True or Magnetic?

[John T1594]

The boat we bought last year came with a Garmin 182C GPS chartplotter that we thoroughly enjoyed using. The compass headings on the GPS unit always differed slightly from the magnetic compass that was mounted on the console and I assume that it was due to the magnetic variation in our region of New Jersey. I have not yet run a deviation chart for the compass and it is somewhere on my to-do list for this summer.Part of the GPS calibration procedure allows for setting the unit to display compass headings as true north or magnetic north. This has always been an issue even with those who use paper charts and keep a dead reckoning plot. What do others use on their GPS display - True or Magnetic?

 

[Wright Ellis s/v WhiskyII]

True

I use true while navigating on a paper chart so my hand-held GPS and calculations will agree. I convert to magnetic when I give the helmsman a course to steer.

 

[Benny]

Either one.

You can use either one as long as you know which one is your GPS unit set to.

 

[Seattle Scott]

Hey John, there will usually be a differnce

between the GPS heading and the binicle compass heading. the boat compass will tell you the direction the boat is pointed and the GSP will tell you the direction you are traveling. For example, you're close hauled port tack with the boat pointing at 45 deg/ mag, the GPS set on Mag will show a direction of travel perhaps as 50 degrees, because of leeway. Add some current to the mix and things change more (or less).One time helm compass showed boat pointing at 180 and knot meter showed 2 knots through the water, so we think we are making 2 knts southerly through the water, and we were. But the water was going 4 knts to the north, so the chartplotter said we were going north at 2 knts, over ground, and we were. We were a couple of miles from land so it was not obvious that we were being swept north. To answer your question, set the gps to magnetic otherwise you have no make something like a 17-23 degree deviation calulation when in NJ between true and magnetic.