To add to Dave's previous question- I have a magnetic compass at the helm, a fluxgate compass for the autohelm, boat mounted GPS (Autohelm) and a handneld GPS. Magnetic compass is one reading, boat GPS and handheld read the same for a second reading, fluxgate is yet a third reading. I can understand the GPS and magentic compass reading different headings, and can live with it. Should/can I adjust the fluxgate to one of the other 2? If so, which one? Should I adjust all 3 to the GPS? Thanks!
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Rich
My Fluxgate Matches GPS
Although I haven't had it calibrated my Fluxgate matches the heading on my GPS pretty consistently. The helm compass varies between dead on and as much as 10 degrees off depending on where I am and what my heading is. I'm assuming the fluxgate was calibrated to match the GPS when it was installed. That would seem to be the most logical choice if you use your autopilot to any degree, especially over long distances. I still check my GPS vs. my Autohelm setting about every 30 minutes or so just to be sure... I'm not sure how to go about calibrating the fluxgate. You may want to call the factory. As for the helm, you might try having it calibrated to match but my guess is what is dead on in one location may be pretty far off 50 miles away from your calibration point.
Although I haven't had it calibrated my Fluxgate matches the heading on my GPS pretty consistently. The helm compass varies between dead on and as much as 10 degrees off depending on where I am and what my heading is. I'm assuming the fluxgate was calibrated to match the GPS when it was installed. That would seem to be the most logical choice if you use your autopilot to any degree, especially over long distances. I still check my GPS vs. my Autohelm setting about every 30 minutes or so just to be sure... I'm not sure how to go about calibrating the fluxgate. You may want to call the factory. As for the helm, you might try having it calibrated to match but my guess is what is dead on in one location may be pretty far off 50 miles away from your calibration point.
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Dan
GPS Heading
Bruce, I would not adjust anything to the GPS heading. The GPS heading is the direction you are going over the ground. You may well be pointing (compass direction) significantly different than that depending upon currents and lee way. The GPS determines your heading by comparing your change in position. I'm not sure which fluxgate compass you have, but I beleive that we swung ours to get it set up. And it does agree with the magnetic compass on some headings and is off on others. You could always use a hand bearing compass and do a deviation chart for each. I'm guessing that our flux gate compass is the most accurate of the magnetic indicators, but except in slack conditions and no wind, I don't expect them to agree with the GPS. Dan Jonas (S/V Feije II)
Bruce, I would not adjust anything to the GPS heading. The GPS heading is the direction you are going over the ground. You may well be pointing (compass direction) significantly different than that depending upon currents and lee way. The GPS determines your heading by comparing your change in position. I'm not sure which fluxgate compass you have, but I beleive that we swung ours to get it set up. And it does agree with the magnetic compass on some headings and is off on others. You could always use a hand bearing compass and do a deviation chart for each. I'm guessing that our flux gate compass is the most accurate of the magnetic indicators, but except in slack conditions and no wind, I don't expect them to agree with the GPS. Dan Jonas (S/V Feije II)
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Dakota Jim Russell
Fluxgate HEADING and GPS Magnetic HEADING Same
Your Fluxgate HEADING and GPS Magnetic HEADING should be reading the same. If not, look for a new location for the fluxgate. This may not always be true depending on local magnetic variation. If you really have a good well adjusted helm compass, that may read close.
Your Fluxgate HEADING and GPS Magnetic HEADING should be reading the same. If not, look for a new location for the fluxgate. This may not always be true depending on local magnetic variation. If you really have a good well adjusted helm compass, that may read close.
