Setting up a new GPS/Chartplotter and began to wonder if I should select "True" or "magnetic" for the display of course/bearing information from the GPS. I guess I'd also have to enter a "deviation" value into the GPS. My heart leans toward "True", but I'm starting to think about it and you know what happens when that starts. Of course the compass reads magnetic, the Autopilot displays magnetic and I'm just wondering if the resulting confusion between the two is just going to be my little issue, or is there more to it that I should think about?
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dj
Your choice, but
Your GPS may automatically calculate the variation from your location. I would not think that you would make a correction for deviation which is the compass error caused by boat metal. (source Chapman) Sorry, had to say that <grin> I keep my gps on true to align with the charts, and mentally correct the compass reading. I'll be curious to see what others say.
Your GPS may automatically calculate the variation from your location. I would not think that you would make a correction for deviation which is the compass error caused by boat metal. (source Chapman) Sorry, had to say that <grin> I keep my gps on true to align with the charts, and mentally correct the compass reading. I'll be curious to see what others say.
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Jose Venegas
Magnetic on board, true at home
Eddie, I keep the cockpit chart plotter/GPS display in magnetic which it can periodically check against the compass. Also, I like to orient the chart display with the course up, since it lets me compare the plotter it with the location of naviation aids and topography relative to the boat. On my laptop I use Map Source (Garmin charting program) to generate routes and waypoints that I transfer to my GPS units. Map Source orients the charts with true North up making it easy to compare them agianst paper charts. Uless you have both the auto pilot and GPS of the same brand, I believe that for proper interface with the auto-pilot, the GPS needs to be reporting magnetic.
Eddie, I keep the cockpit chart plotter/GPS display in magnetic which it can periodically check against the compass. Also, I like to orient the chart display with the course up, since it lets me compare the plotter it with the location of naviation aids and topography relative to the boat. On my laptop I use Map Source (Garmin charting program) to generate routes and waypoints that I transfer to my GPS units. Map Source orients the charts with true North up making it easy to compare them agianst paper charts. Uless you have both the auto pilot and GPS of the same brand, I believe that for proper interface with the auto-pilot, the GPS needs to be reporting magnetic.
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Tim
I agree with DJ on one thing...
Eddie, What DJ says is correct. VARiation and DEViation are two different things. A lot of people confuse the two. I, however, like to have the GPS set up to read magnetic courses, because that is what the compass reads, i.e., GPS says "course to x 045", so you steer 045 without having to do the mental math. If you think about how most people use a GPS, that is, enter the waypoint and select go-to, rather than plotting a course on a paper chart and then making the calculations, it makes it a little easier. Just remember if you do plot a course first, make the appropriate addition/subtraction from true before steering it by compass. Just my .02! Tim
Eddie, What DJ says is correct. VARiation and DEViation are two different things. A lot of people confuse the two. I, however, like to have the GPS set up to read magnetic courses, because that is what the compass reads, i.e., GPS says "course to x 045", so you steer 045 without having to do the mental math. If you think about how most people use a GPS, that is, enter the waypoint and select go-to, rather than plotting a course on a paper chart and then making the calculations, it makes it a little easier. Just remember if you do plot a course first, make the appropriate addition/subtraction from true before steering it by compass. Just my .02! Tim
Question for Tim
Your approach has one potential flaw; it doesn't appear to take sideslip/leeway or set from current into consideration. Under those conditions, the GPS bearing to mark (based on the measured track over ground) and the compass heading (the direction in which the bow of the boat is pointing) will not be the same. The safest thing to do would be to use the GPS "go-to" feature and follow the display, with periodic glances at the compass heading and landmarks on the horizon (if visible) just in case the GPS craps out. Peter H23 "Raven"
Your approach has one potential flaw; it doesn't appear to take sideslip/leeway or set from current into consideration. Under those conditions, the GPS bearing to mark (based on the measured track over ground) and the compass heading (the direction in which the bow of the boat is pointing) will not be the same. The safest thing to do would be to use the GPS "go-to" feature and follow the display, with periodic glances at the compass heading and landmarks on the horizon (if visible) just in case the GPS craps out. Peter H23 "Raven"
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Roger
GPS/COmpass and Piloting
I learned the hard way(almost) to keep an eye on the compass and not just follow the GPS arrow. I had plotted the course in advance by the chart (magnetic) but I use a small hand-held GPS to 'point the way'. The wind picked up, visibility closed down to about a mile. My next waypoint was a green can marking the south side of the channel in open water. When I finally spotted it, I found that over the four or five nm leg I was maybe a quarter mile off course to the south. I could see water breaking over shoals ahead! Since then I have learned to set the compass course at each waypoint, and, while still using the GPS, keep an eye on the compass to make sure that the heading is not changing, marking drift off course. (I also learned to be less cavalier about shoals in apparently open water, and to write myself lots of notes on each course leg info sheet that I prepare before getting under way)
