With the motor running I checked my compass against my Garnin 72 GPS Compass --Garmin GPS 0--- 358 45--- 35 90--- 75 135--- 113 180--- 157 225--- 210 270--- 265 315--- 315 So when navigating with a compass, should I build in a compensation based upon my check against my GPS? Can an onboard compass be adjusted or is this just the way it is? Some have suggested that I keep a handy compass card at the helm with the various steering postions. Comments....
Compass Check
- Thread starter Paul8753
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Is your GPS using Magnetic or true North?
Some GPS have the ability to use True and Magnetic North. That may change your deviation
Some GPS have the ability to use True and Magnetic North. That may change your deviation
True or Magnetic to confirm
I will check that. I was sure that the GPS was in magnetic mode. More to follow...
I will check that. I was sure that the GPS was in magnetic mode. More to follow...
A little research will help you.
I've included a link to the Sailnet.com section on piloting. Here is an excerpt from Jim Sexton's series of articles on the compass that discusses DEVIATION. "Secondary magnetic fields in the vicinity of the compass can also affect the compass readings. These secondary magnetic fields are caused by the presence of ferromagnetic objects, electronics, and electrical wires in the boat. This error can be reduced by changing the position of the small compensating magnets in the compass case. However, it is not possible to remove all of these errors on all headings. The end result will be a compass card showing the number of degrees of error in the compass when you are on various compass headings. This error is called deviation, and like variation must also be considered when determining heading relationships." I might also recommend you pick up a copy of John Rousmaniere's "The Annapolis Book of Seamanship" to further your research.
I've included a link to the Sailnet.com section on piloting. Here is an excerpt from Jim Sexton's series of articles on the compass that discusses DEVIATION. "Secondary magnetic fields in the vicinity of the compass can also affect the compass readings. These secondary magnetic fields are caused by the presence of ferromagnetic objects, electronics, and electrical wires in the boat. This error can be reduced by changing the position of the small compensating magnets in the compass case. However, it is not possible to remove all of these errors on all headings. The end result will be a compass card showing the number of degrees of error in the compass when you are on various compass headings. This error is called deviation, and like variation must also be considered when determining heading relationships." I might also recommend you pick up a copy of John Rousmaniere's "The Annapolis Book of Seamanship" to further your research.
Course or Ship's Head
Paul, Depending upon which model of Garmin GPS you have it will be reading your boat's track when moving at more than 3 knots but your ship's compass will be reading the ship's head. These two will differ if there is any cross current. Looking at your results I do not think this is the cause of the differences. If your boat is stationary - or nearly so - GPS derived directional readings are rubbish unless the GPS has an inbuilt Magnetic Compass, which some Garmin Hand Helds have. If this is the case then either compass could be in error. You need to find a transit and take its bearing off a chart. Then run the boat along this imaginary line and check the compass against the chart. Do it again in the opposite direction and check again. Repeat with another transit at approx 90° to see if the errors vary. If the ship's compass is in error by more than a few degrees then you need the services of a Compass Adjuster who can usually put matters to rights.
Paul, Depending upon which model of Garmin GPS you have it will be reading your boat's track when moving at more than 3 knots but your ship's compass will be reading the ship's head. These two will differ if there is any cross current. Looking at your results I do not think this is the cause of the differences. If your boat is stationary - or nearly so - GPS derived directional readings are rubbish unless the GPS has an inbuilt Magnetic Compass, which some Garmin Hand Helds have. If this is the case then either compass could be in error. You need to find a transit and take its bearing off a chart. Then run the boat along this imaginary line and check the compass against the chart. Do it again in the opposite direction and check again. Repeat with another transit at approx 90° to see if the errors vary. If the ship's compass is in error by more than a few degrees then you need the services of a Compass Adjuster who can usually put matters to rights.
Not Deviation (again...) {GPS vs Compass}
If there is current, your compass will be different than your GPS. Donalex is right. We've had this discussion before. See the link. This is basic navigation 101. Suggest you go to the above mentioned reference sources. There's nothing wrong with your compass or GPS. Stu
If there is current, your compass will be different than your GPS. Donalex is right. We've had this discussion before. See the link. This is basic navigation 101. Suggest you go to the above mentioned reference sources. There's nothing wrong with your compass or GPS. Stu
Compass Errors
Your data indicate both a constant offset and a more or less standard sinusoidal deviation. The constant error is 11.5 deg West (to be subtracted from magnetic to get true). I have listed it in the table below as a variation although you say you have set your GPS to deliver magnetic bearings. It sure looks like a variation but you know the variation in Toronto better than I do. Constant offset errors in compasses are probably not due to local (shipboard) magnetic influences. Another possible source of an error like this is misalignment of the compass with the boat's axis. In this case, the compass reading is constantly off by the amount of misalignment +/- the deviation because the lubber's line is not pointing where the boat is pointing/going. Eleven and a half degrees is pretty small and might not be noticed. Check by stretching a string over the compass from the mast to the back stay. Your deviation (without the constant error) is still up to 12 degrees but it looks pretty sinusoidal and you can probably adjust most of it out using the adjusting magnets on the compass. TRUE VARIATION MAGNETIC DEVIATION COMPASS 358 11.5 W 369.5 9.5 E 360 035 11.5 W 046.5 1.5 E 045 075 11.5 W 086.5 3.5 W 090 113 11.5 W 124.5 10.5 W 135 157 11.5 W 168.5 11.5 W 180 210 11.5 W 221.5 3.5 W 225 265 11.5 W 276.5 6.5 E 270 315 11.5 W 326.5 11.5 E 315 358 11.5 W 369.5 9.5 E 360 Sorry but the table doesn't print very well.
Your data indicate both a constant offset and a more or less standard sinusoidal deviation. The constant error is 11.5 deg West (to be subtracted from magnetic to get true). I have listed it in the table below as a variation although you say you have set your GPS to deliver magnetic bearings. It sure looks like a variation but you know the variation in Toronto better than I do. Constant offset errors in compasses are probably not due to local (shipboard) magnetic influences. Another possible source of an error like this is misalignment of the compass with the boat's axis. In this case, the compass reading is constantly off by the amount of misalignment +/- the deviation because the lubber's line is not pointing where the boat is pointing/going. Eleven and a half degrees is pretty small and might not be noticed. Check by stretching a string over the compass from the mast to the back stay. Your deviation (without the constant error) is still up to 12 degrees but it looks pretty sinusoidal and you can probably adjust most of it out using the adjusting magnets on the compass. TRUE VARIATION MAGNETIC DEVIATION COMPASS 358 11.5 W 369.5 9.5 E 360 035 11.5 W 046.5 1.5 E 045 075 11.5 W 086.5 3.5 W 090 113 11.5 W 124.5 10.5 W 135 157 11.5 W 168.5 11.5 W 180 210 11.5 W 221.5 3.5 W 225 265 11.5 W 276.5 6.5 E 270 315 11.5 W 326.5 11.5 E 315 358 11.5 W 369.5 9.5 E 360 Sorry but the table doesn't print very well.
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