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Les Andersen
GPS Hdg
Bruce, The GPS produces an artificial True Track by establishing the ground track between a series of satellite derived fixes and comparing that track against a celestial grid of latitude and longitude contained inside the GPS. It derives an artificial mag hdg by applying a mathematically derived variation based on where you have been, computed from inside the GPS. The satellite doesn't know anything about mag hdg or variation and the GPS doesn't measure mag hdg. When you enter a destination the GPS will compare actual position against your entered point to compute velocity made good, course made good, etc. Each time you look at the correction to course arrow in your GPS you are looking at the correction from your GPS computed actual position to the destination you entered. On some GPS units you can display the variation the unit is using to compute the mag heading and compare it against the chart variation to see the difference. It is not necessarily the best heading to get to the point because it uses derived variation from previous fixes. When you measure the track off a chart and apply variation half way down track and compute a mag heading to the point you will have a much better heading to follow. If you carry a running fix you will find that you maintain a better course using your computed mag heading. But you will not maintain center line because you used an average variation instead of a point variation. Bottom line, the GPS should not be used as a primary source to derive a mag heading. The compass senses the magnetic forces directly and mechanically displays your heading on a compass card. Many things can throw it off but it is generally going to be your best primary source of mag heading if it has been installed properly. A flux gate electronically senses the magnetic heading and displays your heading, generally digitally, but that is not its primary purpose. It is supposed to sense the magnetic heading you are on and establish a baseline when you press the button to "lock" the heading into your auto pilot. It then senses any deviation from the baseline and attempts to "0" out the difference. The auto pilot doesn't really care what your actual heading is just what is the difference. Most flux gates are not properly aligned to serve as a mag compass. Think about your cruise control. The car isn't really trying to hold 55mph but trying to maintain a zero difference between the actual speedometer reading and the 55mph that was set. If you go slower it speeds up. If you go faster it slows down. It is constantly checking and correcting but it is seldom actually doing 55mph. Tell that to the trooper. If you were going to align anything, and assuming your helm compass was correct, then you could set your flux gate to the compass. The hand helds are not going to be as accurate simply because they generally contain a smaller card than the helm and allow too much movement to use as an accurate setting source. To accurately set your compass wait until your boat is pulled and do it on dry land with no movement. It will be much more accurate. But when you are not moving your GPS will not give you a good heading because there is no difference between fix points. Just my 2 bits worth. Les s/v Mutual Fun
Bruce, The GPS produces an artificial True Track by establishing the ground track between a series of satellite derived fixes and comparing that track against a celestial grid of latitude and longitude contained inside the GPS. It derives an artificial mag hdg by applying a mathematically derived variation based on where you have been, computed from inside the GPS. The satellite doesn't know anything about mag hdg or variation and the GPS doesn't measure mag hdg. When you enter a destination the GPS will compare actual position against your entered point to compute velocity made good, course made good, etc. Each time you look at the correction to course arrow in your GPS you are looking at the correction from your GPS computed actual position to the destination you entered. On some GPS units you can display the variation the unit is using to compute the mag heading and compare it against the chart variation to see the difference. It is not necessarily the best heading to get to the point because it uses derived variation from previous fixes. When you measure the track off a chart and apply variation half way down track and compute a mag heading to the point you will have a much better heading to follow. If you carry a running fix you will find that you maintain a better course using your computed mag heading. But you will not maintain center line because you used an average variation instead of a point variation. Bottom line, the GPS should not be used as a primary source to derive a mag heading. The compass senses the magnetic forces directly and mechanically displays your heading on a compass card. Many things can throw it off but it is generally going to be your best primary source of mag heading if it has been installed properly. A flux gate electronically senses the magnetic heading and displays your heading, generally digitally, but that is not its primary purpose. It is supposed to sense the magnetic heading you are on and establish a baseline when you press the button to "lock" the heading into your auto pilot. It then senses any deviation from the baseline and attempts to "0" out the difference. The auto pilot doesn't really care what your actual heading is just what is the difference. Most flux gates are not properly aligned to serve as a mag compass. Think about your cruise control. The car isn't really trying to hold 55mph but trying to maintain a zero difference between the actual speedometer reading and the 55mph that was set. If you go slower it speeds up. If you go faster it slows down. It is constantly checking and correcting but it is seldom actually doing 55mph. Tell that to the trooper. If you were going to align anything, and assuming your helm compass was correct, then you could set your flux gate to the compass. The hand helds are not going to be as accurate simply because they generally contain a smaller card than the helm and allow too much movement to use as an accurate setting source. To accurately set your compass wait until your boat is pulled and do it on dry land with no movement. It will be much more accurate. But when you are not moving your GPS will not give you a good heading because there is no difference between fix points. Just my 2 bits worth. Les s/v Mutual Fun
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Bruce Hill
Les...A follow up
Thanks all for the good answers. Les- wanted to follow up on yours. To summarize (using the cruise control example) I should throw the fluxgate out of the eqation...it doesn't matter since its only purpose is to drive the autohelm to 0 variance. OK, but since the reading on the fluxgate is irrelevant, if the difference bothers me I could adjust it to the magnetic compass (or just leave it alone). Net is, it's not a nvigational tool, it's an autohelm control. Did I get it right?