I learned the hard way(almost) to keep an eye on the compass and not just follow the GPS arrow. I had plotted the course in advance by the chart (magnetic) but I use a small hand-held GPS to 'point the way'. The wind picked up, visibility closed down to about a mile. My next waypoint was a green can marking the south side of the channel in open water. When I finally spotted it, I found that over the four or five nm leg I was maybe a quarter mile off course to the south. I could see water breaking over shoals ahead! Since then I have learned to set the compass course at each waypoint, and, while still using the GPS, keep an eye on the compass to make sure that the heading is not changing, marking drift off course. (I also learned to be less cavalier about shoals in apparently open water, and to write myself lots of notes on each course leg info sheet that I prepare before getting under way)
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Tim
Good point Peter
Peter, You are correct, sir! I forgot about leeway's effect on the whole process. Of course, the farther 'off the wind' you are, the less leeway makes an effect, and for motoring when the wind dies the compass should agree with GPS (absent any current). I'd be willing to bet a lot of people don't even use compasses anymore, other than for initial direction. Once underway, I'll bet most people "steer" by GPS! Tim
Peter, You are correct, sir! I forgot about leeway's effect on the whole process. Of course, the farther 'off the wind' you are, the less leeway makes an effect, and for motoring when the wind dies the compass should agree with GPS (absent any current). I'd be willing to bet a lot of people don't even use compasses anymore, other than for initial direction. Once underway, I'll bet most people "steer" by GPS! Tim
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Brian
navigation
I have done some magnetic compass adjusting, and agree with anyone who says your compass is the most important tool to use. I have read some key words in other replies and want to point out that deviation and variation are not the same thing. Variation is read from the chart, and tells you how much magnetic north has shifted since the chart was published. It is usually calculated anually by minutes (not full degrees) as there is only a small shift per year. You use this info to plot your course no matter what navigational tool you use. Deviation is unique to each boat and compass and has nothing to do with charts physically. A compass can only be adjusted on the cardinal points N, S, E, & W, with it's internal compensation magnets. All points in between can be off a couple of degrees due to the boats set-up. Insturments, speakers, and anything metal around the compass can cause interference at these points. When you have your compass adjusted, the adjuster should give you a deviation table telling you what your actual magnetic heading is in relation to your compass. For example, if you steer 45 deg. on your compass, you may actually be heading 43 deg. There typically isn't much difference, but keep in mind that for every 60 miles (statute) you travel, a one degree error in course puts you one mile off. If I still have anyone's attention, I will get to the GPS. Keep in mind that a compass tells you what direction the boat is pointing, and a GPS tells you what the direction the boat is moving in. When you are crabbing due to strong current, they won't read the same. I don't set any special info on my GPS, and use it as a back up. I have it set on magnetic, and steer the course I plotted on the chart. I check that the GPS track matches what my compass should be reading (including deviation if necessary, and considering the wind and tide), then steer by the compass. Every couple hours, a change in watch is a good time, I use the GPS to check my position on the chart by Lat. and Long. to make sure I am still on course. I hope this is helpful and my wording hasn't confused anyone!
I have done some magnetic compass adjusting, and agree with anyone who says your compass is the most important tool to use. I have read some key words in other replies and want to point out that deviation and variation are not the same thing. Variation is read from the chart, and tells you how much magnetic north has shifted since the chart was published. It is usually calculated anually by minutes (not full degrees) as there is only a small shift per year. You use this info to plot your course no matter what navigational tool you use. Deviation is unique to each boat and compass and has nothing to do with charts physically. A compass can only be adjusted on the cardinal points N, S, E, & W, with it's internal compensation magnets. All points in between can be off a couple of degrees due to the boats set-up. Insturments, speakers, and anything metal around the compass can cause interference at these points. When you have your compass adjusted, the adjuster should give you a deviation table telling you what your actual magnetic heading is in relation to your compass. For example, if you steer 45 deg. on your compass, you may actually be heading 43 deg. There typically isn't much difference, but keep in mind that for every 60 miles (statute) you travel, a one degree error in course puts you one mile off. If I still have anyone's attention, I will get to the GPS. Keep in mind that a compass tells you what direction the boat is pointing, and a GPS tells you what the direction the boat is moving in. When you are crabbing due to strong current, they won't read the same. I don't set any special info on my GPS, and use it as a back up. I have it set on magnetic, and steer the course I plotted on the chart. I check that the GPS track matches what my compass should be reading (including deviation if necessary, and considering the wind and tide), then steer by the compass. Every couple hours, a change in watch is a good time, I use the GPS to check my position on the chart by Lat. and Long. to make sure I am still on course. I hope this is helpful and my wording hasn't confused anyone!
