Adjusting/Setting your magnetic compass?
- Thread starter Brian D
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Lance..
Magnetic deviation is not generally fixed on your compass. I have mine professionally swung every two years or when ever I add electronics near it. The compass is not off by say 7 degrees on all points of sail. At 359 degrees it may be off 6 degrees and a 180 degrees it may be off only 4 degrees. It's always wise to have your compass swung and a deviation card made for at least N, S, E and W... We have a bunch of magnetic disturbances on the Maine coast that are marked on the chart so just counting on one variation for your area is not always a safest bet unless you have no known disturbances but it's still only half the story your compasses internal deviation are the second half..
Magnetic deviation is not generally fixed on your compass. I have mine professionally swung every two years or when ever I add electronics near it. The compass is not off by say 7 degrees on all points of sail. At 359 degrees it may be off 6 degrees and a 180 degrees it may be off only 4 degrees. It's always wise to have your compass swung and a deviation card made for at least N, S, E and W... We have a bunch of magnetic disturbances on the Maine coast that are marked on the chart so just counting on one variation for your area is not always a safest bet unless you have no known disturbances but it's still only half the story your compasses internal deviation are the second half..
I think the terms are a little confused
Variation is the nautical term used to define the difference between the true direction and the magnetic direction. it is caused by the location of the magnetic (north) pole in contrast to the true pole. This figure is shown on your navigational charts with the date it was recorded and printed, plus an annual figure that has to be used to adjust the printed figure for the creep in the magnetic pole about the earth. This amount as Main Sail said can change with local magnetic conditions or anomalies that are also described on your chart. That's not the only thing you need to find your compass heading or bearing however. There is another term called Deviation, which is the amount or difference there is between your magnetic bearing or heading, and your compass reading. Deviation is determined by the magnetic signature of your own boat, and it changes with the heading (the direction your bow points) of your boat within the magnetic field of the earth. this is what has to be determined by swinging ship, and it is normally the deviation card (a list of the deviation on each heading of the boat) that is kept on the compass. This will probably be different for each magnetic compass on board. In basic piloting they teach "C D M V T" remembered by the phrase Can Dead Men Vote Twice, to make corrections to your various bearings. C is Compass direction, D is Compass deviation from your boats heading (even if the bearing you are interested in is not dead ahead), M is Magnetic direction, Variation is taken from the chart, and T is the true direction. When making corrections in the direction from C to T, you add Easterly errors and subtract westerly errors (Deviation and variation). When correcting the other way you do the opposite. So your link is based on land locations so you should use your charts and build your deviation card. Sail Safely Joe S
Variation is the nautical term used to define the difference between the true direction and the magnetic direction. it is caused by the location of the magnetic (north) pole in contrast to the true pole. This figure is shown on your navigational charts with the date it was recorded and printed, plus an annual figure that has to be used to adjust the printed figure for the creep in the magnetic pole about the earth. This amount as Main Sail said can change with local magnetic conditions or anomalies that are also described on your chart. That's not the only thing you need to find your compass heading or bearing however. There is another term called Deviation, which is the amount or difference there is between your magnetic bearing or heading, and your compass reading. Deviation is determined by the magnetic signature of your own boat, and it changes with the heading (the direction your bow points) of your boat within the magnetic field of the earth. this is what has to be determined by swinging ship, and it is normally the deviation card (a list of the deviation on each heading of the boat) that is kept on the compass. This will probably be different for each magnetic compass on board. In basic piloting they teach "C D M V T" remembered by the phrase Can Dead Men Vote Twice, to make corrections to your various bearings. C is Compass direction, D is Compass deviation from your boats heading (even if the bearing you are interested in is not dead ahead), M is Magnetic direction, Variation is taken from the chart, and T is the true direction. When making corrections in the direction from C to T, you add Easterly errors and subtract westerly errors (Deviation and variation). When correcting the other way you do the opposite. So your link is based on land locations so you should use your charts and build your deviation card. Sail Safely Joe S
The other way
The way I learned it was T V M D C (A W) or True Virgins Make Dull Company (Add Wine) Start with True and add Variation from the charts, giving you Magnetic. Add/Subtract your Deviation to get Compass. Add for West and subtract for East.