Thanks all for the good answers. Les- wanted to follow up on yours. To summarize (using the cruise control example) I should throw the fluxgate out of the eqation...it doesn't matter since its only purpose is to drive the autohelm to 0 variance. OK, but since the reading on the fluxgate is irrelevant, if the difference bothers me I could adjust it to the magnetic compass (or just leave it alone). Net is, it's not a nvigational tool, it's an autohelm control. Did I get it right?
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Daniel Jonas
Handheld
Les, Just need to take one exception to your well thought out explanation. Unless the mgnetic compass has been swung professionally and a deviation card is already plotted, I would use the hand bearing compass as the tool for preparing a deviation card. In fact, Imight do that just to prove the deviation card was right. It is true that they have smaller cards, but I was taught that you don't trust an installed magnetic compass until you swing it against a handbearing compass. I've completed this process on several boats that we have sailed offshore (out of sight of land) and am surprised sometimes how far off the magnetic compass is. For what it is worth. How do you swing a boat on the hard? I agree that would be better, but on the lift it has a lot of iron around the boat, and most yards don't set them down on wheels. And if you did swing it, what are you using for cardinal headings (handbearing compass?). Just curious. Dan Jonas (S/V Feije II)
Les, Just need to take one exception to your well thought out explanation. Unless the mgnetic compass has been swung professionally and a deviation card is already plotted, I would use the hand bearing compass as the tool for preparing a deviation card. In fact, Imight do that just to prove the deviation card was right. It is true that they have smaller cards, but I was taught that you don't trust an installed magnetic compass until you swing it against a handbearing compass. I've completed this process on several boats that we have sailed offshore (out of sight of land) and am surprised sometimes how far off the magnetic compass is. For what it is worth. How do you swing a boat on the hard? I agree that would be better, but on the lift it has a lot of iron around the boat, and most yards don't set them down on wheels. And if you did swing it, what are you using for cardinal headings (handbearing compass?). Just curious. Dan Jonas (S/V Feije II)
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Dakota Jim Russell
Les ??
Not sure I understand what you said when you stated: It derives an artificial mag hdg by applying a mathematically derived variation based on where you have been, computed from inside the GPS. The satellite doesn't know anything about mag hdg or variation and the GPS doesn't measure mag hdg. The Garminmap 76 and similar models have a built in adjustment to allow the gps to provide a magnetic heading as well as a true heading. This has nothing to do where one has been. It is as accurate as the magnetic variation provided on charts. Thus it would appear that using true headings to compare with a chart would be more accurate, while comparing GPS mag headings to the various compasses on board would provide a reasonable check on their accuracy. Am I missing something?
Not sure I understand what you said when you stated: It derives an artificial mag hdg by applying a mathematically derived variation based on where you have been, computed from inside the GPS. The satellite doesn't know anything about mag hdg or variation and the GPS doesn't measure mag hdg. The Garminmap 76 and similar models have a built in adjustment to allow the gps to provide a magnetic heading as well as a true heading. This has nothing to do where one has been. It is as accurate as the magnetic variation provided on charts. Thus it would appear that using true headings to compare with a chart would be more accurate, while comparing GPS mag headings to the various compasses on board would provide a reasonable check on their accuracy. Am I missing something?
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Les Andersen
Headings
The directions on swinging/compensating a compass on the water are available at www.ritchienavigation.com. They are easier said than done. I have used the procedure and cross checked the result the next winter when the boat was hauled and placed on stands. I got behind the boat about 100 feet and used a spotting scope to ensure I was lined up with the mast and backstay. I used a hand bearing to measure the heading from where I was standing to the mast. I read the helm compass and would have adjusted the compass by rotating it if there had been a difference but there wasn’t. Part of the problem is the movement on the water in taking a reading along with the inherent wobble in the small hand held compass. But if you can support the compass on something that won’t move and measure something that is not moving, the readings are going to be much more accurate. Measuring a heading with a hand bearing compass requires you to exactly stand in the center towards the stern and sight through the forestay. Generally and hopefully at sea the mast will be in the way. You can try to fudge and guestimate the reading but along with the hand compass movements that will cause even more error. Ideally you could stand behind the boat and sight through the backstay, mast, and forestay to ensure you are looking down the centerline of the boat but that is not possible. Hand bearing compasses are used to measure mag bearings from the boat to a visible object. I would not use them for a heading unless I was very desperate. There are 2 procedures that must be completed for an accurate compass. 