If I use my GPS to find true magnetic north, and read the difference between it and my on board compass, will this give me a correct compass variation to go by? Being new to using a gps, I am trying to integrate both instruments! thanks Dennis
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Variation
Dennis ???true magnetic north??? It's either one or the other. If your GPS is set for magnetic, you could conceivably use it to "cross check" your compass, except for current set and drift. Your boat heading could be one direction, shown on your compass, and the boat could actually be moving in another direction because of current. I recommend reading some Chapman's and other basic navigation material to get a better understanding of the dynamics and variables. Think "complement" rather than "integrate." Stu
Dennis ???true magnetic north??? It's either one or the other. If your GPS is set for magnetic, you could conceivably use it to "cross check" your compass, except for current set and drift. Your boat heading could be one direction, shown on your compass, and the boat could actually be moving in another direction because of current. I recommend reading some Chapman's and other basic navigation material to get a better understanding of the dynamics and variables. Think "complement" rather than "integrate." Stu
Deviation
At least for my older GPS, the GPS points to true north. Your compass, if you disregard deviation, points to magnetic north. You need to factor in both deviation and variation. Variation is the difference between magnetic north and true north. Deviation is the difference between the direction your magnetic compass is pointing, if heading north (east/west/south), and magnetic north (east/west/south). I figured that magnetic north was about 358 degrees true on my lake. In order to swing my compass, on a calm day with the sails down, I motored magnetic north (358 true) at about 5k. I swung my compass to point true north. I then did the same going south except that rather than correcting the the compass to true south, I removed only half of the error. I then did the same east and west. Then I went back and sailed the cardinal directions and calculated the deviation. You can find magnetic variation on most charts. There is a chance that I have confused variation and deviation. If I have I am sure someone will correct me.
At least for my older GPS, the GPS points to true north. Your compass, if you disregard deviation, points to magnetic north. You need to factor in both deviation and variation. Variation is the difference between magnetic north and true north. Deviation is the difference between the direction your magnetic compass is pointing, if heading north (east/west/south), and magnetic north (east/west/south). I figured that magnetic north was about 358 degrees true on my lake. In order to swing my compass, on a calm day with the sails down, I motored magnetic north (358 true) at about 5k. I swung my compass to point true north. I then did the same going south except that rather than correcting the the compass to true south, I removed only half of the error. I then did the same east and west. Then I went back and sailed the cardinal directions and calculated the deviation. You can find magnetic variation on most charts. There is a chance that I have confused variation and deviation. If I have I am sure someone will correct me.
Coastal Navigation
Hi Dennis, I understand your confusion with various navigation terms. Variation is the difference between where your compass would point if there were no other magnetic fields acting on it and true north. Variation exists because the magnetic field near the top of the globe is not exactly at the north pole. Variation is consistent for a geographic area so it is marked on your chart in the compass rose. Since the magnetic pole is actually moving, variation changes from year to year and this factor is also on the chart. Deviation is the difference from where your compass points when it is on your vessel from where it would point if no other magnets were acting upon it. The metal and electronics on your boat produce the deviation and to make it more confusing, deviation varies for all points on the compass. Using your GPS can help to sort this out but knowing how to navigate in case of electronics failure is crucial as well. In addition to Chapmans, I would recommend taking the Coast Guard Auxiliary courses in Basic and Advanced Coastal Navigation. They are very inexpensive, provide excellent reference material and they are even fun. I met a lot of great people when I took the course(even though power boaters are permitted). Gregg
Hi Dennis, I understand your confusion with various navigation terms. Variation is the difference between where your compass would point if there were no other magnetic fields acting on it and true north. Variation exists because the magnetic field near the top of the globe is not exactly at the north pole. Variation is consistent for a geographic area so it is marked on your chart in the compass rose. Since the magnetic pole is actually moving, variation changes from year to year and this factor is also on the chart. Deviation is the difference from where your compass points when it is on your vessel from where it would point if no other magnets were acting upon it. The metal and electronics on your boat produce the deviation and to make it more confusing, deviation varies for all points on the compass. Using your GPS can help to sort this out but knowing how to navigate in case of electronics failure is crucial as well. In addition to Chapmans, I would recommend taking the Coast Guard Auxiliary courses in Basic and Advanced Coastal Navigation. They are very inexpensive, provide excellent reference material and they are even fun. I met a lot of great people when I took the course(even though power boaters are permitted). Gregg
Compass Compensation Explained
This link will explain the 'how-to' of it for their units....it may also work on yours or at least further your understanding.
