wardialer
Topic Author
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Joined: Fri Sep 14, 2001 1:08 pm

True Vs MAG Course?

Wed Jun 20, 2012 4:33 pm

I am using a mapping tool software for plotting my tracks.

Question: If I want to use a MAGNETIC course option instead of TRUE, do I have to select the Great Circle option instead of Rhumlbline? There is an option to select either Great Circle or Rhumbline, but do I have to select Great Circle in order to use MAGNETIC course instead of TRUE course?

Please help. Thanks.

[Edited 2012-06-20 09:38:33]
 
Mir
Posts: 19108
Joined: Mon Jan 05, 2004 3:55 am

RE: True Vs MAG Course?

Wed Jun 20, 2012 5:04 pm

Whether you plot a course as a rhumb line or a great circle is independent from whether you measure the direction of that course by referencing true north or magnetic north. So it shouldn't matter.

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
wardialer
Topic Author
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Joined: Fri Sep 14, 2001 1:08 pm

RE: True Vs MAG Course?

Wed Jun 20, 2012 5:13 pm

So for example, when they plan to launch a test ICBM from lets say Vandenberg AFB, do they use TRUE course or MAG course on their GPS/INS system?

And recently, the X-37B landed into Vandenberg so what course type do they use in order for the space plane guide itself to the runway approach course? Was MAGNETIC or TRUE course was used?

[Edited 2012-06-20 10:15:05]

[Edited 2012-06-20 10:21:53]
 
SAAFNAV
Posts: 365
Joined: Wed Mar 17, 2010 5:41 pm

RE: True Vs MAG Course?

Wed Jun 20, 2012 5:21 pm

When you draw a line on a map, it is a True Track.

Then you add the local variation to get Magnetic Track. From that you can derive you Magnetic Heading to steer, correcting for drift.

A Ring Laser Gyro INS and GPS will measure your True Track, then add the Magnetic Variation from an on-board database.

Hope this helps.

Erich
On-board Direction Consultant
 
wardialer
Topic Author
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RE: True Vs MAG Course?

Wed Jun 20, 2012 5:22 pm

Well, thanks.

And recently, the X-37B landed into Vandenberg so what course type do they use in order for the space plane guide itself to the runway approach course? Was MAGNETIC or TRUE course was used?
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: True Vs MAG Course?

Wed Jun 20, 2012 6:01 pm

Quoting wardialer (Reply 4):
And recently, the X-37B landed into Vandenberg so what course type do they use in order for the space plane guide itself to the runway approach course? Was MAGNETIC or TRUE course was used?

I have no idea what they actually used but they could have used either. TRUE and MAG course varies by a known amount everywhere on earth; even the most basic aviation GPS units know both headings at all times (and you can bet the X-37B has a better than basic nav system).

The runway approach track is fixed in space by the runway orientation; the actual path the aircraft wants to fly doesn't change if you're working in true or magnetic heading. All that matters is you're consistent...if you specify runway heading in true you want to make sure the aircraft is flying true, if you specify in magnetic you want the aircraft flying magnetic.

Tom.
 
wardialer
Topic Author
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RE: True Vs MAG Course?

Thu Jun 21, 2012 4:17 am

I know that Google Earth uses True heading in the Ruler measurement tool.
 
SAAFNAV
Posts: 365
Joined: Wed Mar 17, 2010 5:41 pm

RE: True Vs MAG Course?

Thu Jun 21, 2012 4:33 am

I'll try again.

Magnetic track is what you see on the compass, corrected for drift.

You use both in way. Runway numbering is Magnetic, so it will line up with a magnetic compass. But to get anywhere you need to fly a True Track over the ground, by steering a Magnetic Track..

Google Earth gives you a True Track, not a True Heading.

I'll try find a diagram.

Regards,
Erich
On-board Direction Consultant
 
timz
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Joined: Fri Sep 17, 1999 7:43 am

RE: True Vs MAG Course?

Thu Jun 21, 2012 5:09 pm

Additional complication: a rhumbline is usually considered to be a line of contstant true azimuth-- right? So along a "rhumbline", the mag azimuth will be changing. Can any software actually give you a "rhumbline" of constant mag azimuth, or do they just fake it somewhat?
 
timz
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Joined: Fri Sep 17, 1999 7:43 am

RE: True Vs MAG Course?

Thu Jun 21, 2012 8:33 pm

Quoting wardialer (Thread starter):
If I want to use a MAGNETIC course option instead of TRUE...

So far I doubt anyone can figure out what you're asking. You're hoping to draw a line on a map that has a constant magnetic azimuth, between two given points? If so you obviously want nothing to do with a great circle. Or, if you're trying to draw some other line, what defines that line? It's not a great circle, and it's not a rhumb line (i.e. constant true azimuth)? So what is it?
 
SAAFNAV
Posts: 365
Joined: Wed Mar 17, 2010 5:41 pm

RE: True Vs MAG Course?

Thu Jun 21, 2012 8:47 pm

No it won't be constant magnetic, because the variation changes from place to place.

They don't fake it, they use the True Track/Course/Azimuth and correct the value for Magnetic.

So if you are flying a Rhumbline Track (assuming no Wind), you will actually make heading changes to stay on the same Track.

Erich
On-board Direction Consultant
 
timz
Posts: 6161
Joined: Fri Sep 17, 1999 7:43 am

RE: True Vs MAG Course?

Thu Jun 21, 2012 9:59 pm

Sure you can draw a line between two points that's constant magnetic azimuth. It'll be a lot more work than drawing a rhumb line, since there's no reasonably simple formula for it (i.e. there's no reasonably simple formula giving the mag variation at every lat-lon).

Quoting saafnav (Reply 10):
if you are flying a Rhumbline Track (assuming no Wind), you will actually make heading changes

Just to clarify: if you trying to follow a course of constant true direction, your magnetic heading will need to change as you move along that intended "rhumbline" course. That "rhumbline" course will be a different line from the one that has a constant magnetic direction, but the latter still exists.

(I see at http://www.ngdc.noaa.gov/geomag-web/#declination and
http://www.ngdc.noaa.gov/geomag-web/#ushistoric they claim half-degree accuracy for mag-minus-true. Do other sites claim to do better?

[Edited 2012-06-21 15:04:56]
 
Mir
Posts: 19108
Joined: Mon Jan 05, 2004 3:55 am

RE: True Vs MAG Course?

Fri Jun 22, 2012 12:10 am

Quoting saafnav (Reply 7):
Google Earth gives you a True Track, not a True Heading.

If there's no wind (which is the case if you're looking at a map or mapping software), then then two are the same.

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
SAAFNAV
Posts: 365
Joined: Wed Mar 17, 2010 5:41 pm

RE: True Vs MAG Course?

Fri Jun 22, 2012 4:32 am

Depends on if you put one or two arrows on the line?  
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