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Daniel Jonas
Original Question
The original question was whether to set your GPS to magnetic or true. I agree with those who picked magnetic, to "match" the compass. It makes sense to eliminate the need while "Navigating" to do the conversion as an additional check. Much of the more recent conversation has really been about navigation. Setting out to a GPS waypoint, or any GPS destination, by setting a direct course without compensation for leeway, current, etc. may cause your route (or course over ground) to form an arc. You will eventually reach the GPS waypoint, but you will be changing the heading to compensate for aforementioned drift. This arc may take you into dangerous waters. The original question was simple. As some have pointed out, simply aiming at a heading does not produce true navigation. One responder discussed watching the heading move as drift entered the navigational picture. One way to keep yourself out of troubled waters is to fully understand the concept of danger bearings. Although these are taught mostly in visual situations, there is no reason that one can not substitute the use of radar or GPS bearing to a known point as a check on position in much the same way a danger bearing is used visually. GPS, the compass, radar, charts, etc. are all tools. If you have a chartplotter with your GPS, life can get easier for the uninitiated, but when that isn't available (they do break), you need to know how the whole system actually works. I wonder how many know the fog signals for that little air horn, but haven't a clue how to ded reckon there way to safety. I sail on San Francisco Bay and outside the gate. Fog can move into the bay very quickly, and if you have not been caught in it, you don't sail much here. That's why my boat has radar. The currents here can have you pointing significantly off your destination while making direct progress over the ground towards it (or the reverse if you are doing it wrong). That's not why I have a GPS, that's why I think it is important to actually understand the concepts of navigation. So set your GPS to magnetic. Then turn it off and learn how to navigate with your compass, a clock, the knotmeter and depth finder (or combinations thereof). You will be glad you did. Dan Jonas (S/V Feije II)
The original question was whether to set your GPS to magnetic or true. I agree with those who picked magnetic, to "match" the compass. It makes sense to eliminate the need while "Navigating" to do the conversion as an additional check. Much of the more recent conversation has really been about navigation. Setting out to a GPS waypoint, or any GPS destination, by setting a direct course without compensation for leeway, current, etc. may cause your route (or course over ground) to form an arc. You will eventually reach the GPS waypoint, but you will be changing the heading to compensate for aforementioned drift. This arc may take you into dangerous waters. The original question was simple. As some have pointed out, simply aiming at a heading does not produce true navigation. One responder discussed watching the heading move as drift entered the navigational picture. One way to keep yourself out of troubled waters is to fully understand the concept of danger bearings. Although these are taught mostly in visual situations, there is no reason that one can not substitute the use of radar or GPS bearing to a known point as a check on position in much the same way a danger bearing is used visually. GPS, the compass, radar, charts, etc. are all tools. If you have a chartplotter with your GPS, life can get easier for the uninitiated, but when that isn't available (they do break), you need to know how the whole system actually works. I wonder how many know the fog signals for that little air horn, but haven't a clue how to ded reckon there way to safety. I sail on San Francisco Bay and outside the gate. Fog can move into the bay very quickly, and if you have not been caught in it, you don't sail much here. That's why my boat has radar. The currents here can have you pointing significantly off your destination while making direct progress over the ground towards it (or the reverse if you are doing it wrong). That's not why I have a GPS, that's why I think it is important to actually understand the concepts of navigation. So set your GPS to magnetic. Then turn it off and learn how to navigate with your compass, a clock, the knotmeter and depth finder (or combinations thereof). You will be glad you did. Dan Jonas (S/V Feije II)
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Brian
Time to learn...
