Having gotten caught in a fog not long ago, and being in relatively shallow water, I decided to anchor and wait until the weather improved. I had been navigating with my GPS and that worked quite well, but I didn't want to risk a grounding or collision with other vessels in the area, two of the few things that a GPS won't help much with. After getting the anchor down, I sat in the cockpit and listened the the VHF (I also rang my bell every few minutes to let others know where I was). I monitored a local conversation between another boat and the Coast Guard. The other boat was aground in the fog a few miles away and was asking for assistance from the USCG (which offered to refer him to a local tow boat company as no danger was apparently involved). But what got my interest was the Coast Guard asking the other boat operator if he had a GPS aboard. He did. Then the Coasties asked him for his position, which was then provided. I'm assuming that since this other boat was not moving, whatever position he was reading on his GPS would not be very accurate. Is this correct?
GPS and position
- Thread starter Warren M
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Why?
Why do you think it wouldn't be accurate? GPS gives you your current lat/long, pretty much real time. I would think it might in fact be somewhat less accurate if he were moving, since he would have moved from the position the GPS displayed during the time it took teh GPS to display it, and for him to read it, then to transmit it to the CG.
Why do you think it wouldn't be accurate? GPS gives you your current lat/long, pretty much real time. I would think it might in fact be somewhat less accurate if he were moving, since he would have moved from the position the GPS displayed during the time it took teh GPS to display it, and for him to read it, then to transmit it to the CG.
GPS accuracy when stopped
I agree with Peter - position is more accurate when stopped as there is no movement between reading and reporting. Warren, perhaps you are thinking of the inaccuracy caused in the GPS direction (course) reading. With a GPS, direction is inaccurate when you are stopped (or moving slowly) as GPS deduces direction from the change in position between readings. If position changes little between readings directional information is poor. That said, some hiking GPSs have a built in compass which is used to correct the GPS readings so that you can get a good direction reading on the GPS when standing still.
I agree with Peter - position is more accurate when stopped as there is no movement between reading and reporting. Warren, perhaps you are thinking of the inaccuracy caused in the GPS direction (course) reading. With a GPS, direction is inaccurate when you are stopped (or moving slowly) as GPS deduces direction from the change in position between readings. If position changes little between readings directional information is poor. That said, some hiking GPSs have a built in compass which is used to correct the GPS readings so that you can get a good direction reading on the GPS when standing still.
Warren, those survey transits that you see being
used these days are all GPS. The longer they are left on, in one spot, the more accurate the reading and thusly the maps they produce. Stop by a survey crew and ask them. They probably won't bite. And this method was also used to update the height of Mt. Everest. It grew two feet. Just goes to show how good the original surveyors were. It was on PBS. P.S. That first survey was about a hundred years ago. Mt. Everest has grown, what, 15,000 feet in 10,000 years. (Sea shells are that high now and can be dated) Somebody help me here, how many feet does Mt. Everest grow every hundred years? Those first surveyors were spot on!? Wow, who needs GPS?
used these days are all GPS. The longer they are left on, in one spot, the more accurate the reading and thusly the maps they produce. Stop by a survey crew and ask them. They probably won't bite. And this method was also used to update the height of Mt. Everest. It grew two feet. Just goes to show how good the original surveyors were. It was on PBS. P.S. That first survey was about a hundred years ago. Mt. Everest has grown, what, 15,000 feet in 10,000 years. (Sea shells are that high now and can be dated) Somebody help me here, how many feet does Mt. Everest grow every hundred years? Those first surveyors were spot on!? Wow, who needs GPS?
Position vs Direction
Malcolm is right on. GPS is extremely accurate (depends on the model of GPS, signal strength and other features) on Lat and Long. It can't tell direction if you aren't moving because it looks at point A to point B to guess that when you are moving. I'd say that the GPS was probably within 10-15 feet accurate or better.
Malcolm is right on. GPS is extremely accurate (depends on the model of GPS, signal strength and other features) on Lat and Long. It can't tell direction if you aren't moving because it looks at point A to point B to guess that when you are moving. I'd say that the GPS was probably within 10-15 feet accurate or better.
Fog?
I agree with everything that has been said already, but back to the original question. Could the grounded boat's GPS been off, not because of the lack of movement, but because of the fog? I am a hiker and have a GPS and love it. When I am in thick trees, however, it doesn't work worth a darn. Could the fog have been blocking some of the satellite signal, thus throwing off the location?
I agree with everything that has been said already, but back to the original question. Could the grounded boat's GPS been off, not because of the lack of movement, but because of the fog? I am a hiker and have a GPS and love it. When I am in thick trees, however, it doesn't work worth a darn. Could the fog have been blocking some of the satellite signal, thus throwing off the location?
Not
Chris, Anything that interferes with line of sight to the satelites can impair the signal. For example, thick trees, narrow valley that restricts the horizon, etc. Even indoors, it may only be necessary to get near a window to allow it to work. Water vapor in the form of clouds or fog won't do it, unless the fog is so thich the user can't see the display. Dan Jonas (S/V Feije II)
Chris, Anything that interferes with line of sight to the satelites can impair the signal. For example, thick trees, narrow valley that restricts the horizon, etc. Even indoors, it may only be necessary to get near a window to allow it to work. Water vapor in the form of clouds or fog won't do it, unless the fog is so thich the user can't see the display. Dan Jonas (S/V Feije II)
Chris, Warren in original post said
his GPS has working real well in the fog, so there you go. And remember you guys, that GPS doesn't tell you where your bow is pointed when you are underway, it tells you the heading for the whole boat. So if your helm compass is at 350 and your GPS says you are on a 355 heading, both are correct and not conflicting.
his GPS has working real well in the fog, so there you go. And remember you guys, that GPS doesn't tell you where your bow is pointed when you are underway, it tells you the heading for the whole boat. So if your helm compass is at 350 and your GPS says you are on a 355 heading, both are correct and not conflicting.
