How do you determine true north with a GPS system? GPS only gives you your position, not your orientation.

> How do you determine true north with a GPS system?

  def true_bearing(magnetic_bearing, location)
    magnetic_bearing + gma(location)
  end

And that's the point - your true bearing is a function of your location.

Ah. Right. Duh. (I must be having an even worse day than I thought.)

Wouldn't it be possible to broadcast local magnetic declination over ATIS or other automated broadcast systems?

As far as I know (and according to the article), modern navigation systems contain databases of the local magnetic declination anyway; instead of updating maps and navigational databases, we could just update these declination database instead every once in a while if I understand it correctly.

How do you apply these things without knowing where you are?

True/magnetic deviation is localised. You need to know where you are to know what the localised value is.

Wouldn't a very rough location suffice, i.e. something that could be either manually set (for general aviation and shorter distances), derived from a VOR station identifier or similar, or just estimated via dead reckoning/an INS?

In other words, if you don't even have a rough idea of where you are, what good will a magnetic heading do?

Rough yeah - but I think it does change by integer degrees from actual map sheet to map sheet, so you could probably drift a couple of degrees without knowing it within an hour or so of flying.