While viewing the message board, and particularly the message regarding GPS, I recalled a problem that I observed with my Garmin GPS handheld. A few weeks ago, while sailing, I noticed that the GPS compass displayed a bearing about twenty degrees different from my magnetic compass. Last week, I took out my peloris and my Ritchie handheld direction finder to check the accuracy of my magnetic compass, only to find out that my magnetic compass was correct. Is this normal? that there be such a difference from GPS to magnetic? While I can trust the GPS for positioning, which is why I have it, I cannot trust their compass.
If anyone could explain the reason for the difference, I would appreciate it. Maybe I am doing something wrong.
Thanks,
Gary Lapine
Red Witch III
CD30C, #339
dory26@attbi.com
Why is it?
Moderator: Jim Walsh
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Jacob Bogatch
Re: Why is it?
Hi Gary
GPS cannot replace your compass. It is BETTER in high velocity and BAD near STOP.
I read in one of the magazines that Furono came up with a compass that works on the same satellites as the GPS but it will cost you another $3,500 to enjoy an accurate bearing.
Jacob Bogatch
"Fhloston Paradise"
CD30C Hull# 363
Haversraw on the Hudson
jbogatch@yahoo.com
GPS cannot replace your compass. It is BETTER in high velocity and BAD near STOP.
I read in one of the magazines that Furono came up with a compass that works on the same satellites as the GPS but it will cost you another $3,500 to enjoy an accurate bearing.
Jacob Bogatch
"Fhloston Paradise"
CD30C Hull# 363
Haversraw on the Hudson
Gary L. wrote: While viewing the message board, and particularly the message regarding GPS, I recalled a problem that I observed with my Garmin GPS handheld. A few weeks ago, while sailing, I noticed that the GPS compass displayed a bearing about twenty degrees different from my magnetic compass. Last week, I took out my peloris and my Ritchie handheld direction finder to check the accuracy of my magnetic compass, only to find out that my magnetic compass was correct. Is this normal? that there be such a difference from GPS to magnetic? While I can trust the GPS for positioning, which is why I have it, I cannot trust their compass.
If anyone could explain the reason for the difference, I would appreciate it. Maybe I am doing something wrong.
Thanks,
Gary Lapine
Red Witch III
CD30C, #339
jbogatch@yahoo.com
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Matt Cawthorne
Re: Why is it?
Gary,
The compass measured the direction you are pointed (relative to the earth). The GPS should be measuring the direction you are going (relative to the earth). Not necessarily the same thing. (It is a good metaphor for life) Could it possibly be that there was a current where you were sailing? 20 degrees sounds like too much to be leeway, but 8 or 10 degrees of leeway would not surprise me, particularly if you had the boat heeling way over.
Matt
mcawthor@bellatlantic.net
The compass measured the direction you are pointed (relative to the earth). The GPS should be measuring the direction you are going (relative to the earth). Not necessarily the same thing. (It is a good metaphor for life) Could it possibly be that there was a current where you were sailing? 20 degrees sounds like too much to be leeway, but 8 or 10 degrees of leeway would not surprise me, particularly if you had the boat heeling way over.
Matt
mcawthor@bellatlantic.net
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Neil Gordon
Re: Why is it?
>>Could it possibly be that there was a current where you were sailing? 20 degrees sounds like too much to be leeway, but 8 or 10 degrees of leeway would not surprise me, particularly if you had the boat heeling way over.<<
Careful observation of the difference between the GPS and the compass will teach you lots about your boat and how it sails relative to the wind. Information that's especially useful when the electronics fail and you're subject to dead reconing in the fog searching for the next buoy.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com
Careful observation of the difference between the GPS and the compass will teach you lots about your boat and how it sails relative to the wind. Information that's especially useful when the electronics fail and you're subject to dead reconing in the fog searching for the next buoy.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com
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Gary L.
Re: Why is it?
Matt,Matt Cawthorne wrote: Gary,
The compass measured the direction you are pointed (relative to the earth). The GPS should be measuring the direction you are going (relative to the earth). Not necessarily the same thing. (It is a good metaphor for life) Could it possibly be that there was a current where you were sailing? 20 degrees sounds like too much to be leeway, but 8 or 10 degrees of leeway would not surprise me, particularly if you had the boat heeling way over.
Matt
Thanks for your reply. No, I was probably doing 4-5kts, heeled no more than 10 degrees in Mt. Hope Bay, which does not have terribly strong currents to be set that far off.
