GPS Heading and Compass heading

[Lets_Fish]

GPS works in geodetic north while your compass is using magnetic north. The declination for San Diego is around 13°+/-. Your's appears to be out by 22°, not sure why.

Here is a link to the USGS http://rockyweb.cr.usgs.gov/outreach...gps_north.html

Here is a link to help determine your declination http://www.thecompassstore.com/decvar.html

Hope this helps.

[wiredantz]

My manual says I have to change the gps from true north to magnetic north so I'll try that

[flydigital]

Originally Posted by wiredantz

My manual says I have to change the gps from true north to magnetic north so I'll try that

The north pole is further "east" than "true" north (straight up) by 13 degrees or so in this region. So when your compass reads N (0deg) your true heading is 13. I always set my GPS to magnetic to keep it simple.

Other factors include wind and current. GPS only shows your track over the ground, while your compass shows your actual magnetic heading. So its not uncommon to see a variance there unless it is dead calm.

Finally, magnetic deviation and interference (sonar and power cables, other magnetic structures in the area, for example) can cause a variance.

But if you keep your GPS in magnetic mode it shouldn't vary more than 5-10 degrees in most circumstances, close enough for a kayak.

[wiredantz]

So I set mygps to magnetic north and now I am off by 6 degrees if I slow down to 1 mph it can't tell which way I am facing because of the wind and swell once I pick up speed it almost matches my compass

[bubblehide]

Frank, many digital compasses (it sounds like your GPS compass is digital) only work while moving, and you might need that 1+ MPH for it to function. You should be able to check if this is correct in the owners manual.

[GregAndrew]

As Damian (Flydigital) said, with current, chop and wind you are not necissarily moving straight in the direction that your kayak is pointing even when you may be paddling hard. So, the reading on your compass is going to show you the direction you are pointing, while the GPS shows you the direction you are actually traveling.

[flydigital]

Originally Posted by GregAndrew

As Damian (Flydigital) said, with current, chop and wind you are not necissarily moving straight in the direction that your kayak is pointing even when you may be paddling hard. So, the reading on your compass is going to show you the direction you are pointing, while the GPS shows you the direction you are actually traveling.

Yep, to be technical, HEADING as defined as what direction you are pointing, regardless of where you are actually moving. TRACK is the direction/bearing over the ground. Compass shows heading, GPS shows track.

Think of it like this... if you are HEADING north, 0deg magnetic, going slowly at 1mph, and there is a combined wind/current pushing you at 2mph toward the south, your HEADING is 0 but your TRACK is 180 (and at a speed of 1mph). You're going backwards. Fishing for halibut this happens all the time at very slow "troll" speeds. The track becomes meaningless, and the magnetic heading becomes important. It can all get very complicated and frustrating when trying to return to a waypoint, especially when the wind and current are doing 2 different things. This is when you have to forget about the GPS for navigating TO the waypoint, use your compass or your eyes, and use the GPS simply for reference.

Some GPS units have a setting where you can change the speed threshold of track updates. I.e. if you set it to 3mph you will only see your track "direction" update after you go over 3mph. Worth playing with for your preference if you have that option.