The way I learned it was T V M D C (A W) or True Virgins Make Dull Company (Add Wine) Start with True and add Variation from the charts, giving you Magnetic. Add/Subtract your Deviation to get Compass. Add for West and subtract for East.
who cares?
Hey, Does anyone still use a compass? When I am trying to get somewhere (as opposed to just day sailing wherever the wind blows) I have waypoints set in my GPS and I go where it tells me. I use the compass to help keep going in a straight line (if I can't see a point of reference somewhere else), but I don't care if I'm going 10 degrees or 15 degrees. The magnetic compass is off about 10 degrees from true, so I try to take that into account. Lastly, I sail on the Long Island sound. It's over 100 miles E to W, but only 10 - 20 miles N - S. If the GPS system ever went down when I was trying to get somewhere, I would sail close to shore until I found a bouy, locate my position on a chart, then go E or W until I found the next bouy. Barry
Hey, Does anyone still use a compass? When I am trying to get somewhere (as opposed to just day sailing wherever the wind blows) I have waypoints set in my GPS and I go where it tells me. I use the compass to help keep going in a straight line (if I can't see a point of reference somewhere else), but I don't care if I'm going 10 degrees or 15 degrees. The magnetic compass is off about 10 degrees from true, so I try to take that into account. Lastly, I sail on the Long Island sound. It's over 100 miles E to W, but only 10 - 20 miles N - S. If the GPS system ever went down when I was trying to get somewhere, I would sail close to shore until I found a bouy, locate my position on a chart, then go E or W until I found the next bouy. Barry
Barry , as long as you have a gps and 10mile visibility
you don't need a magnetic compass. But when the gps runs away from home and the fog settles in and visibility drops to a quarter mile that compass is very comforting.
you don't need a magnetic compass. But when the gps runs away from home and the fog settles in and visibility drops to a quarter mile that compass is very comforting.
Barry..
Barry come try that in Maine and you'll really be in some serious trouble when your GPS gives up the ghost.. Fog moves in up here in a mater of minutes sometimes and with 60 feet of vis you'd better have your compass dialed in if you want to see that next buoy.
Barry come try that in Maine and you'll really be in some serious trouble when your GPS gives up the ghost.. Fog moves in up here in a mater of minutes sometimes and with 60 feet of vis you'd better have your compass dialed in if you want to see that next buoy.
as I like to use the term...
Brian, some specificity with actual terms might be in order here. If you don't have and use a compass and depend on the GPS or Loran, then when it hits the fan you're SOL.] Even the instructions that come with the GPS say not to depend on only ONE means of navigation. Please, let's support cautious and prudent seamanship.
Brian, some specificity with actual terms might be in order here. If you don't have and use a compass and depend on the GPS or Loran, then when it hits the fan you're SOL.] Even the instructions that come with the GPS say not to depend on only ONE means of navigation. Please, let's support cautious and prudent seamanship.
BarryL I think everyone else who has common sense uses a compass
and I find it a little sad to think anyone would be proud of the fact that they don't care about correcting a compass. To rely solely on GPS is a little limited when sailing and tacking to windward. It may work OK if you power everywhere and your GPS doesn't fail. May you never sail in fog And Kathy, Your way works just as well as mine but I learned mine a long long time ago. Joe S
and I find it a little sad to think anyone would be proud of the fact that they don't care about correcting a compass. To rely solely on GPS is a little limited when sailing and tacking to windward. It may work OK if you power everywhere and your GPS doesn't fail. May you never sail in fog And Kathy, Your way works just as well as mine but I learned mine a long long time ago. Joe S
Stu...