Gary Lapine
dory26@attbi.com
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Jim Myers
Re: Why is it?
Is your GPS giving you a true (map) heading taking into account the variation in your area and the deviation in your compass?
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Richard Hill
Re: Why is it?
I think it's an operator error problem. Sorry. Not to be a smartass, but I think it's highly likely that the GPS wasn't aligned perfectly with the centerline of the boat - especially if you had it in your hand. Next time out, put the GPS down somewhere where you can be certain it is perfectly aligned, then check your compass heading (corrected for variation and deviation, of course). This same problem used to crop up with the old RDF units, except that they only gave a relative bearing, not true.Gary L. wrote: While viewing the message board, and particularly the message regarding GPS, I recalled a problem that I observed with my Garmin GPS handheld. A few weeks ago, while sailing, I noticed that the GPS compass displayed a bearing about twenty degrees different from my magnetic compass. Last week, I took out my peloris and my Ritchie handheld direction finder to check the accuracy of my magnetic compass, only to find out that my magnetic compass was correct. Is this normal? that there be such a difference from GPS to magnetic? While I can trust the GPS for positioning, which is why I have it, I cannot trust their compass.
If anyone could explain the reason for the difference, I would appreciate it. Maybe I am doing something wrong.
Thanks,
Gary Lapine
Red Witch III
CD30C, #339
captrahill@comcast.net
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Jeff Funston
Re: Why is it?
Your GPS does not know where the pointy end of the boat is, its only aware of the direction you are heading, remember you are being pushed by the wind and the force of the water under/besides the keel/rudder can only partially offset that force. I would suguest the GPS is showing the true direction of the craft.
jefffNospam@pinn.net
jefffNospam@pinn.net
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Bob Loewenstein
Re: Why is it?
Huh? Am I missing something? Why does it matter where the GPS is as long as it's on the boat? The boat is a rigid body so no matter where you are on it your moving in the direction the boat is, whether on the centerline or not.
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Gary L.
Re: Why is it?
That's what I thought too. However, when I checked all the tools, magnetic compass, peloris, direction finder and the GPS, I was at my mooring and checking bearings to a fixed object on land and within site! The only tool out was the GPS.Bob Loewenstein wrote: Huh? Am I missing something? Why does it matter where the GPS is as long as it's on the boat? The boat is a rigid body so no matter where you are on it your moving in the direction the boat is, whether on the centerline or not.
Gary Lapine
dory@attbi.com
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Olli Wendelin
Re: Why is it?
Gary,
A GPS measures position relative to it's satelite constellation. It then gives you it's position in lat and long. Bearing to a waypoint can be accurately computed from this.
Speed and ships bearing are computed by comparing ships position at two different times. It may also be averaged over several times. If the ship is stopped whatever bearing the GPS gives is meaningless. The time interval or averaging scheme can change the accuracy of the GPS "Compass Bearing" greatly. A GPS in neither a magnetic or gyro compass. I wouldn't use it at all as a compass.
Olli Wendelin
BLUE MOON
Charleston, SC
wendelin@spawar.navy.mil
A GPS measures position relative to it's satelite constellation. It then gives you it's position in lat and long. Bearing to a waypoint can be accurately computed from this.
Speed and ships bearing are computed by comparing ships position at two different times. It may also be averaged over several times. If the ship is stopped whatever bearing the GPS gives is meaningless. The time interval or averaging scheme can change the accuracy of the GPS "Compass Bearing" greatly. A GPS in neither a magnetic or gyro compass. I wouldn't use it at all as a compass.
Olli Wendelin
BLUE MOON
Charleston, SC
wendelin@spawar.navy.mil
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Gary L.
Re: Why is it?
Thank you. Now it all makes sense.Olli Wendelin wrote: Gary,
A GPS measures position relative to it's satelite constellation. It then gives you it's position in lat and long. Bearing to a waypoint can be accurately computed from this.
Speed and ships bearing are computed by comparing ships position at two different times. It may also be averaged over several times. If the ship is stopped whatever bearing the GPS gives is meaningless. The time interval or averaging scheme can change the accuracy of the GPS "Compass Bearing" greatly. A GPS in neither a magnetic or gyro compass. I wouldn't use it at all as a compass.
Olli Wendelin
BLUE MOON
Charleston, SC
Gary
dory@attbi.com
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Clay Stalker
Re: Why is it?
