The easiest rule would surely be based on the spin of the planet (the equator). Anyway, maps sometimes do have a Y axis that is not pointed towards north, and

The easiest rule would surely be based on the spin of the planet (the equator). Anyway, maps sometimes do have a Y axis that is not pointed towards north, and the north arrow is used to orient yourself (even Google maps can do this). Changing that would just confuse everyone. BTW, GPS devices initially calculate their location in geocentric coordinates from the centre of the earth with the reference plane aligned to the equator. It is calculated from the orbital parameters of the satellites.