1 is ensuring the compass rose is aligned to the centerline of the boat. You fix that by rotating the compass body but you must be able to measure an accurate heading to set. 2 ensure the compass reading is accurate by swinging the compass and adjust the ½ differences out on the cardinal headings if possible but at least 180 and 90 degrees different. Each of these takes care of a different set of errors. You can read more about all this on www.irbs.com/bowditch/. It has been the authority for some 200 years. As for using your fluxgate as a compass I would make sure I aligned the gate with an accurate helm compass before I placed any/much reliance on it. If you can get a reading off of the fluxgate and it is accurate then it is a navigation instrument. If it is not, and it doesn’t need to be to steer a boat, I would not use it. That being said it should be relatively easy to rotate the fluxgate to match the helm. Swinging your fluxgate may not be possible. Not sure how it is compensated. However there are helm compasses that use a fluxgate and that can be compensated and adjusted. I just don’t know if the autopilot gate is one. Another 2 bits worth, sorry for the ramble. Les S/v Mutual Fun
The directions on swinging/compensating a compass on the water are available at www.ritchienavigation.com. They are easier said than done. I have used the procedure and cross checked the result the next winter when the boat was hauled and placed on stands. I got behind the boat about 100 feet and used a spotting scope to ensure I was lined up with the mast and backstay. I used a hand bearing to measure the heading from where I was standing to the mast. I read the helm compass and would have adjusted the compass by rotating it if there had been a difference but there wasn’t. Part of the problem is the movement on the water in taking a reading along with the inherent wobble in the small hand held compass. But if you can support the compass on something that won’t move and measure something that is not moving, the readings are going to be much more accurate. Measuring a heading with a hand bearing compass requires you to exactly stand in the center towards the stern and sight through the forestay. Generally and hopefully at sea the mast will be in the way. You can try to fudge and guestimate the reading but along with the hand compass movements that will cause even more error. Ideally you could stand behind the boat and sight through the backstay, mast, and forestay to ensure you are looking down the centerline of the boat but that is not possible. Hand bearing compasses are used to measure mag bearings from the boat to a visible object. I would not use them for a heading unless I was very desperate. There are 2 procedures that must be completed for an accurate compass. 1 is ensuring the compass rose is aligned to the centerline of the boat. You fix that by rotating the compass body but you must be able to measure an accurate heading to set. 2 ensure the compass reading is accurate by swinging the compass and adjust the ½ differences out on the cardinal headings if possible but at least 180 and 90 degrees different. Each of these takes care of a different set of errors. You can read more about all this on www.irbs.com/bowditch/. It has been the authority for some 200 years. As for using your fluxgate as a compass I would make sure I aligned the gate with an accurate helm compass before I placed any/much reliance on it. If you can get a reading off of the fluxgate and it is accurate then it is a navigation instrument. If it is not, and it doesn’t need to be to steer a boat, I would not use it. That being said it should be relatively easy to rotate the fluxgate to match the helm. Swinging your fluxgate may not be possible. Not sure how it is compensated. However there are helm compasses that use a fluxgate and that can be compensated and adjusted. I just don’t know if the autopilot gate is one. Another 2 bits worth, sorry for the ramble. Les S/v Mutual Fun
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Les Andersen
Jim?
Jim, The satellites provide lines of position that are used by your GPS Receiver to compute your present position. They essentially plot this position in their memory and keep on plotting positions. Depending on the model they can use more satellites (4 necessary to produce an accurate fix, 2 necessary for any fix) and plot fixes more frequently (some every 6 seconds). To determine your true track or course they compute the track between a last series of fixes. The series depends on the model and determines the accuracy of the track. Just like when you plot 2 fixes and measure the track off a chart. To turn the true track into a mag heading, the receiver applies the mathematically derived variation from an internal table (like your calculator computing roots and squares) using the current present position, but the track was based on previous fixes. That’s why I said it uses past data. That’s why it depends on where you have been ‘cus that’s what determines the track it uses. That’s also why if you turn on the GPS and set it down on the ground you will find that the heading will be very inaccurate. The GPS will plot fixes and measure the course between them but since there is no movement, all it can sense are the normal built in system errors. The heading will bounce around or may not even be displayed. Once you pick up the unit and walk around it will sense movement and begin displaying better track data. That’s also why it doesn’t matter how you orientate the unit. As long as the antenna can receive, the unit will display accurate position data and compute headings based on track whether you point the top forward of backwards. It doesn’t measure heading but computes it. If you hold a hand held compass backwards or turn the helm compass out of alignment they will measure a bad heading because they measure the heading based on their reference. To get a mag heading from a true track you have to apply drift to the true track to get true heading. Then apply actual spot position variation to produce an actual mag heading or a mid point variation ½ way to your next destination to produce a corrected mag heading to get to the