This link will explain the 'how-to' of it for their units....it may also work on yours or at least further your understanding.
GPS as ac "Compass"
A GPS doesn't "point" to anything. If you are under way, and have a reliable signal, the GPS receiver can calculate the net heading made good. "Net" in the sense that effects of wind and current are factored into that calculation. I suggest investing in Nigel Calder's new book "GPS for Mariners" ($15.95 retail). It's good. A recent edition of either "Ocean Navigator" or "Ocean Voyager" had a short piece written about using GPS to get a calibration for your compass. --RonD
A GPS doesn't "point" to anything. If you are under way, and have a reliable signal, the GPS receiver can calculate the net heading made good. "Net" in the sense that effects of wind and current are factored into that calculation. I suggest investing in Nigel Calder's new book "GPS for Mariners" ($15.95 retail). It's good. A recent edition of either "Ocean Navigator" or "Ocean Voyager" had a short piece written about using GPS to get a calibration for your compass. --RonD
Am I right?
My understanding is that the compass rose on marine charts is in reference to magnetic north for the spot on the earth shown on the chart. My pedistal compass needle points to magnetic north. So if I am off of Green point and want to get a bearing to Red Island, I put the ruler on the chart for the line I want to travel and then walk the ruler over to the compass rose and get my bearing. Then I stear this bearing using the commpass on the boat and don't do any adjustments for true north/ mag north. If I do the same exercise on my eletronic chart for the same course, the computer will give me the same compass heading I calculated manually. My GPS handheld will show the same direction of travel as the boat compass. And the GPS plugged into the PC will show the same magnetic heading as the boat compass heading, assuming no magnetic interference affecting the compass. In my local waters the "declination" not "deviation" between true and magnetic north is about 22 degrees east, but I don't care about this since the GPS and charts are both referencing magnetic north as is the boat compass I steer by. I assume that the GPS is somehow programed to "know" magnetic direction given wherever it "finds" itself in the world. Now if I go hiking the topograghic map references true north and the top of the map is always true north. So to make my handheld compass work with the land map I adjust the setting on the compass so magnetic north reads 22 decrees. So when I take bearings to the tops of Red Green and Blue Peaks to find out where I am along Cold River, I can transfer those bearings to the map without having to make allowances for the 22 degrees deviation. I was traveling cross county in the mountains in Western Washington with a guy who had flown out from Denver. We used my compass to determine a heading through the forest and started walking. The Denver guy kept walkiing about 10 degrees off of our heading. Finally I inquired whether were trying to head 260 degrees and he said yep. When we compared the heading of our compasses they were about 10 degres off because the Denver guy had not adjusted his compass for the diference in declination between Denver and Western Washington. So am I correct, or just lucky so far? Scott
My understanding is that the compass rose on marine charts is in reference to magnetic north for the spot on the earth shown on the chart. My pedistal compass needle points to magnetic north. So if I am off of Green point and want to get a bearing to Red Island, I put the ruler on the chart for the line I want to travel and then walk the ruler over to the compass rose and get my bearing. Then I stear this bearing using the commpass on the boat and don't do any adjustments for true north/ mag north. If I do the same exercise on my eletronic chart for the same course, the computer will give me the same compass heading I calculated manually. My GPS handheld will show the same direction of travel as the boat compass. And the GPS plugged into the PC will show the same magnetic heading as the boat compass heading, assuming no magnetic interference affecting the compass. In my local waters the "declination" not "deviation" between true and magnetic north is about 22 degrees east, but I don't care about this since the GPS and charts are both referencing magnetic north as is the boat compass I steer by. I assume that the GPS is somehow programed to "know" magnetic direction given wherever it "finds" itself in the world. Now if I go hiking the topograghic map references true north and the top of the map is always true north. So to make my handheld compass work with the land map I adjust the setting on the compass so magnetic north reads 22 decrees. So when I take bearings to the tops of Red Green and Blue Peaks to find out where I am along Cold River, I can transfer those bearings to the map without having to make allowances for the 22 degrees deviation. I was traveling cross county in the mountains in Western Washington with a guy who had flown out from Denver. We used my compass to determine a heading through the forest and started walking. The Denver guy kept walkiing about 10 degrees off of our heading. Finally I inquired whether were trying to head 260 degrees and he said yep. When we compared the heading of our compasses they were about 10 degres off because the Denver guy had not adjusted his compass for the diference in declination between Denver and Western Washington. So am I correct, or just lucky so far? Scott
Compass rose shows both
true and magnetic directions. You are correct so long as you use the right indicator.
