Hello Bob,

I'm unable to provide the answer as to the technology employed by
computer-generated displays to display magnetic information, but I would expect
something like a Hall device might be employed. As you implied, it could as
easily be derived from GPS data corrected for magnetic variation at the
particular location.

Magnetic/compass headings are not true, but magnetic. Navigation Sectional
charts http://skyvector.com/ are depicted in True coordinates, and the
pilot/navigator must correct for magnetic variation in drafting his flight
plan. ATC expects all course/heading numbers to be magnetic.

You'll find more information he
http://www.faa.gov/regulations_policies/handbooks_manuals/, and he
http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2015.pdf.

Hope that helps.

Best regards,
Larry Dighera



On Mon, 13 Jul 2015 18:36:54 -0400, Bob wrote:

Hello,

Not a Pilot, but always interested in the engineering aspects.

This question came up, and I realize that I am not sure of a correct answer.

Regarding the compass heading that shows in the glass displays:

How is it derived ?
GPS ?

If not GPS, what ?

Is it then Magnetic or True ?

Does the Pilot have to correct for variation/declination changes during
a flight ?

Does aviation use True or Magnetic when discussing or flying ?

etc. ?

Thanks,
Bob