GPS's are wonderful things to check your location. If you set up your GPS to guide you to your destination, what happens if you blow a fuse, or the power goes out? I use my GPS as a back up to plot postion and a chart, compass and tools to to plan my journey. I check my dead reckoning against the GPS and make the neccesary corrections. That way it doesn't matter if it's magnetic or true. And when things go bad, as electronics seem to do when you most need them, you won't be stuck.
GPS's are wonderful things to check your location. If you set up your GPS to guide you to your destination, what happens if you blow a fuse, or the power goes out? I use my GPS as a back up to plot postion and a chart, compass and tools to to plan my journey. I check my dead reckoning against the GPS and make the neccesary corrections. That way it doesn't matter if it's magnetic or true. And when things go bad, as electronics seem to do when you most need them, you won't be stuck.
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Todd Osborne (Windwalker)
Bearing vs Course/Heading?
So am I assuming the terminology correctly that a GPS gives the course/heading (physical direction of travel) and a compass gives a bearing (physical direction the boat is pointed)? Both not necessarily being the same. If this is true, then set & drift are built into the GPS course data and not into the compass?
So am I assuming the terminology correctly that a GPS gives the course/heading (physical direction of travel) and a compass gives a bearing (physical direction the boat is pointed)? Both not necessarily being the same. If this is true, then set & drift are built into the GPS course data and not into the compass?
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d j
GPS works at night for me.
I enjoy my gps the most (two on board at any time) on pitch black nights navigating up an unmarked river to my docks. But since there are virtually no lights along the river, and no navigational lights or aids, you learn to follow your gps route with the same confidence a pilot brings a plane in on instruments. Works for me . . .but then I keep two on board at all times.
I enjoy my gps the most (two on board at any time) on pitch black nights navigating up an unmarked river to my docks. But since there are virtually no lights along the river, and no navigational lights or aids, you learn to follow your gps route with the same confidence a pilot brings a plane in on instruments. Works for me . . .but then I keep two on board at all times.
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Dan
Set & Drift
Todd, The GPS does not compensate for set and drift exactly (at least I have a problem with that exact language). The GPS simply compares a progress of positions and relates a course from that calculation. It turns out that that result does reflect the direction you are actually going (which you can also arrive at by calculating set and drift in a more conventiaonl navigational solution). It's the word "compensates" I have concern over. You can't use the GPS heading to steer by on the compass because that eliminates any compensation. Its a fine line here and I can understand the usage as long as we agree that compensate in this usage relates only to the GPS course. The relevance of which does not provide you with a heading to steer by unless you "compensate" for set, drift, and perhaps a few other things depending on if the GPS is set for true or magnetic and if it automatically adjusts for variation. If you have a chartplotter, things get easy, as I said before. The thing about using GPS headings to input to the compass is that if you don't understand how the tools work, and you aren't watching your progress on a chartplotter, your course may become an arc. Let's say you start out with a GPS course to the waypoint of 300 degrees. You steer that on the compass. After a little while you notice that the GPS says the bearing to the waypoint is 310 degress, so now you steer that. Finally, you notice that the GPS says the bearing to the waypoint is 325 degress so you steer that and actually get there. If you had properly compensated for set and drift initially (by steering something between 300 and 325 degress (what depends on the navigational math) you would have sailed a direct course between the primary position and the waypoint rather than an arc (all other issues being equal). Nothing wrong with an arc, just make sure that the arc stays out of any danger areas. Comments I made in an earlier post on danger bearings are relevant. I wish that navigation issues were easier, but frankly they take a bit of study and practical application. Again, if you have an operational chartplotter, it gets easy. As one poster said, just like flying an approach (although not every aircraft has a moving map, so there are some differences). What if the chartplotter goes out (both) and you are on the unlighted river. Ded reckoning and a depth gauge would probably be pretty handy tools to understand. My order of preference for navigational tools puts the radar above the GPS chartplotter. Just my thoughts. Dan Jonas (S/V Feije II)