true and magnetic directions. You are correct so long as you use the right indicator.
Scott,
You are correct only if you set up your GPS to give you the headings in Magnetic. In reality, the GPS calculates the true heading from the satellites data, and it uses internal data to derive the magnetic heading at your present location. Because the variation varies between different locations on earth, if you set a long enough route, your actual magnetic heading may change as you move towards your destination in a straight line. This, should not happen in short costal cruising unless you are very close to the poles.
You are correct only if you set up your GPS to give you the headings in Magnetic. In reality, the GPS calculates the true heading from the satellites data, and it uses internal data to derive the magnetic heading at your present location. Because the variation varies between different locations on earth, if you set a long enough route, your actual magnetic heading may change as you move towards your destination in a straight line. This, should not happen in short costal cruising unless you are very close to the poles.
depends on GPS
Hello, My GPS (Garmen ETREX Legend) allows you to select true north OR magnetic north. It doesn't matter which one you use as long as you use the same one for your chart and your compass. Barry
Hello, My GPS (Garmen ETREX Legend) allows you to select true north OR magnetic north. It doesn't matter which one you use as long as you use the same one for your chart and your compass. Barry
OK, now I get it
The GPS prompts, at set up, for land or marine. So when I select marine, it must go into mag north mode. If I go hiking I will need to select "land" mode i.e. true north, so it "fits" with the USGS true north land map. Thanks guys. Great feed back. Scott
The GPS prompts, at set up, for land or marine. So when I select marine, it must go into mag north mode. If I go hiking I will need to select "land" mode i.e. true north, so it "fits" with the USGS true north land map. Thanks guys. Great feed back. Scott
Scott's comments
Scott wrote: "My GPS handheld will show the same direction of travel as the boat compass." If your boat was caught in a vicious tidal rip as you were traveling down a river, and it forced the bow to turn 40 degrees to port, but you were still moving down the river with the flow, your GPS heading and boat compass would differ by 40 degrees. The original question was can you use the GPS to check your compass and vice versa. The answer is only under controlled conditions. Currents will alter your boat's compass heading compared to your actual direction "over ground," and has nothing whatsoever to do with deviation, variation, true or magnetic. Please, just think about it. Stu
Scott wrote: "My GPS handheld will show the same direction of travel as the boat compass." If your boat was caught in a vicious tidal rip as you were traveling down a river, and it forced the bow to turn 40 degrees to port, but you were still moving down the river with the flow, your GPS heading and boat compass would differ by 40 degrees. The original question was can you use the GPS to check your compass and vice versa. The answer is only under controlled conditions. Currents will alter your boat's compass heading compared to your actual direction "over ground," and has nothing whatsoever to do with deviation, variation, true or magnetic. Please, just think about it. Stu
Question about Declination
Reference to Scott's use of the term declination. I thought, and I certainly may be wrong, is that declination is the tendency of the compass needle to point down towards the magnetic center of the earth. Variation, again I may be wrong, but don't believe I am, is the difference between magnetic north and true north. After years of flying the term declination was never used, except for the reminder "ANDS" which is "Accelerate North, Decelerate South, which is not a issue, at least not in my boat. This referred to the tendency of the magnetic compass to swing when accelerating and decelerating. Unfortunately I am away from home right now and have no way of checking this, unless I go onto the internet and search it out, which I don't have time to do at this time.
