However, GPS reception is fine over the poles too and is not related to the magnetic variation, so it’s not clear to me why these limits exist. The GPS boxes I know of use a lookup table for mag-true conversion so aren’t constrained by the loss of accuracy by the other commonly used method – the polynomial.
Peter wrote:
However, GPS reception is fine over the poles too and is not related to the magnetic variation, so it’s not clear to me why these limits exist. The GPS boxes I know of use a lookup table for mag-true conversion so aren’t constrained by the loss of accuracy by the other commonly used method – the polynomial.
GPS position accuracy and availability is reduced over the poles as the satellite orbits are designed to give better coverage in areas where people are more likely to need positioning data. The G1000 is an integrated system which also provide heading and attitude information. Getting a reliable heading at high latitudes can be tricky not only because of high magnetic variation but also because of high inclination – i.e. the magnetic field is almost perpendicular to the surface, leaving a very small component in the lateral plane. In the G1000 reliable attitude information depends on either accurate position or both heading and air data information being available. Garmin (or the FAA) may consider the likelihood of losing both position and heading is too great in the areas mentioned.
Has Canada’s decision anything to do with the North Magnetic Pole moving from Canada to Russia of its own volition? :-)
They are probably thinking “good riddance”. Having a magnetic pole in the vicinity is a major PITA, or any pole, really, which is why almost nobody lives near the geographic poles…
Well, there is a difference between a magnetic pole and a geomagnetic pole – the former is where the magnetic dip reaches 90°, and the latter is where the compass would point to, assuming the Earth to be a magnetic dipole (which it really isn’t). The former is moving towards Russia, whereas the latter is staying in Canada. The farther they come apart, the more confusing is the magnetic field picture in the vicinity…
Airborne_Again wrote:
Isn’t it over international waters!?
Allegedly it’s on Ellesmere Island in Nunavut.
Ultranomad wrote:
there is a difference between a magnetic pole and a geomagnetic pole – the former is where the magnetic dip reaches 90°, and the latter is where the compass would point to, assuming the Earth to be a magnetic dipole (which it really isn’t).
Very interesting! I didn’t realise that they were different. So actually the geomagnetic pole is a purely theoretical construction and compasses do indeed point to the magnetic pole?
Airborne_Again wrote:
Very interesting! I didn’t realise that they were different. So actually the geomagnetic pole is a purely theoretical construction and compasses do indeed point to the magnetic pole?
Both are approximations, geomagnetic being more global, magnetic being more local. The real magnetic moment of the Earth can be expanded into a Fourier-like series: a dipole component, a weaker quadrupole component, a still weaker octupole component and so on. The arrangement of poles on these components is shown in the figure, but keep in mind that individual components are not aligned to each other, they differ in both amplitude and orientation. The geomagnetic pole represents the first of them (dipole), and this is where the compass will point from far away (e.g., from the equator).
On the other hand, at those faraway locations, the real field lines (i.e. the lines along which the compass needle would arrange itself if it were rotating freely in 3D) are more or less parallel to the ground. The closer to the poles, the bigger is the vertical component of the field and the smaller is the horizontal one. The magnetic pole is the place where the field lines become perpendicular to the ground, so the free needle would point directly downward. In the polar region, a compass that only rotates around a vertical axis will roughly tend to point toward the magnetic pole, but its sensitivity will decrease as you approach it because the horizontal field component becomes very small. Worse yet, in the region between the two poles (magnetic and geomagnetic), the field lines can form fairly irregular patterns, so all bets as to where the compass points are off. Furthermore, as the two poles come apart, the effect of the quadrupole and higher components increases, so these patterns become more intricate.
Ultranomad wrote:
The geomagnetic pole … is where the compass will point from far away (e.g., from the equator). …
In the polar region, a compass … will roughly tend to point toward the magnetic pole
That makes sense. Thanks.
ICAO has published some information materials on this issue and launched a survey.