Todd, The GPS does not compensate for set and drift exactly (at least I have a problem with that exact language). The GPS simply compares a progress of positions and relates a course from that calculation. It turns out that that result does reflect the direction you are actually going (which you can also arrive at by calculating set and drift in a more conventiaonl navigational solution). It's the word "compensates" I have concern over. You can't use the GPS heading to steer by on the compass because that eliminates any compensation. Its a fine line here and I can understand the usage as long as we agree that compensate in this usage relates only to the GPS course. The relevance of which does not provide you with a heading to steer by unless you "compensate" for set, drift, and perhaps a few other things depending on if the GPS is set for true or magnetic and if it automatically adjusts for variation. If you have a chartplotter, things get easy, as I said before. The thing about using GPS headings to input to the compass is that if you don't understand how the tools work, and you aren't watching your progress on a chartplotter, your course may become an arc. Let's say you start out with a GPS course to the waypoint of 300 degrees. You steer that on the compass. After a little while you notice that the GPS says the bearing to the waypoint is 310 degress, so now you steer that. Finally, you notice that the GPS says the bearing to the waypoint is 325 degress so you steer that and actually get there. If you had properly compensated for set and drift initially (by steering something between 300 and 325 degress (what depends on the navigational math) you would have sailed a direct course between the primary position and the waypoint rather than an arc (all other issues being equal). Nothing wrong with an arc, just make sure that the arc stays out of any danger areas. Comments I made in an earlier post on danger bearings are relevant. I wish that navigation issues were easier, but frankly they take a bit of study and practical application. Again, if you have an operational chartplotter, it gets easy. As one poster said, just like flying an approach (although not every aircraft has a moving map, so there are some differences). What if the chartplotter goes out (both) and you are on the unlighted river. Ded reckoning and a depth gauge would probably be pretty handy tools to understand. My order of preference for navigational tools puts the radar above the GPS chartplotter. Just my thoughts. Dan Jonas (S/V Feije II)
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Vic
I've read this string and notice one thing not ...
talked about ... cross track error and its correction. We are making a lot of noise about something that is already programmed into most GPSs. After a short stretch you can pretty quickly figure out a corrected setting and stay solid on your course line. Or am I missing something about an obsession to need to use a GPS like a magnetic compass like on the tall ships? I use a Garmin 162 and a 48 as backup [if there should ever be a problem with the 162]with the chart marked as often as needed to know where we are and that the GPS is doing it's job. The biggest problem using a GPS is inputing a waypoint wrong or forgetting to put one in... or accidently using a wrong setting that you are not aware of its influence, or overshooting a waypoint. [operator error]. Vic
talked about ... cross track error and its correction. We are making a lot of noise about something that is already programmed into most GPSs. After a short stretch you can pretty quickly figure out a corrected setting and stay solid on your course line. Or am I missing something about an obsession to need to use a GPS like a magnetic compass like on the tall ships? I use a Garmin 162 and a 48 as backup [if there should ever be a problem with the 162]with the chart marked as often as needed to know where we are and that the GPS is doing it's job. The biggest problem using a GPS is inputing a waypoint wrong or forgetting to put one in... or accidently using a wrong setting that you are not aware of its influence, or overshooting a waypoint. [operator error]. Vic
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Daniel Jonas
Experience
Vic, I agree with you. Most modern GPS's include a chartplotter, even if rudimentary and cross track error anunciation is common. Using it properly and knowing how to program waypoints can make for easy navigation work. You referenced putting your GPS position on a chart as often as necessary to make sure it is doing the job. Thats more than most do I suspect and supports my point of actually navigating rather than watching it move by on the screen. I suspect that you have a pretty good idea how it actually works (navigation and maximum utilization of the tool). I have dual Raytheon chartplotters on my boat, but seldom plug in a waypoint. I suspect that few sailors do, because, unless the wind is pretty cooperative, you probably aren't going there directly anyway. On San Francisco Bay, even if the wind is cooperating, something else will manage to be in the way anyway, be it a bridge abutement, an island or shallow area, or just another vessel constrained by draft. Sometimes the currents alone are enough to make more than a few tacks necessary just to get somewhere. Its more likely that I'll steer by compass and use the GPS to keep track of position and maybe even to setup a set and drift correction by comparing the GPS heading with the compass over time. Although the cross track and other features are useful...I think if someone is asking if the GPS should be set to true north or magnetic north, they might want to get a better understanding of navigational basics before really focusing in on full dependence upon the gee whiz stuff in a GPS. With a little bit of aviation instrument training and experience under my belt, I can tell you that it isn't just about knowing how to make the electronics work. Sometimes there are subtle clues that something isn't right. Malfunctions are not always a completely blank screen. Maybe it shows up as just an erroneous instrument reading. The instrument failures that scare pilots the most