I learned about Magnetic Declination while doing land navigation in the Marines. I also teach how one can determine true north versus magnetic north when one is searching for a satellite in a particular geo orbit. All headings for locating satellites is given as true. Everywhere I go to do a dish point, and I am talking 5m dishes and up (not TVRO or DTH dishes) I check for the delta between true and magnetic at that Lat/Long. I just thought the web page would be useful to someone, especially if they want to correct the compass rose on their charts.
I learned about Magnetic Declination while doing land navigation in the Marines. I also teach how one can determine true north versus magnetic north when one is searching for a satellite in a particular geo orbit. All headings for locating satellites is given as true. Everywhere I go to do a dish point, and I am talking 5m dishes and up (not TVRO or DTH dishes) I check for the delta between true and magnetic at that Lat/Long. I just thought the web page would be useful to someone, especially if they want to correct the compass rose on their charts.
compass
Hi Tom, I have a nice Ritchie Globemaster mounted in the binnacle. It works fine. I have never had it 'swung' or corrected or anything like that. Compared to true north (from the GPS) the Ritchie magnetic compass appears to be about 10 degree off. I have no idea how accurate it really is, but it's certainly good enough for me to find east or west. I also have a hand bearing compass and a small boy scout type compass. I haven't ever used them, but they seem to work. Some of you guys crack me up by showing off how 'salty' you can be. I enjoy sailing, talking about sailing, reading about sailing, and helping others when I can. I don't care at all how 'salty' I am. I try to be a prudent safe sailer. I don't believe that I need to have my compass corrected every two years to do that. Maybe if I sailed in Maine, with fog, rocks, and other hazards I would feel differently. I have sailed in fog exactly twice. The first time I was single handling from Mt. Sinai to Northport. I left in about 100 yards visibility. I followed the route I had set in my GPS, being careful to stay far enough off shore to avoid any obstacles. If the GPS died, my compass would have been accurate enough to keep me off shore. After a few hours the fog lifted and I could see where I was. The second time I was bringing home my new to me O'day from Newport RI to Mt. Sinai. As we (me, my dad, and a delivery captain) were leaving Narragansett bay, the delivery captain pointed out a fog bank rolling in. So we went a little further out to sea and kept on moving until the fog lifted. BTW, the delivery captains I interviewed (or was it they who were interviewing me) asked me a lot of questions about the boat including what gear was on it, how the engine was, etc. None asked me the last time the compass was swung. Barry
Hi Tom, I have a nice Ritchie Globemaster mounted in the binnacle. It works fine. I have never had it 'swung' or corrected or anything like that. Compared to true north (from the GPS) the Ritchie magnetic compass appears to be about 10 degree off. I have no idea how accurate it really is, but it's certainly good enough for me to find east or west. I also have a hand bearing compass and a small boy scout type compass. I haven't ever used them, but they seem to work. Some of you guys crack me up by showing off how 'salty' you can be. I enjoy sailing, talking about sailing, reading about sailing, and helping others when I can. I don't care at all how 'salty' I am. I try to be a prudent safe sailer. I don't believe that I need to have my compass corrected every two years to do that. Maybe if I sailed in Maine, with fog, rocks, and other hazards I would feel differently. I have sailed in fog exactly twice. The first time I was single handling from Mt. Sinai to Northport. I left in about 100 yards visibility. I followed the route I had set in my GPS, being careful to stay far enough off shore to avoid any obstacles. If the GPS died, my compass would have been accurate enough to keep me off shore. After a few hours the fog lifted and I could see where I was. The second time I was bringing home my new to me O'day from Newport RI to Mt. Sinai. As we (me, my dad, and a delivery captain) were leaving Narragansett bay, the delivery captain pointed out a fog bank rolling in. So we went a little further out to sea and kept on moving until the fog lifted. BTW, the delivery captains I interviewed (or was it they who were interviewing me) asked me a lot of questions about the boat including what gear was on it, how the engine was, etc. None asked me the last time the compass was swung. Barry
BarryL it may seem salty to you to be prudent but it's not salty
or trying to be salty, or trying to be important , or anything like that. It's just trying to be smart. Having a piece of gear on board that doesn't work as it is supposed to, or has an error of an unknown magnitude and repeatability isn't smart. You have a GPS but is it WAAS enabled? Do you have a differential beacon receiver to make it more accurate? What will you do if selective availability is re-instituted by the government? It would seem to me that you are sailing a less than smart course, which is your right, but please don't become an advocate for it. Joe S
or trying to be salty, or trying to be important , or anything like that. It's just trying to be smart. Having a piece of gear on board that doesn't work as it is supposed to, or has an error of an unknown magnitude and repeatability isn't smart. You have a GPS but is it WAAS enabled? Do you have a differential beacon receiver to make it more accurate? What will you do if selective availability is re-instituted by the government? It would seem to me that you are sailing a less than smart course, which is your right, but please don't become an advocate for it. Joe S
selective what!?
