Dan Thomas wrote:

Remember that GPS give you track, not heading, and if there's any
appreciable wind you won't get anywhere near accurate heading
indications for compass calibration purposes. Use the heading
indicator instead.

Yep, forget the GPS. Use this method instead:

Line up on a taxiway or runway with a known, published heading and carefully set
the HI. Go fly on gyro headings 360, 030, 060, etc. and carefully record the
magnetic compass reading. You might want to try it with and without lights,
pitot heat, any other major power consumers. Return to your known taxiway or
runway and verify the HI hasn't drifted.

Dave
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