Magnetic North vs. True North....?

[BergerBoy]

Please forgive me if this question has been asked before but I am having a hard time finding the answer to it.....

As far as ELR shooting goes what has the greatest effect on the bullets flight or rather which setting will help put my round on target better. My Kestrel asks for "true north" or "magnetic north" and to be honest I am not sure what setting would be most accurate. I am leaning toward true north since the earth is spherical. What say you guys????
Thanks for your time and God Bless.

 

[Trnelson]

True north is true north. Magnetic north varies, and depending on your present location the difference between true north and magnetic north changes. On VFR flight charts these isogonic lines of variation are marked and the value has to be included for navigation planning. Your compass displays magnetic north.

 

[Greyfox]

Technically, you are right, true north would align with the axis, therefore the rotation of the earth which relates to the Coriolis Effect. Practically, I think it's highly unlikely you would see a difference in your POI by choosing one over the other.

 

[Trnelson]

If you live in Ft. Brag California the difference is 14• East, if you live in Cedar Rapids Iowa there is no variation, east of there the variation changes to westerly. The only time this will make a substantial change for you is if you take your magnetic compass from the east coast and go to the west coast, with no correction. Even then, you'd likely never be able to ascertain the difference at normal shooting ranges. If you can get true north from your GPS, use it to minimize tollerence stacking errors. If not, just check your poi change at the new location.

 

[BergerBoy]

Thanks gentlemen. I appreciate the intel.

 

[Garycrow]

The kestrel default setting is magnetic north because that's what it reads from it's digital compass. If you use the true north setting you'll need to input the variation for your location to make everything match up. Wind direction is calculated in magnetic north by the instrument.

I agree with Greyfox that you'd probably never notice a difference and I'd probably leave it at the default setting. However, the true north setting would potentially be more accurate because of the mentioned reason of the coriolis effect being determined by the earth's axis (true north) rather than the magnetic poles. You just have to be sure you know your location's magnetic declination angle to enter.

The magnetic declination of your location is easy to find by going to this website:

Magnetic Declination

 

[BergerBoy]

Thanks Garycrow.
I just hit the link....
Once again, I appreciate all the intel.

 

[Bravo 4]

Technically, you are right, true north would align with the axis, therefore the rotation of the earth which relates to the Coriolis Effect. Practically, I think it's highly unlikely you would see a difference in your POI by choosing one over the other.

+1, the difference in declination probably anywhere in the continental U.S. wouldn't be big enough for a scope adjustment @ 1000 yards. An extreme would be if you were on the most NW tip of Washington or the most NE tip of Maine (both of which have a deviation of somewhere around 19) and your scope had 1/10 moa adjustments, and you were shooting longer ranges. For the ELR you mentioned, however, I would want to negate any variables I could to achieve first round hits.
Without adjusting anything else, I just punched some numbers into the Coriolis Acceleration tab on my Ballistics AE app and the difference in firing a 90 vs 270 degree azimuth may be a .5 moa in elevation @ 1000. Firing due E compared to due W is about as extreme difference in azimuth as you'll get.

 

[Bravo 4]

If you live in Ft. Brag California the difference is 14

Oh Good Lord, they moved Bragg to California?! What's this country coming to?
I blame Obama and Hillary for this. Next thing you know they'll tell us to remove the American Flag from our uniforms, so as not to offend anybody!

Disclaimer: I had my attourney look this over and was advised to add- That was all a joke! (Insert smiley face for confirmation)

 

[Trnelson]

[/QUOTE]

Oh Good Lord, they moved Bragg to California?! What's this country coming to?
I blame Obama and Hillary for this. Next thing you know they'll tell us to remove the American Flag from our uniforms, so as not to offend anybody!

Disclaimer: I had my attourney look this over and was advised to add- That was all a joke! (Insert smiley face for confirmation)

It's just a small private airstrip that has the 15• isogonic line running near to it.