4.15. Using True North vs. Magnetic North
For the average geocacher, it should not matter whether your GPS is set to True North or Magnetic North as the GPS should be accurate either way. True North can sometimes be better if you frequently use maps over long distances on your GPS.
Magnetic North is only really required when you're following a magnetic compass. If your GPS is set to True North, and it says to go 15°, and you pull out your magnetic compass and shoot a bearing of 15°, you may be in trouble based on your position if you are using the type of map pictured here. (This is an old one.)
However, if your GPS tells you to go 15° and you follow the arrow on the GPS (and the numbers get smaller), you'll be fine.
Two scenarios illustrate this:
Scenario 1: simple, short distance
In Chicago, Illino, I have a geocache 750 feet (229 meters) away.
My GPS is set to True North, and the GPS says that the geocache is 45°. I take out my trusty compass and shoot a bearing of 45° and walk for 750 feet. But wait! I didn't take into account the magnetic declination, which around Chicago is somewhere between 2 and 3 degrees. Let's say it is 3.
According to my calculations, after traveling 750 feet, I will be off 41 feet (about 12.5 meters). It's not a big deal. Many GPS units are off by that much anyway. I will still arrive at the geocache.
Scenario 2: big declination, long distance
I'm hiking on Cape Cod, Massachusetts and there is a geocache 3 miles (5 km) away.
My GPS is set to True North, and the GPS says that the geocache is 45°. I take out my trusty compass and shoot a bearing of 45° and walk for 3 miles. But wait! I didn't take into account the magnetic declination, which around Cape Cod is somewhere around 16 degrees.
With that much difference in the declination and with traveling that far, I would be off by 0.78 miles (1.2 km). I may not get close to the geocache's location at all.
In a real situation, I would take out my GPS many times during the 3 mile (5 km) hike. Many small adjustments would be made over that distance. I would see the displays and adjust my own direction.
That is why magnetic declination doesn't really enter into it. Follow the arrow on your GPS and watch the numbers go down.
Many thanks to Volunteer Forum Moderator Markwell for initially developing this text.