Suddenly paranoid that the government was on the verge of spying on where and when I anchor, who I'm with, what we might have to drink, and whether we're dressed, I had to Wiki that "selective availability" reference. It appears that our government does not plan to throw off the scores of US consumers who now have, and rely on GPS equipment. In fact, retired satellites are being replaced without SA. Point taken though, Joseph, in that I'd hate to rely on GPS only if I was in unfamiliar shallow water and grounded because my GPS was 100 meters off. As with most arguments on this board, it all depends on what kind of sailing you do, and where you do it.
Suddenly paranoid that the government was on the verge of spying on where and when I anchor, who I'm with, what we might have to drink, and whether we're dressed, I had to Wiki that "selective availability" reference. It appears that our government does not plan to throw off the scores of US consumers who now have, and rely on GPS equipment. In fact, retired satellites are being replaced without SA. Point taken though, Joseph, in that I'd hate to rely on GPS only if I was in unfamiliar shallow water and grounded because my GPS was 100 meters off. As with most arguments on this board, it all depends on what kind of sailing you do, and where you do it.
Just 5 degrees of error can put you 1000 feet off
your mark in 2 miles. If you steer carefully and know your compass error you can still be off by 1/2 that much. But it is nice to know. The currents in long island sound combined with fog and a compass of unknown accuracy can turn you into tomorrows headlines.
your mark in 2 miles. If you steer carefully and know your compass error you can still be off by 1/2 that much. But it is nice to know. The currents in long island sound combined with fog and a compass of unknown accuracy can turn you into tomorrows headlines.
Salty
Thanks BrianD for the link. I have a compass. Got the charts and my Weems and Plath bits-o-platic decoder kit and rely on the deviation information on the charts; so many degrees per year etc. I use GPS. Or not. Depends on the day. I go sailing and enjoy the day safely without making the enterprise a complete fetish. I'm with Barry.
Thanks BrianD for the link. I have a compass. Got the charts and my Weems and Plath bits-o-platic decoder kit and rely on the deviation information on the charts; so many degrees per year etc. I use GPS. Or not. Depends on the day. I go sailing and enjoy the day safely without making the enterprise a complete fetish. I'm with Barry.
