A compass points to magnetic north, not true north: to walk a true bearing set the compass to the true bearing minus the declination (east positive), which in 2026 is +1.2° in London, +12.8° in Sydney and −12.5° in New York.
The Earth’s field comes from the liquid iron of the core and its north pole is not at the spin axis: the field is like a bar magnet tilted about 9° from it, with much more detail on top, and the magnetic pole has moved to the Arctic Ocean north of Siberia, wandering by tens of kilometres a year. The declination is the angle between the two, and so changes from place to place and year to year; maps print its value with a date.
The World Magnetic Model gives the declination to about half a degree, and not at all near the poles: within about 1,000 km of a magnetic pole the horizontal field is too weak for a compass. Steel, rocks, a car or a phone close by deflect the needle by more than the declination does, and the value drifts by up to a degree in a few years.
Estimate with the rule, then check it against the calculator that models it properly.
Open Magnetism →