destination. Since the receiver is using true track and applying spot variation and calling it mag heading it is basically assuming that there is no drift. Since we are in sail boats where we crab into the wind and worry about cross course corrections when we navigate, using this system to produce a mag heading that assumes there is no drift makes no sense. A GPS receiver is a great way to find your way to a destination with great accuracy, however it uses some short cuts when determining the true heading and mag heading it displays. If you use it as a heading indicator it will generally be in conflict with your corrected compass and create confusion. The greater the distance to destination and the greater the drift, the greater the confusion. That’s why I wouldn’t use it is a primary source of heading. Certainly it can be used as a reference but I would tend to rely on a corrected compass to measure my mag heading not something that computes it. If I computed my mag heading to a point from a chart and entered the point coordinates into my GPS and compared the GPS “course to destination” against my computed mag heading I would expect them to be very close but I would not expect them to be exactly the same. If you use GPS to determine the course to a point, i.e. follow the arrow then it will get you there. But it doesn’t look forward to the destination to factor in changes in variation and drift. It takes a fix, compares that fix to your destination, determines the delta, points the arrow at the delta, and does it all again. With any amount of drift and variation you will sail a curved path to destination. That is the long way to go. The short way is to factor in the drift, account for the mid point variation and compute a corrected heading to drive you straight to the point. Sorry for the length, I’ll be quiet, Les S/v Mutual Fun
Jim, The satellites provide lines of position that are used by your GPS Receiver to compute your present position. They essentially plot this position in their memory and keep on plotting positions. Depending on the model they can use more satellites (4 necessary to produce an accurate fix, 2 necessary for any fix) and plot fixes more frequently (some every 6 seconds). To determine your true track or course they compute the track between a last series of fixes. The series depends on the model and determines the accuracy of the track. Just like when you plot 2 fixes and measure the track off a chart. To turn the true track into a mag heading, the receiver applies the mathematically derived variation from an internal table (like your calculator computing roots and squares) using the current present position, but the track was based on previous fixes. That’s why I said it uses past data. That’s why it depends on where you have been ‘cus that’s what determines the track it uses. That’s also why if you turn on the GPS and set it down on the ground you will find that the heading will be very inaccurate. The GPS will plot fixes and measure the course between them but since there is no movement, all it can sense are the normal built in system errors. The heading will bounce around or may not even be displayed. Once you pick up the unit and walk around it will sense movement and begin displaying better track data. That’s also why it doesn’t matter how you orientate the unit. As long as the antenna can receive, the unit will display accurate position data and compute headings based on track whether you point the top forward of backwards. It doesn’t measure heading but computes it. If you hold a hand held compass backwards or turn the helm compass out of alignment they will measure a bad heading because they measure the heading based on their reference. To get a mag heading from a true track you have to apply drift to the true track to get true heading. Then apply actual spot position variation to produce an actual mag heading or a mid point variation ½ way to your next destination to produce a corrected mag heading to get to the destination. Since the receiver is using true track and applying spot variation and calling it mag heading it is basically assuming that there is no drift. Since we are in sail boats where we crab into the wind and worry about cross course corrections when we navigate, using this system to produce a mag heading that assumes there is no drift makes no sense. A GPS receiver is a great way to find your way to a destination with great accuracy, however it uses some short cuts when determining the true heading and mag heading it displays. If you use it as a heading indicator it will generally be in conflict with your corrected compass and create confusion. The greater the distance to destination and the greater the drift, the greater the confusion. That’s why I wouldn’t use it is a primary source of heading. Certainly it can be used as a reference but I would tend to rely on a corrected compass to measure my mag heading not something that computes it. If I computed my mag heading to a point from a chart and entered the point coordinates into my GPS and compared the GPS “course to destination” against my computed mag heading I would expect them to be very close but I would not expect them to be exactly the same. If you use GPS to determine the course to a point, i.e. follow the arrow then it will get you there. But it doesn’t look forward to the destination to factor in changes in variation and drift. It takes a fix, compares that fix to your destination, determines the delta, points the arrow at the delta, and does it all again. With any amount of drift and variation you will sail a curved path to destination. That is the long way to go. The short way is to factor in the drift, account for the mid point variation and compute a corrected heading to drive you straight to the point. Sorry for the length, I’ll be quiet, Les S/v Mutual Fun
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Daktoa Jim Russell
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