are those slow subtle failures. If you understand what should be happening, you most likely won't get caught. I guess that the point I keep coming back to is that understanding the concepts allows you to utilize the tools effectively. If you don't understand, or understand just enough to get a lttile out of the tools, the result may not be what is expected (arcs versus straight lines). The real back-up isn't another GPS, it is understanding good navigational concepts. When I did my coastal cruising navigational classes, we were not allowed to turn a GPS on. Radar use was allowed occasionally based on conditions and as part of gaining experience with radar based navigational concepts. The rest was just good basic navigational concepts. Once you get the basics down, then the gee whiz stuff like cross track error, waypoints, various kinds of trend lines, heading versus course will all make sense and you will know enough to become suspicious when it seems wrong, and maybe avoid that "human error". Dan Jonas (S/V Feije II)
Vic, I agree with you. Most modern GPS's include a chartplotter, even if rudimentary and cross track error anunciation is common. Using it properly and knowing how to program waypoints can make for easy navigation work. You referenced putting your GPS position on a chart as often as necessary to make sure it is doing the job. Thats more than most do I suspect and supports my point of actually navigating rather than watching it move by on the screen. I suspect that you have a pretty good idea how it actually works (navigation and maximum utilization of the tool). I have dual Raytheon chartplotters on my boat, but seldom plug in a waypoint. I suspect that few sailors do, because, unless the wind is pretty cooperative, you probably aren't going there directly anyway. On San Francisco Bay, even if the wind is cooperating, something else will manage to be in the way anyway, be it a bridge abutement, an island or shallow area, or just another vessel constrained by draft. Sometimes the currents alone are enough to make more than a few tacks necessary just to get somewhere. Its more likely that I'll steer by compass and use the GPS to keep track of position and maybe even to setup a set and drift correction by comparing the GPS heading with the compass over time. Although the cross track and other features are useful...I think if someone is asking if the GPS should be set to true north or magnetic north, they might want to get a better understanding of navigational basics before really focusing in on full dependence upon the gee whiz stuff in a GPS. With a little bit of aviation instrument training and experience under my belt, I can tell you that it isn't just about knowing how to make the electronics work. Sometimes there are subtle clues that something isn't right. Malfunctions are not always a completely blank screen. Maybe it shows up as just an erroneous instrument reading. The instrument failures that scare pilots the most are those slow subtle failures. If you understand what should be happening, you most likely won't get caught. I guess that the point I keep coming back to is that understanding the concepts allows you to utilize the tools effectively. If you don't understand, or understand just enough to get a lttile out of the tools, the result may not be what is expected (arcs versus straight lines). The real back-up isn't another GPS, it is understanding good navigational concepts. When I did my coastal cruising navigational classes, we were not allowed to turn a GPS on. Radar use was allowed occasionally based on conditions and as part of gaining experience with radar based navigational concepts. The rest was just good basic navigational concepts. Once you get the basics down, then the gee whiz stuff like cross track error, waypoints, various kinds of trend lines, heading versus course will all make sense and you will know enough to become suspicious when it seems wrong, and maybe avoid that "human error". Dan Jonas (S/V Feije II)
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Tim
Clarification
In the previous reply "navigation", Brian writes: "Variation is read from the chart, and tells you how much magnetic north has shifted since the chart was published. It is usually calculated anually by minutes (not full degrees) as there is only a small shift per year. " Not sure what you were meaning to say here Brian, but more succinctly: Variation is the angular difference between the bearing to true north and magnetic north. The annual increase/decrease listed on the chart is the amount variation changes over time, not variation itself. The variation is listed in the compass rose on the chart, and the increase/decrease to that is listed below it. Tim
In the previous reply "navigation", Brian writes: "Variation is read from the chart, and tells you how much magnetic north has shifted since the chart was published. It is usually calculated anually by minutes (not full degrees) as there is only a small shift per year. " Not sure what you were meaning to say here Brian, but more succinctly: Variation is the angular difference between the bearing to true north and magnetic north. The annual increase/decrease listed on the chart is the amount variation changes over time, not variation itself. The variation is listed in the compass rose on the chart, and the increase/decrease to that is listed below it. Tim
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Bob Camarena
I like magnetic
I like magnetic so that my GPS, autopilot, and compass all show (approximately)the same heading.
I like magnetic so that my GPS, autopilot, and compass all show (approximately)the same heading.
