The Thing With Magnetic North
When I was introduced to map and compass work, I learned about “true” and “magnetic” north. Say .. what? There are two “Norths” ? It was puzzling.
As it turns out, there are actually three “Norths”, though only the first two have relevance for SAR team members:
1) Geographic North (True North)
2) Magnetic North
3) Geomagnetic North
So .. what is what?
Let’s forget about the “Geomagnetic North” (or look it up here, cool video) . For map & compass purposes we look at the Geographic North and the Magnetic North only. Geographic North (“True” North) is what is used on a map. As you may know the earth has an axis it spins around. The top of the world so to speak. This is where the map north points to.
It is called “geographic” or “true” north.
But this is not where our compass needles point to. The compass points to the magnetic north pole. Our earth produces its magnetic properties because it has a molten iron core which is rotating and sloshing around thus creating the magnetic shield around earth which protects us against harmful radiation from space. All that molten stuff is why the magnetic north pole is moving around. It’s not at a static location and it’s (currently) not at the location of earth’s axis.
It’s those magnetic field lines that our compass needle lines up with.
As you can see, the Magnetic North Pole is located (in our area) to the left of the True North Pole. If we are measuring a magnetic bearing with our compass, we get a degree in relation to the Magnetic North Pole.
It we want to transfer this bearing to a map, we have to add what is called the “declination” which is a fancy word for the difference we have to add so that both angles (the magnetic and true) are the same. It’s like adding “minutes” to a clock so that both pointers show the same time.
The “declination” changes not only depend on your position on the earth, but also because – as mentioned before – the magnetic north pole meanders around. The current (2022) declination in Roswell, Nm is +6°59’ – pretty much 7 degrees.
That is what you need to add to the red magnetic bearing to make it overlap the orange true bearing.
So .. why do the maps use “True” instead of “Magnetic” bearings? Simple. They would have to re-print new maps every year because the magnetic bearings change.
Do GPS devices show “true” or “magnetic” bearings? We can configure that on our “InReach” satellite and GPS tracker.
Lastly: If you want to play around with a cool “Online” compass (mobile devices only) try this one from the National Oceanic and Atmospheric Administration (NOAA).