Want to see if I understand something
Hello, Perhaps I'm missing something. Are you old salts, (who can navigate by compass, knot log, and chronometer) writing that you can navigate from a known position, to a spot some distance away (lets make it something small like 2 nm), by COMPASS alone (no GPS, no visual confirmation), and arrive within 20' of your destination? If so, my hat is off to you, and you must be one of the best navigators on the planet. You can compensate for tides, currents, wind, waves, leeway, and still steer accurately enough to arrive within 20' of your desired destination? That is simply amazing. Or, when you get close enough, you start sounding with your lead line and tallow, examining the bits of bottom you bring up, to see where you really are. If I did the math correctly, a 1 degree difference in course would put you about 70 yards off course at the end of 2 nm. In my case, without constant feedback (either a visual reference mark or GPS assisted) I find it difficult to stay within 10 degrees of where I *think* I want to go, and that doesn't include outside factors like waves, tide, wind, etc. For me personally, even with a GPS, if I had to be within 20' of somewhere, and I can't see it with my own eyeballs, I'm staying away until I can, or I give up and go somewhere else. I like to sail at night. With GPS I know where the NAVAIDS are, I like to get close to confirm that I'm where I think I am, but not so close that I hit them (that would be embarrassing). Depending on how dark it is, I can be 50 yards away from a buoy, and still not see it. If I were closer than that to a real hazard, and could not see it, that would make me real uncomfortable. You guys are definitely better sailors than I am. Barry
Hello, Perhaps I'm missing something. Are you old salts, (who can navigate by compass, knot log, and chronometer) writing that you can navigate from a known position, to a spot some distance away (lets make it something small like 2 nm), by COMPASS alone (no GPS, no visual confirmation), and arrive within 20' of your destination? If so, my hat is off to you, and you must be one of the best navigators on the planet. You can compensate for tides, currents, wind, waves, leeway, and still steer accurately enough to arrive within 20' of your desired destination? That is simply amazing. Or, when you get close enough, you start sounding with your lead line and tallow, examining the bits of bottom you bring up, to see where you really are. If I did the math correctly, a 1 degree difference in course would put you about 70 yards off course at the end of 2 nm. In my case, without constant feedback (either a visual reference mark or GPS assisted) I find it difficult to stay within 10 degrees of where I *think* I want to go, and that doesn't include outside factors like waves, tide, wind, etc. For me personally, even with a GPS, if I had to be within 20' of somewhere, and I can't see it with my own eyeballs, I'm staying away until I can, or I give up and go somewhere else. I like to sail at night. With GPS I know where the NAVAIDS are, I like to get close to confirm that I'm where I think I am, but not so close that I hit them (that would be embarrassing). Depending on how dark it is, I can be 50 yards away from a buoy, and still not see it. If I were closer than that to a real hazard, and could not see it, that would make me real uncomfortable. You guys are definitely better sailors than I am. Barry
You don't understand how useful your brain is!!!!
I once went 3 nm in fog and cross current and hit the mark dead on. Am I the Uber-navigator? No. But I did note crab pots along the way and kept the compass as centered as I could. 3 degrees on way, 3 degrees the other, gusetimate into the current..... I have done land navigation in the Army for over 20 years and the power of the human brain to intigrate all the varables is truly AMAZING!!! Everyone should try this and see for themselves. Pick a mark on the water about a nm away and head toward it. Note the tide's set and drift and guestimate how much you should crab into it to compensate. When you get to the point note how far and which way your where off. Your brain learned something! The next time you will do better The real key is to try. Without trying you will never know how easy it really is. Also I note that we got along very well without GPS for most of recorded history! How hard can it be?
I once went 3 nm in fog and cross current and hit the mark dead on. Am I the Uber-navigator? No. But I did note crab pots along the way and kept the compass as centered as I could. 3 degrees on way, 3 degrees the other, gusetimate into the current..... I have done land navigation in the Army for over 20 years and the power of the human brain to intigrate all the varables is truly AMAZING!!! Everyone should try this and see for themselves. Pick a mark on the water about a nm away and head toward it. Note the tide's set and drift and guestimate how much you should crab into it to compensate. When you get to the point note how far and which way your where off. Your brain learned something! The next time you will do better The real key is to try. Without trying you will never know how easy it really is. Also I note that we got along very well without GPS for most of recorded history! How hard can it be?
no one..
