Hello to all, I have a Richie navigator compass on my Cat 30 and a Garmin GPS. The compass numbers does not match the GPS, it is off by 10degrees. Could I just set the compass to the GPS? It seems like this should be accurate. Thanks, Russ
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compass deviation
Check for compass deviation, this can change with each heading. Deviation can caused by several factors, usually electical/metal near compass. You might consider a deviation chart, if the source of deviation cannot be neutralized.
Check for compass deviation, this can change with each heading. Deviation can caused by several factors, usually electical/metal near compass. You might consider a deviation chart, if the source of deviation cannot be neutralized.
Too many options
This is too complex an issue to reduct it to matching the GPS. You have to contend with variation and deviation, on different points of sail. You also have to consider that when sailing the boat, the compass is reading the direction the boat is facing, and the GPS shows the direction the boat is actually moving in. If it were me and I was concerned, I would take it to someone who swings compass for a living, and knows what they are doing. Run a known, straight, channel, and plot the compass bearing on a chart. Check the local variation, then run the channel and compare your compass reading.
This is too complex an issue to reduct it to matching the GPS. You have to contend with variation and deviation, on different points of sail. You also have to consider that when sailing the boat, the compass is reading the direction the boat is facing, and the GPS shows the direction the boat is actually moving in. If it were me and I was concerned, I would take it to someone who swings compass for a living, and knows what they are doing. Run a known, straight, channel, and plot the compass bearing on a chart. Check the local variation, then run the channel and compare your compass reading.
They will never match
It's not just the numbers on the card that tell you what direction that you are going. Did you figure in the deviation between true north and magnetic north? I think that it is about 6 deg. in St Louis. but the long and short of it is that you keep track of your progress on a chart or GPS and make changes in your compass heading to get you there. They will never match.
It's not just the numbers on the card that tell you what direction that you are going. Did you figure in the deviation between true north and magnetic north? I think that it is about 6 deg. in St Louis. but the long and short of it is that you keep track of your progress on a chart or GPS and make changes in your compass heading to get you there. They will never match.
There is vertually no variation between
true and magnetic north in Gulfport MS. Your GPS can be programed to show magnetic. Often times drift or set will show a course over ground on your GPS quite different than your compass heading.
true and magnetic north in Gulfport MS. Your GPS can be programed to show magnetic. Often times drift or set will show a course over ground on your GPS quite different than your compass heading.
Magnetic declination in our PNW waters differs...
by up to almost 20 degrees in places. Seldom do our binnacle compass, autopilot and GPS headings match. Most of the time they are pretty close. I tend to rely more on our Garmin than anything else. Maybe I should be more anal about it but all too often I'm just wrapped up in enjoying the surroundings and sail trim. Terry
by up to almost 20 degrees in places. Seldom do our binnacle compass, autopilot and GPS headings match. Most of the time they are pretty close. I tend to rely more on our Garmin than anything else. Maybe I should be more anal about it but all too often I'm just wrapped up in enjoying the surroundings and sail trim. Terry
It is always interesting to compare the precise
with the common usage. All of the compass roses on my charts show variation of magnetic to true. The topo zone shows a pair of arrows below each map and explains that they show geographic north and the declination of magnetic north. I guess that declination is the precise term but variation is the common term.
with the common usage. All of the compass roses on my charts show variation of magnetic to true. The topo zone shows a pair of arrows below each map and explains that they show geographic north and the declination of magnetic north. I guess that declination is the precise term but variation is the common term.
Once again:
1. An EXAMPLE: Going through Deception Pass, current is 7 knots heading west, boat is heading north being tossed by currents. Compass reads 0 degrees (heading of boat is N), GPS reads 270 - boat's goin' west. 2. They have written, oh my gosh, BOOKS about this stuff. Read Chapman's or any of the other 5,987 books.
1. An EXAMPLE: Going through Deception Pass, current is 7 knots heading west, boat is heading north being tossed by currents. Compass reads 0 degrees (heading of boat is N), GPS reads 270 - boat's goin' west. 2. They have written, oh my gosh, BOOKS about this stuff. Read Chapman's or any of the other 5,987 books.
Compass and GPS will disagree MOST of the time.
