Creating a deviation table from a GPS.

RopeBurn · #1 (**permalink**) 09-04-2001

Is it possible to construct a deviation table for a magnetic compass using headings recorded from a GPS?

tliveright · #2 (**permalink**) 09-04-2001

Absolutely as long as you have a 12 channel GPS. I can''t remember which of the popular sailing books discusses it but it''s there.

EscapeArtist · #3 (**permalink**) 09-04-2001

How would you compensate for deviation caused by tidal current?

RopeBurn · #4 (**permalink**) 09-04-2001

I''m not sure what tidal current has to due with compass deviation. Compass deviation is particular to a compass, and is due to magnetic influences from it''s surroundings.

EscapeArtist · #5 (**permalink**) 09-05-2001

Coming home last weekend I had to steer 280 degrees magnetic to maintain a "GPS heading" of 275. Should I look at that and conclude that my compass has a 5 degree error?

GPSs don''t know much about compass directions, they only know where they are and where they were.

RopeBurn · #6 (**permalink**) 09-05-2001

I understand. But in the slack between tides or on a lake, it should be possible to create a deviation table using GPS headings.

Seems like tidal current would also interfere with creating a deviation table via sun bearings. I think you''d need a still body of water in either case.

alwilt · #7 (**permalink**) 09-05-2001

Surely a GPS heading is True and therefore
both Variation and Deviation have to be tak.en into account to compare with your compass.
Tides ,currents etc have nothing to do with the compas but may influence a gps Track

Leon-T · #8 (**permalink**) 09-13-2001

Some GPS devices let you set headings to magnetic. I have a Garmin GPS 76: it lets you set the heading to magnetic, and it automatically adds (subtracts) variation. You can also manually input the variation.

I''ll plan on using my GPS 76 to check my compass deviation (another to-do item). It definitely seems like like the most economical alternative.

posmentier · #9 (**permalink**) 12-22-2001

The difference between your GPS and your compass is the sum of variation (difference between magnetic and geographical North) plus deviation (compass "error" caused by ferromagnetic metals on the boat) plus leeway (current plus leeward slippage of the boat through the water). To find deviation from GPS and compass readins you need to know variation and leeway, or to sail in one direction and then reverse course by 180.