Reference to Scott's use of the term declination. I thought, and I certainly may be wrong, is that declination is the tendency of the compass needle to point down towards the magnetic center of the earth. Variation, again I may be wrong, but don't believe I am, is the difference between magnetic north and true north. After years of flying the term declination was never used, except for the reminder "ANDS" which is "Accelerate North, Decelerate South, which is not a issue, at least not in my boat. This referred to the tendency of the magnetic compass to swing when accelerating and decelerating. Unfortunately I am away from home right now and have no way of checking this, unless I go onto the internet and search it out, which I don't have time to do at this time.
Dan, I think you are right
At my high school the physics teacher had a special vertically oriented compass that measured local declination. At the magnetic poles the field is vertical and so a compass will want to point downward. Only at the magnetic equator is the magnetic field parallel to the earth's surface; the further you are away from the equator, the stronger is the vertical component. Which raises an interesting point: are compasses cards balanced to compensate for the declination at a particular lattitude?
At my high school the physics teacher had a special vertically oriented compass that measured local declination. At the magnetic poles the field is vertical and so a compass will want to point downward. Only at the magnetic equator is the magnetic field parallel to the earth's surface; the further you are away from the equator, the stronger is the vertical component. Which raises an interesting point: are compasses cards balanced to compensate for the declination at a particular lattitude?
Some confusion in terms...
Okay, I may be totally wrong here, but aren't the terms 'declination' and 'variation' describing the same thing, that is, the difference between True North and Magnetic North? Deviation is a particular compass's readings that differ from Magnetic North, usually due to nearby items that have or generate magnetic fields (engine, speakers, etc.) You will often find a 'deviation' card mounted on an aircraft compass that gives the corrections to be used for various headings. Even once a compass has been properly calibrated, you still need the deviation card because no compass can ever be full compensated and adjusted when it is surrounded by magnetic anomolies. As to GPS, the other posters are right in saying that it will give your course in degrees, but cannot compensate for current and wind drift, although some GPS models also incorporate a flux gate compass (my E-Trex Vista has one) that acts like a regular compass, although it too is affected by any magnetic anomolies nearby just like any other compass. One answer might be in a new system that uses a GPS antenna at the bow and another at the stern to measure the phase angle of the incoming GPS signal to calculate heading. This system is highly accurate (or so they claim) and is unaffected by magnetic anomoly. Kind of ironic, in this age of super computers and Mars rovers, that we are still trying to get our compasses to point in the right direction! Jeff
Okay, I may be totally wrong here, but aren't the terms 'declination' and 'variation' describing the same thing, that is, the difference between True North and Magnetic North? Deviation is a particular compass's readings that differ from Magnetic North, usually due to nearby items that have or generate magnetic fields (engine, speakers, etc.) You will often find a 'deviation' card mounted on an aircraft compass that gives the corrections to be used for various headings. Even once a compass has been properly calibrated, you still need the deviation card because no compass can ever be full compensated and adjusted when it is surrounded by magnetic anomolies. As to GPS, the other posters are right in saying that it will give your course in degrees, but cannot compensate for current and wind drift, although some GPS models also incorporate a flux gate compass (my E-Trex Vista has one) that acts like a regular compass, although it too is affected by any magnetic anomolies nearby just like any other compass. One answer might be in a new system that uses a GPS antenna at the bow and another at the stern to measure the phase angle of the incoming GPS signal to calculate heading. This system is highly accurate (or so they claim) and is unaffected by magnetic anomoly. Kind of ironic, in this age of super computers and Mars rovers, that we are still trying to get our compasses to point in the right direction! Jeff
Using a Compass
When using a map on land, the term declination refers to the local magnetic field found in the environment that will affect the compass's reading. When using a nautical chart, the term for local magnetic field in the environment is called variation. Another term used on the water is deviation. Deviation is the magnetic field found on the ship which will affect the ship's compass such as speakers (magnets) in the cockpit. Compass deviation card or compass error cards shows the deviation the compass reading must be adjusted to by adding or subtracting the error. A compass deviation card is generated by "Swinging" the compass. This is done by sailing a complete 360 degrees and using fixed land points to check the ship's deviation with a hand held compass. For inshore sailing you can swing your own compass, but if you are going to sail blue water, I would have a professional compass adjuster do the swinging and certify the compass deviation card. For sailors, only the terms variation and deviation are used to describe errors added or