Does one have to calibrate the compass on the Petrel before each dive?
No, just an initial calibration that takes very little time.
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Does one have to calibrate the compass on the Petrel before each dive?
The other error is more subtle...at best, the compass points toward magnetic north, which is not the same as true north; the difference is called magnetic deviation.
Yes, you are correct, I was being sloppy. Declination is exactly the problem of true vs magnetic heading. Deviation is the problem of local fields causing an additional error; unfortunately, like DCS and SAC being used to mean different things by different people, deviation is sloppily used to mean both declination and the local/deviation problem. My bad.I have always known it as magnetic declination. It is not really an error per say but a geophysical phenomena that one must account for when using a compass:
Magnetic Declination (Variation) Home