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Steve LeBlanc
Mines usually set to magnetic
I usually like to use my GPS to verify against the compass. Face it Electronics do fail, hopefully your compass won't
I usually like to use my GPS to verify against the compass. Face it Electronics do fail, hopefully your compass won't
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Geof Tillotson
Thanks for the thread - Mystery Solved!
I was going crazy with my compass not matching the GPS some of the time. I bought the boat used so I had little faith in the on-baord Plastimo compass compared to the old tried and true GPS. I had all but decided to redo the compensation on the compass, or replace it, because the quick checks I made against the gps were really frustrating me. They were almost always different in that the variations seemed to range a different number of degrees depending on when I compared the two. I have moved anything that might interfere with the compass from the boat, and I have compared it against neighbors compasses. The compasses almost always were close enough. Understand, I have hiked, biked and off-roaded with the GPS in the past and it has always proved correct and reliable. Then it hit me - I wonder if the GPS is set to Magnetic or True North. Of course I would set it to magnetic and leave it there. However, my kids are in middle school, and they are known to go hiking and biking etc. It wasn't until we were going away for a ski trip a few weeks ago and I was using the GPS in the car that I noticed it was reading headings I didn't expect. Viola! The kids had changed it to True North to test their understanding of declination and never put it back. Last year one of them did the same thing and then remembered to correct it. So, I'm pretty sure I'm not just getting old and forgetting the course to the marks I always see anymore, the GPS was set to True and it disagreed with the compass (DUH!) I can't wait to get to the boat this weekend to see if they agree again. Thanks for the "kick in the old brain-box" it might have saved me from dishing out another $175 this year. Geof
I was going crazy with my compass not matching the GPS some of the time. I bought the boat used so I had little faith in the on-baord Plastimo compass compared to the old tried and true GPS. I had all but decided to redo the compensation on the compass, or replace it, because the quick checks I made against the gps were really frustrating me. They were almost always different in that the variations seemed to range a different number of degrees depending on when I compared the two. I have moved anything that might interfere with the compass from the boat, and I have compared it against neighbors compasses. The compasses almost always were close enough. Understand, I have hiked, biked and off-roaded with the GPS in the past and it has always proved correct and reliable. Then it hit me - I wonder if the GPS is set to Magnetic or True North. Of course I would set it to magnetic and leave it there. However, my kids are in middle school, and they are known to go hiking and biking etc. It wasn't until we were going away for a ski trip a few weeks ago and I was using the GPS in the car that I noticed it was reading headings I didn't expect. Viola! The kids had changed it to True North to test their understanding of declination and never put it back. Last year one of them did the same thing and then remembered to correct it. So, I'm pretty sure I'm not just getting old and forgetting the course to the marks I always see anymore, the GPS was set to True and it disagreed with the compass (DUH!) I can't wait to get to the boat this weekend to see if they agree again. Thanks for the "kick in the old brain-box" it might have saved me from dishing out another $175 this year. Geof
D
Dennis Thomas
Learn to Navigate
I’ve taught piloting for the USPS for about 12 years. When I started GPS (satnav) was just getting popular. We avoided even talking about it because nobody was sure how it would all turn out. Then we began to use it ourselves and quickly appreciated how it could supplement our navigational tools. Today you have to start with the assumption that the student is going to be using GPS as the primary navigational tool and try to teach them how to use it wisely. If you haven’t understood every word in this thread PLEASE sign up for a class with your local Power Squadron or Coastes! This winter I’ve been stuck in Moscow wanting to be back in Texas and on the water. The only water here is frozen. The only thing flowing is the vodka. It is really a treat to follow fellow sailors on line. Just a few years ago I thought CNN in a hotel was just like being home. How the world is changing! Thanks for being there. Dennis Thomas S/V Anodyne
I’ve taught piloting for the USPS for about 12 years. When I started GPS (satnav) was just getting popular. We avoided even talking about it because nobody was sure how it would all turn out. Then we began to use it ourselves and quickly appreciated how it could supplement our navigational tools. Today you have to start with the assumption that the student is going to be using GPS as the primary navigational tool and try to teach them how to use it wisely. If you haven’t understood every word in this thread PLEASE sign up for a class with your local Power Squadron or Coastes! This winter I’ve been stuck in Moscow wanting to be back in Texas and on the water. The only water here is frozen. The only thing flowing is the vodka. It is really a treat to follow fellow sailors on line. Just a few years ago I thought CNN in a hotel was just like being home. How the world is changing! Thanks for being there. Dennis Thomas S/V Anodyne
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