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Paul Bradbury · 05-02-2006, 06:10

Learning Coastal Navigation using my compass as a primary and the GPS as a backup. I know what causes deviation (metal close by the compass) but how can I create a compass card? I was thinking that I take the boat for a motor and write to differences between by on board compass and the hand held GPS. Write all the difference at incriments of 10 degrees and go from there.

Will that work?

Cheers

**GordMay** · 05-02-2006, 07:33

Swinging the Compass, to develop a Compass Deviation Card is usually tabulated in increments of 4 bearings (8 @ 45 deg, 16 @ 22.5 deg, or 32 @ 11.25 degree increments). These match the Cardinal and inter-cardinal compass points (N @ 0, NNE @ 22.5, NE @ 45, ENE 67.5, then E, ESE, SE, SSE, and so on around the card).

Lodesman · 05-02-2006, 07:38

Paul,

Deviation can also be caused by magnetic fields induced by energized electrical equipment and wires. It's not a bad idea to check for compass changes when you flash up and shut down engines, radios etc while you're alongside. Your GPS method will probably give you a reasonably accurate card, but I assume you'll be taking True Course so current and drift will give you an error. If your GPS has a built in fluxgate compass, it too will suffer from some deviation from just being on board, but that may be less, more or equal to the compass' deviation - no way to tell. Some harbours have a charted swinging area - it would likely have some form of transit mark in the vicinity so you could line up on a true bearing and compare it visually to the compass bearing as you swing the boat through the cardinal and sub-cardinal points.
http://education.qld.gov.au/curricul...nav/cnswi.html
Good luck with whatever you choose.

Kevin

Jerry · 05-02-2006, 10:59

Are the directional readouts on a GPS referring to magnetic north or true north.
BY the way I am located in the northern hemisphere should any down-under folks be reading this...LOL