I don’t follow this at all please explain further.
On an analog compass the magnets point to magnetic north. The needle is magnetic or not, they are not and cannot be calibrated for a particular region. The variation between magnetic north and true north is declination, this does vary from location to location and an analog compass needs to be adjusted to account for this, but the needle is always pointing to magnetic north. A digital compass may or may not account for this and need to be re calibrated. Now magnetic north does move and in-fact has and will again flip from one poll to the next,
but let’s not worry about that for the moment.
Sorry for the confusion. You are talking about declination; I am talking about inclination.
For example:
Suunto Compass Balancing
The vertical intensity and direction of the earth's magnetic field, the inclination, influences the horizontal plane of a compass needle according to the latitude where it is used. Due to inclination, compasses must be balanced for different geographical zones in order to keep the needle in a horizontal position.
Previously, the globe was divided into five different balance zones in which specific compasses functioned. However, as people travel more and more, the need for a compass with a greater geographical range has increased. Thanks to Suunto's 'Two Zone System' there are only two balance zones to consider in compass use — the Northern Hemisphere and the Southern Hemisphere. In practice, this means that a Suunto compass designed for the northern hemisphere will still work for a fair distance into the southern hemisphere, and vice-versa.
And: http://www.mapworld.co.nz/global.html
Short story:
I was teaching the PADI navigation class once upon a time; it ahs three dives: (1) the one everybody is familiar with, from AOW, namely swimming a square; (2) swimming a presignated course with multiple non-right-angle turns and varying distances between the turns; and (3) swimming a "treasure hunt" with pre-laid hidden markers, each giving the distance and bearing to the next marker; you start by telling the student the distance and bearing to the first marker. Dives 1 and 2 went fine, exceedingly well, in fact. the 3rd was awful; the student started out 10-15 deg off from the intended bearing, so couldn't find the first marker. This happened twice, with some discussion and verification of compass-holding technique in between the two tries. Then we took his compass and laid it out on the wooden dock along with half a dozen others compasses borrowed from the observers. All pointed the same direction, except his, which was 10-15 deg off. We loaned him someone else's compass, and he swam the course without issues. Afterwards, he said he'd bought that compass on a trip to Australia. Dives 1 and 2 -- and most UW navigation -- is all relative....you take a sighting on a buoy and then swim to it underwater, for example. the compass could be way off, and that technique would still work. But not all navigation is like that.
