the magnetic north swung west of north, or declination became more negative or more west. Change in declination is negative, or counterclockwise, or west of north.

I would pick A. It is not appropriate to refer to a change in declination using a bearing angle (as in D). It should just be an angle (not a direction).

 

[civilized_naah]

YESSS! (PFIAADCD: Pumps fist in air and does crazy dance!) Surveying was never my favorite subject

 

[ptatohed]

By convention the declination is positive when magnetic north is east of true north, and negative when it is to the west.So, at first the line was N 7d25m W and later was N 2d30m E. In other words the magnetic north was N 7d25m E and later was N2d30m W with respect to the line (which is fixed). Therefore, the magnetic north swung west of north, or declination became more negative or more west. Change in declination is negative, or counterclockwise, or west of north.

I would pick A. It is not appropriate to refer to a change in declination using a bearing angle (as in D). It should just be an angle (not a direction).

I'm still confused. I agree that the mag dec is positive when mag north is east of true north (and vice versa). But nothing in this problem statement tells us where true north is, right? True North could be west of, east of, or in the middle of the two given mag bearings. The question is asking the change in mag dec over the last X decades. In my view, we have no idea what the mag dec is or was, all we know is it moved 9o55' to the east from yesteryear to today. Oh well, I give up. :whiteflag:

YESSS! (PFIAADCD: Pumps fist in air and does crazy dance!) Surveying was never my favorite subject

LOL. Actually surveying is one of my fav's - and I can't figure this one out! So congrats.

 

[Lungshen]

I would pick A too.

The easy way to see it is this, the bearing line is actually a fixed line it doesn't move. What is moving is the MN, and thus the Bearing description change, the referenced datum line ( MN) is moving. Draw your diagram, there are actually two MN lines, old and new.

 

[Lungshen]

Oh and this problem has nothing to do with TN....don't draw TN on your diagram. Your referenced datum line MN Has to be move from Right side to the left side of your fixed bearing line if the description changes from N7'25'W to N2'30'E. You see it now?

 

[Lungshen]

This is my graphical explanation to the problem. Hope it makes sense.

This question got me too when I first looked at it because when reading the problem we see the picture in our head, and when Magnetic Bearing changes from N7'25'W to N2'30E, we visualize that the Bearing Line is moving - so its an easy trap to pick option B.

Bearing Line is always fixed. True north line is alway sIt is always fixed. What is not fixed is the Magnetic North.

On a side note, good to see you guys posting question regarding to Survey. I almost feel alone on taking the survey exam but now that I know there are people on the same boat

Keep questions coming - it refreshes my memories and help me for the final review. Magnetic Bearing Sketch.pdf

 

[ptatohed]

Thanks a lot Lung. I do think I see it now. This one hurt my head for a while! But, I get it now, thanks.