In article , lid, Bob
says...

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 ?

Not sure what you really mean by glass display, but I'll assume old
school (real) compass - a magnetic pointer).

They point North! - magnetic north, always, of course!

I believe a LAME checks and swings the compass (Licenced Aircraft
Mechanical Engineer). (Ie. a mechanic! - but a plane mechanic! )
On doing so, they may adjust a placard beside the compass to indicate
local deviation, normally quadrants and sub-quadrants only. So maybe +1
for East for example. This is of course, local to each plane.

If not GPS, what ?

Is it then Magnetic or True ?

A compass always points magnetically.

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

Yes, both chart variation, and compass deviation (per above). On top of
that, there's wind to adjust for.

Does aviation use True or Magnetic when discussing or flying ?

Both of course. Charts are True, the compass is Magnetic.

--
Duncan.