GPS Heading and Compass heading

[wiredantz]

Does anyone have any idea why the GPS on my fishfinder would read differently than my two compass.


Example my fishfinder would say that the Artificial reef is directly ahead of me at 212 degrees as i am paddling at 2.5 mph

I then looked at my two compasses and compass will say 190 degrees same direction i have a humminbird 161. I don't remember very well i have to test again i just remember that the GPS Heading was wayyyyyyyyyyy offfff

[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.

[Jimmyz123]

Does your system have a way to load updates on it from Himminbird? A friend of mine had a Raymarine GPS that was doing similar to that and they told him he had to pay for an upgrade to the GPS unit. Let us know how that change works for you.