When using a GPS, you must use a go-to funtction to a waypoint etc. to expect any compass corelation !!!!! The GPS averages the course, the compass is 'real-time'. You must also match the 'datum' of the chart you are using for reference with your GPS 'system' datum .... NAD29 and WGS84 etc. are common 'datums' used for charting for North America When performing compass corrections via GPS, always use the BEARING "To or From" mode on theGPS. Do NOT use the HEADING Information because it is VERY inaccurate in ***real time***. SimpleSpeak: follow the GPS to a WAYPOINT to estabalish the BEARING. further info: http://www.ritchienavigation.com/service/compensation.html
When using a GPS, you must use a go-to funtction to a waypoint etc. to expect any compass corelation !!!!! The GPS averages the course, the compass is 'real-time'. You must also match the 'datum' of the chart you are using for reference with your GPS 'system' datum .... NAD29 and WGS84 etc. are common 'datums' used for charting for North America When performing compass corrections via GPS, always use the BEARING "To or From" mode on theGPS. Do NOT use the HEADING Information because it is VERY inaccurate in ***real time***. SimpleSpeak: follow the GPS to a WAYPOINT to estabalish the BEARING. further info: http://www.ritchienavigation.com/service/compensation.html
Russ....
My suggestion to you is to get a good basic seamanship manual such as "Chapman's" or the "Annapolis Book of Seamanship". Both have excellent sections on the magnetic compass. You need to understand the difference between "true" and "magnetic". Studying the sections on coastal piloting will help you make sense of all there terms being thrown around here.
My suggestion to you is to get a good basic seamanship manual such as "Chapman's" or the "Annapolis Book of Seamanship". Both have excellent sections on the magnetic compass. You need to understand the difference between "true" and "magnetic". Studying the sections on coastal piloting will help you make sense of all there terms being thrown around here.
swing the compass
If you have nothing to do one morning or afternoon... you could swing the compass. Many of the new GPS's are very smart and can calculate Variation. So we have True, Magnetic, Variation and Deviation... you need to look into the bowels of the GPS to see how you are set up.
If you have nothing to do one morning or afternoon... you could swing the compass. Many of the new GPS's are very smart and can calculate Variation. So we have True, Magnetic, Variation and Deviation... you need to look into the bowels of the GPS to see how you are set up.
GPS Bearing
Rich H. Just reading the bearing to a waypoint will not give accurate heading info for the boat. The boat can be heading any direction and the bearing to the waypoint will remain constant. Your own reference instructs you to VISUALLY aim toward a known waypoint that you have established a bearing to and correct your compass from that VISUAL course. I think you understand that, but your post did not read that way to me. fair winds Dave
Rich H. Just reading the bearing to a waypoint will not give accurate heading info for the boat. The boat can be heading any direction and the bearing to the waypoint will remain constant. Your own reference instructs you to VISUALLY aim toward a known waypoint that you have established a bearing to and correct your compass from that VISUAL course. I think you understand that, but your post did not read that way to me. fair winds Dave
I get better results
swinging my compass using a hand-bearing compass rather than the GPS. A few years ago when doing this, I inadvertently discovered that the metal canister for the air horn mounted in a holder on the pedestal -- and only a few inches from the compass -- was causing the compass to be off by a number of degrees. I also would expect that any battery powered device, like a GPS, could also cause your compass to be off by lots of degrees.
swinging my compass using a hand-bearing compass rather than the GPS. A few years ago when doing this, I inadvertently discovered that the metal canister for the air horn mounted in a holder on the pedestal -- and only a few inches from the compass -- was causing the compass to be off by a number of degrees. I also would expect that any battery powered device, like a GPS, could also cause your compass to be off by lots of degrees.
Adding
Someone below said, "You also have to consider that when sailing the boat, the compass is reading the direction the boat is facing, and the GPS shows the direction the boat is actually moving in." This is the perfect explanation, however adding to that: Remember that the centerline of a sailboat is not pointing in the direction of travel. Leeway, current, horizontal keel angle, heel, etc change the centerline direction, thus changing the compass reading, while the GPS will show you your true heading over land.
Someone below said, "You also have to consider that when sailing the boat, the compass is reading the direction the boat is facing, and the GPS shows the direction the boat is actually moving in." This is the perfect explanation, however adding to that: Remember that the centerline of a sailboat is not pointing in the direction of travel. Leeway, current, horizontal keel angle, heel, etc change the centerline direction, thus changing the compass reading, while the GPS will show you your true heading over land.
Warren, you know this but for the rest.
Quote: "I also would expect that any battery powered device, like a GPS, could also cause your compass to be off by lots of degrees" This is why all DC power should be carried in twisted pair wiring. The electromagnetic field reverses at every turn.
Quote: "I also would expect that any battery powered device, like a GPS, could also cause your compass to be off by lots of degrees" This is why all DC power should be carried in twisted pair wiring. The electromagnetic field reverses at every turn.
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