Comment by oersted
13 hours ago
I agree, but the video at the bottom helped.
You can draw a line between your location and the north pole, they talk about three variants:
- Magnetic North: Shortest surface line to the magnetic north pole (simply in the direction of the compass at your location).
- True North: Shortest surface line from where you are to the geographic north pole (based on the rotation axis?).
- Grid North: A line to the same geographic north pole, but aligned to the longitude lines (EDIT: for a local UK grid standard, slightly different from the global one). I didn't fully understand the subtleties of why it's different from True North, something about the projection. Not sure if it's exactly to the same north pole, the rotation axis might also change slightly and I assume that the grid north point is fixed by convention?
They are saying that there's a particular point where all three lines point in the same direction, and that point is moving.
In the UK there's a standard grid used for local-only mapping: https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid
It's a transverse mercator projection rather than a mercator as you might often see because it minimises distortion over the UK as a whole which means that the distortion is as you move away from the meridian, rather than as you move away from the equator (with a regular mercator I think all points have the grid aligned with true North)
This grid is setup such that it's origin is not on the prime meridian (at Greenwich), but 2deg west so only points on the line 2deg west are aligned with true north.
One of the advantages of doing this seemingly weird projection is that you can treat "local" maps (for some definition of local) as flat rectangular grids without introducing a lot of errors: drawing straight lines between two points, measuring the distance / angle between them, etc., just by dealing with a flat piece of paper. VERY convenient, but the farther you are from the center of the projection, the higher the errors that are introduced.
In short, you can treat the local geometry as Euclidean.
3 replies →
> Magnetic North: Shortest surface line to the magnetic north pole (simply in the direction of the compass at your location).
Magnetic North is the local horizontal direction of the magnetic field. But that doesn't generally coincide with the shortest surface line (geodesic) to the magnetic north pole (however you define that - there's several).
If you followed your compass you could end up in a loop without reaching the magnetic north pole.