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understanding true north versus magnetic north

Former Member over 15 years ago

Basic question
On mapsource between two points on a route I created says that the course is 0 degrees true; this is from my apartment to another point. My Garmin summit hc says the point from my apartment is 11 degrees. I checked with the declination site and my zip code is 10 degrees 41 minutes west. This makes some sense since if I subtract 10 degrees 41 minutes I would come close to 0 degrees true north.

But this confuses me since I thought that the gps was set for true north based on latitude and longitude.

Anybode clarify my thinking--thanks a lot

0 Former Member over 15 years ago

I have an old GPSMAP76S with an electronic compass similar to the one in your SummitHC. It needs to be calibrated once in a while (pg 28 in your manual) to keep the magnetic sensing part accurate.

Standing still it works like a magnetic compass so it will be off true north by the magnetic declination just like you thought.

When you are moving the unit uses GPS coords to calculate the bearing of the direction you are heading. That heading is based on true north.

You should be able to go down a straight street and get a heading and then stop for a minute or two to let the unit revert to magnetic and get a compass reading that will differ from the heading by the declination.