Are you suggesting it purely for the reasons of authenticity? Because, while to realism purists and organized players this feature might actually add to the fun element of gameplay, to everyone else it's just another step of complexity in determining your relative location.

In public non-clan games, it's difficult enough to get players to use azimuth in their communication instead of falling down to subjective positioning cues like "the enemy is to my left/right". Imagine, how much more confusion something like this would add.

Don't know, if only as an option... (You have my vote up)

That's silly. None of the theaters/maps are going to be large enough to have any variance in magnetic declination, therefore it cannot and will not be a factor in any gameplay.

Declination would be set once when you arrive in stratis/Limnos and then forgotten about, and would most likely be done using the simple declination method anyway, that being asking someone "Hey buds, what's the declination here?"

If you're going to impliment magnetic declination, you may as well implement a "set watch to this time zone" feature.

You could simulate sun flares, too, disturbing electronical equipment like GPS.
I think it is too much. Maybe sth for ACE, but not the original game.

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Chris2525 said it all: Good compasses have an adjustable declination correction.

Thanks for the idea.
It's unnecessarily complex. Also following the downvotes I'm closing the ticket.