subtracted to get magnetic heading. The other terms are used for non-sailors. "The geomagnetic field can be quantified as total intensity, vertical intensity, horizontal intensity, inclination and declination. The total intensity is the magnetic strength, which ranges from about 23 microteslas (equivalent to 23000 nanoteslas or gammas, or 0.23 oersteds or gauss) around Sao Paulo, Brazil to 67 microteslas near the south magnetic pole near Antarctica. Vertical and horizontal intensities are components of the total intensity. The angle of the field relative to the level ground is the inclination or dip, which is 90° at the north magnetic pole. Finally, the angle of the horizontal intensity with respect to the north geographic pole is the declination, also called variation in mariners' and aviators' jargon. In other words, declination is the angle between where a compass needle points and the true North Pole... A beginners guide to understanding magnetic declination by Chris M. Goulet, Montreal, Canada." The GPS will calculate a true and magnetic heading, depending on how you setup your GPS. Most GPS has a function called "AutoMag" which has stored variation values based on your actual position to convert true to magnetic. The primary purpose of the GPS is to provide a position fix. For heading and speed, the GPS takes two reading to calculate the change in position due to speed and direction to provide an averaged speed value and average course heading. Since it is not instantaneous but must be done over some time interval and movement of the ship, an error factor due to changes occurring during the time interval and ship's movement is introduced. A compass reading is instantaneous, the ship doesn't have to be moving or be measured after some time interval. You can Swing your own compass, there are books you can get providing different methods and ways to swing your own compass to get different degrees of accuracy. Fair Winds, Clyde
When using a map on land, the term declination refers to the local magnetic field found in the environment that will affect the compass's reading. When using a nautical chart, the term for local magnetic field in the environment is called variation. Another term used on the water is deviation. Deviation is the magnetic field found on the ship which will affect the ship's compass such as speakers (magnets) in the cockpit. Compass deviation card or compass error cards shows the deviation the compass reading must be adjusted to by adding or subtracting the error. A compass deviation card is generated by "Swinging" the compass. This is done by sailing a complete 360 degrees and using fixed land points to check the ship's deviation with a hand held compass. For inshore sailing you can swing your own compass, but if you are going to sail blue water, I would have a professional compass adjuster do the swinging and certify the compass deviation card. For sailors, only the terms variation and deviation are used to describe errors added or subtracted to get magnetic heading. The other terms are used for non-sailors. "The geomagnetic field can be quantified as total intensity, vertical intensity, horizontal intensity, inclination and declination. The total intensity is the magnetic strength, which ranges from about 23 microteslas (equivalent to 23000 nanoteslas or gammas, or 0.23 oersteds or gauss) around Sao Paulo, Brazil to 67 microteslas near the south magnetic pole near Antarctica. Vertical and horizontal intensities are components of the total intensity. The angle of the field relative to the level ground is the inclination or dip, which is 90° at the north magnetic pole. Finally, the angle of the horizontal intensity with respect to the north geographic pole is the declination, also called variation in mariners' and aviators' jargon. In other words, declination is the angle between where a compass needle points and the true North Pole... A beginners guide to understanding magnetic declination by Chris M. Goulet, Montreal, Canada." The GPS will calculate a true and magnetic heading, depending on how you setup your GPS. Most GPS has a function called "AutoMag" which has stored variation values based on your actual position to convert true to magnetic. The primary purpose of the GPS is to provide a position fix. For heading and speed, the GPS takes two reading to calculate the change in position due to speed and direction to provide an averaged speed value and average course heading. Since it is not instantaneous but must be done over some time interval and movement of the ship, an error factor due to changes occurring during the time interval and ship's movement is introduced. A compass reading is instantaneous, the ship doesn't have to be moving or be measured after some time interval. You can Swing your own compass, there are books you can get providing different methods and ways to swing your own compass to get different degrees of accuracy. Fair Winds, Clyde
declination or dip
The declination you speak of was termed "dip" (inclination of the needle) in my early sea-going years. In the northern hemisphere the needle will dip downwards, in the southern hemisphere it will point upwards. This I think is caused by an attraction in the north and repelling in the south. Imagine that there is a huge magnet inside the earth. There is a magnetic equator near the geographical equator where there is no dip (the needle will be parallel to the earth's surface). If you go south of the equator with a small "northern" magnetic compass, the compass requires another correction. Without it, if you rely on a magnetic compass, you may not end up where you want. Of course the Polynesians sailed all over without relying on compasses.