The GPS and compass have different functions...the compass, if set up properly on your boat, will tell you in what direction you are pointing. The GPS won't tell you this, it will only give your position, and not always that accurately at that, especially if you are not moving. When underway, the compass gives your heading, the GPS speed and heading....but they usually don't read the same, because the compass, if calibrated properly, will be more accurate than the GPS, which does corrections slower. Don't know if this helps. This is why a compass is so necessary for navigation, and a GPS is a helpful tool with a variety of fun and neat functions....if I have to make a choice, give me a chart and a compass anyday!Gary L. wrote:Bob Loewenstein wrote: Huh? Am I missing something? Why does it matter where the GPS is as long as it's on the boat? The boat is a rigid body so no matter where you are on it your moving in the direction the boat is, whether on the centerline or not.
That's what I thought too. However, when I checked all the tools, magnetic compass, peloris, direction finder and the GPS, I was at my mooring and checking bearings to a fixed object on land and within site! The only tool out was the GPS.
Gary Lapine
Clay Stalker
cstalker@cheshire.net
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Gary L.
Re: Why is it?
Thanks,Clay Stalker wrote:The GPS and compass have different functions...the compass, if set up properly on your boat, will tell you in what direction you are pointing. The GPS won't tell you this, it will only give your position, and not always that accurately at that, especially if you are not moving. When underway, the compass gives your heading, the GPS speed and heading....but they usually don't read the same, because the compass, if calibrated properly, will be more accurate than the GPS, which does corrections slower. Don't know if this helps. This is why a compass is so necessary for navigation, and a GPS is a helpful tool with a variety of fun and neat functions....if I have to make a choice, give me a chart and a compass anyday!Gary L. wrote:Bob Loewenstein wrote: Huh? Am I missing something? Why does it matter where the GPS is as long as it's on the boat? The boat is a rigid body so no matter where you are on it your moving in the direction the boat is, whether on the centerline or not.
That's what I thought too. However, when I checked all the tools, magnetic compass, peloris, direction finder and the GPS, I was at my mooring and checking bearings to a fixed object on land and within site! The only tool out was the GPS.
Gary Lapine
Clay Stalker
I may be getting all the latest toys, but I still can be found with chart in hand, compass and a stop watch.
Gary
dory@attbi.com
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Neil Gordon
Re: Why is it?
>>A GPS in neither a magnetic or gyro compass. I wouldn't use it at all as a compass.<<
The GPS lives in the past, while the compass lives in the present. I sailed from Boston to Provincetown and back last weekend... most of it in the fog so there were no visual marks to steer toward. Using the GPS heading as a "compass" causes oversteering every time... by the time the GPS says your heading and the bearing to the mark are the same, you've swung past your course. You get better at it after a while, though. Still, it's easier to steer by compasss and I found that doing so kept the GPS heading relatively steady.
As for comments relating to the position of the GPS in the boat... head out in heavier seas and position matters. As many have stated, the GPS doesn't know where the pointy end of the boat is. But... don't pretend that the movement of the boat at its center of gravity is the same as the movement of the boat at the bow or at the transom. We all know that's not true. If a boat is yawing through 10 degrees of arc, a GPS on the transom will move through that arc and the speed and heading shown will be effected. This was apparent on the P-town trip where my handheld and the boat's fixed GPS often differed as to speed and heading. Bearings to marks though, were right on, as each GPS was still doing its job of calculating position and the distance between the two units was only about 15 ft.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com
The GPS lives in the past, while the compass lives in the present. I sailed from Boston to Provincetown and back last weekend... most of it in the fog so there were no visual marks to steer toward. Using the GPS heading as a "compass" causes oversteering every time... by the time the GPS says your heading and the bearing to the mark are the same, you've swung past your course. You get better at it after a while, though. Still, it's easier to steer by compasss and I found that doing so kept the GPS heading relatively steady.
As for comments relating to the position of the GPS in the boat... head out in heavier seas and position matters. As many have stated, the GPS doesn't know where the pointy end of the boat is. But... don't pretend that the movement of the boat at its center of gravity is the same as the movement of the boat at the bow or at the transom. We all know that's not true. If a boat is yawing through 10 degrees of arc, a GPS on the transom will move through that arc and the speed and heading shown will be effected. This was apparent on the P-town trip where my handheld and the boat's fixed GPS often differed as to speed and heading. Bearings to marks though, were right on, as each GPS was still doing its job of calculating position and the distance between the two units was only about 15 ft.
Regards, Neil
s/v LIQUIDITY
Cape Dory 28 #167
neil@nrgordon.com