No one ever said hit the mark within 20'. Here's exactly what I said: " Where I sail 20 feet is the difference between a totaled boat and safe passage and a 10 degree error, at N only, who knows what it is at S,W or E, from three miles out, can put you a long way off your mark and we haven't even considered other factors such as set and drift." My point was 20' can be the difference between rocks and a can or nun and if your confounding that upon a compass that's already known to be way off at N and you don't know your corrections for S,E or W you don't even stand a chance of even getting close, let alone ever hitting the mark. My point was also to include the fact that with your compass off then trying to calculate set and drift you're even more screwed.. Having said that I have hit the mark many times, as Bill has, and sailed long before there were GPS and Loran and yes we still sailed in the fog. I'm certainly not as good as I once was at dead/ded reckoning but I can still do a decent job once I get back into the flow. Unfortunately, GPS has made me less practiced at traditional navigation but still plenty capable as anyone sailing the ocean should be unless you stay in protected bays every time you sail....
No one ever said hit the mark within 20'. Here's exactly what I said: " Where I sail 20 feet is the difference between a totaled boat and safe passage and a 10 degree error, at N only, who knows what it is at S,W or E, from three miles out, can put you a long way off your mark and we haven't even considered other factors such as set and drift." My point was 20' can be the difference between rocks and a can or nun and if your confounding that upon a compass that's already known to be way off at N and you don't know your corrections for S,E or W you don't even stand a chance of even getting close, let alone ever hitting the mark. My point was also to include the fact that with your compass off then trying to calculate set and drift you're even more screwed.. Having said that I have hit the mark many times, as Bill has, and sailed long before there were GPS and Loran and yes we still sailed in the fog. I'm certainly not as good as I once was at dead/ded reckoning but I can still do a decent job once I get back into the flow. Unfortunately, GPS has made me less practiced at traditional navigation but still plenty capable as anyone sailing the ocean should be unless you stay in protected bays every time you sail....
no one..
No one ever said hit the mark within 20'. Here's exactly what I said: " Where I sail 20 feet is the difference between a totaled boat and safe passage and a 10 degree error, at N only, who knows what it is at S,W or E, from three miles out, can put you a long way off your mark and we haven't even considered other factors such as set and drift." My point was 20' can be the difference between rocks and a can or nun and if your confounding that upon a compass that's already known to be way off at N and you don't know your corrections for S,E or W you don't even stand a chance of even getting close, let alone ever hitting the mark. My point was also to include the fact that with your compass off then trying to calculate set and drift you're even more screwed.. Having said that I have hit the mark many times, as Bill has, and sailed long before there were GPS and Loran and yes we still sailed in the fog. I'm certainly not as good as I once was at dead/ded reckoning but I can still do a decent job once I get back into the flow. Unfortunately, GPS has made me less practiced at traditional navigation, though I do force myself to practice a number of times per season, but I'm still quite capable, as anyone sailing the ocean should be, unless you stay in protected bays every time you sail.... Quote Barry L.: "You guys are definitely better sailors than I am." No, we're probably not, just better prepared...;D
No one ever said hit the mark within 20'. Here's exactly what I said: " Where I sail 20 feet is the difference between a totaled boat and safe passage and a 10 degree error, at N only, who knows what it is at S,W or E, from three miles out, can put you a long way off your mark and we haven't even considered other factors such as set and drift." My point was 20' can be the difference between rocks and a can or nun and if your confounding that upon a compass that's already known to be way off at N and you don't know your corrections for S,E or W you don't even stand a chance of even getting close, let alone ever hitting the mark. My point was also to include the fact that with your compass off then trying to calculate set and drift you're even more screwed.. Having said that I have hit the mark many times, as Bill has, and sailed long before there were GPS and Loran and yes we still sailed in the fog. I'm certainly not as good as I once was at dead/ded reckoning but I can still do a decent job once I get back into the flow. Unfortunately, GPS has made me less practiced at traditional navigation, though I do force myself to practice a number of times per season, but I'm still quite capable, as anyone sailing the ocean should be, unless you stay in protected bays every time you sail.... Quote Barry L.: "You guys are definitely better sailors than I am." No, we're probably not, just better prepared...;D
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