The declination you speak of was termed "dip" (inclination of the needle) in my early sea-going years. In the northern hemisphere the needle will dip downwards, in the southern hemisphere it will point upwards. This I think is caused by an attraction in the north and repelling in the south. Imagine that there is a huge magnet inside the earth. There is a magnetic equator near the geographical equator where there is no dip (the needle will be parallel to the earth's surface). If you go south of the equator with a small "northern" magnetic compass, the compass requires another correction. Without it, if you rely on a magnetic compass, you may not end up where you want. Of course the Polynesians sailed all over without relying on compasses.
Compass Deviation
Okay everyone, I am just trying to find an easy way to figure my compass deviation, which I believe is caused by it being in a proximity to other metals on the boat! I figured that if I point the standard compass toward its magnetic north, with its deviation, and then checked the gps for its reading on magnetic north, then the difference between the two should be my standard compass deviation! Am I correct about that? I did mistakenly say variation! Thanks for all the imput you are all giving!
Okay everyone, I am just trying to find an easy way to figure my compass deviation, which I believe is caused by it being in a proximity to other metals on the boat! I figured that if I point the standard compass toward its magnetic north, with its deviation, and then checked the gps for its reading on magnetic north, then the difference between the two should be my standard compass deviation! Am I correct about that? I did mistakenly say variation! Thanks for all the imput you are all giving!
compass deviation
Dennis, There is no "standard" compass deviation. Deviation will vary depending on your heading. You have to swing the compass to find out the deviation and create a deviation card. A GPS is not a compass, in can only tell you the direction you have travelled. The direction in which you have travelled may differ from your heading due to set (current) or leeway (wind and sea action).
Dennis, There is no "standard" compass deviation. Deviation will vary depending on your heading. You have to swing the compass to find out the deviation and create a deviation card. A GPS is not a compass, in can only tell you the direction you have travelled. The direction in which you have travelled may differ from your heading due to set (current) or leeway (wind and sea action).
Nope
"that if I point the standard compass toward its magnetic north, with its deviation, and then checked the GPS for its reading on magnetic north, then the difference between the two should be my standard compass deviation! Am I correct about that? " NO< NO< NO< nope. Please get a BIG or small navigation book, like Chapman's and read all about it. You could probably do a Goggle search on Navigation and find it all written up on the INTERNET. Lots of folks have written a lot of interesting information here, and some are almost book writers in their quality and length. Deviation, variation, dip, and basics to keeping course are subjects far more interesting if you get a book, with pictures, that will explain it all. Deviation, as you mention it, is not deviation as defined. Your description is only the difference between your GPS and your compass. It is NOT deviation as defined for compasses. Only a quality book, which will take pages and a full chapter to explain and answer your question, is what you should be looking at, Dennis. Please, understand that we just can't copy it all here. Stu
"that if I point the standard compass toward its magnetic north, with its deviation, and then checked the GPS for its reading on magnetic north, then the difference between the two should be my standard compass deviation! Am I correct about that? " NO< NO< NO< nope. Please get a BIG or small navigation book, like Chapman's and read all about it. You could probably do a Goggle search on Navigation and find it all written up on the INTERNET. Lots of folks have written a lot of interesting information here, and some are almost book writers in their quality and length. Deviation, variation, dip, and basics to keeping course are subjects far more interesting if you get a book, with pictures, that will explain it all. Deviation, as you mention it, is not deviation as defined. Your description is only the difference between your GPS and your compass. It is NOT deviation as defined for compasses. Only a quality book, which will take pages and a full chapter to explain and answer your question, is what you should be looking at, Dennis. Please, understand that we just can't copy it all here. Stu
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