Fog, no - trees yes
Currently, the FAA is testing landing systems for planes. If there was any signal loss with clouds/fog they would not implement it. Trees are another matter, in the woods you could possibly be seeing several satellites with one or more behind a tree. signal degraded. The state of GPS receivers is changing monthly with better accuracy and quicker response. And it all depends on where that antenna is located for clear sight to the satellites. Jim S/V Java
Currently, the FAA is testing landing systems for planes. If there was any signal loss with clouds/fog they would not implement it. Trees are another matter, in the woods you could possibly be seeing several satellites with one or more behind a tree. signal degraded. The state of GPS receivers is changing monthly with better accuracy and quicker response. And it all depends on where that antenna is located for clear sight to the satellites. Jim S/V Java
Scott-maybe no conflict!
If you're seeing 350 on the GPS and 355 on the compass, there could be several explanations. 1) One instrument could be set for a reading of magnetic course, the other true. 2) You could be being set to the side by a current, causing the compass to read the direction the boat's pointing, and the GPS the direction you're actually traveling. 3) The compass could have 5 degrees of deviation on that course. 4) Some combination of all. All the reasons are normal, except for #4. (5 degrees of deviation is high!) The navigator just needs to be aware of them so he doesn't think the boat's doing one thing when it's actually doing something else. With refernce to #1, personally I like to have both my GPS and compass reading magnetic, so I don't get confused. That happens too easily to me as it is!
If you're seeing 350 on the GPS and 355 on the compass, there could be several explanations. 1) One instrument could be set for a reading of magnetic course, the other true. 2) You could be being set to the side by a current, causing the compass to read the direction the boat's pointing, and the GPS the direction you're actually traveling. 3) The compass could have 5 degrees of deviation on that course. 4) Some combination of all. All the reasons are normal, except for #4. (5 degrees of deviation is high!) The navigator just needs to be aware of them so he doesn't think the boat's doing one thing when it's actually doing something else. With refernce to #1, personally I like to have both my GPS and compass reading magnetic, so I don't get confused. That happens too easily to me as it is!
correct
Thanks for the correction, I know, as Jim stated, the technology is constantly improving. But, at one time I remember hearing that thick fog or heave clouds could through off a GPS unit and not knowing how old the unit on the grounded boat is/was....
Thanks for the correction, I know, as Jim stated, the technology is constantly improving. But, at one time I remember hearing that thick fog or heave clouds could through off a GPS unit and not knowing how old the unit on the grounded boat is/was....
Technically a GPS cannot tell you your heading
It actually tells you the direction of your track which is historical data. In other words it determines direction based on where you have already been. Not where you are going. A GPS should never be used in place of a good compass for steering a course. As Peter stated a sideways movement would throw off this value where your GPS heading would look like a diagonal line going through your boat from bow to stern. Very much different from where your boat is actually pointing. Too many times I have seen skippers take out a GPS and point it to an object away from the direction of travel only to think the GPS was malfunctioning because of it not showing the direction he was pointing it. Tim R.
It actually tells you the direction of your track which is historical data. In other words it determines direction based on where you have already been. Not where you are going. A GPS should never be used in place of a good compass for steering a course. As Peter stated a sideways movement would throw off this value where your GPS heading would look like a diagonal line going through your boat from bow to stern. Very much different from where your boat is actually pointing. Too many times I have seen skippers take out a GPS and point it to an object away from the direction of travel only to think the GPS was malfunctioning because of it not showing the direction he was pointing it. Tim R.
GPS is great for steering a course
My GPS has a compass rose and an arrow. The arrow points the direction I need to go and the rose rotates so that the direction that I am traveling is at the top of the unit. But you must always remember that neither have anything to do with the direction that the unit is pointing. Normally I have the unit oriented along the long axis of my boat the same as my compass. Then I use the compass to head in the approximate direction of my waypoint and then watch the arrow on the GPS. Once the arrow on the GPS points directly to the top of the unit then I am heading directly to the waypoint. A sailboat's track (course) rarely is the same as it's heading. We almost always slip some to the leeward and must head up to compensate. Even under power there is some prop walk that effects the course and heading. Tom
My GPS has a compass rose and an arrow. The arrow points the direction I need to go and the rose rotates so that the direction that I am traveling is at the top of the unit. But you must always remember that neither have anything to do with the direction that the unit is pointing. Normally I have the unit oriented along the long axis of my boat the same as my compass. Then I use the compass to head in the approximate direction of my waypoint and then watch the arrow on the GPS. Once the arrow on the GPS points directly to the top of the unit then I am heading directly to the waypoint. A sailboat's track (course) rarely is the same as it's heading. We almost always slip some to the leeward and must head up to compensate. Even under power there is some prop walk that effects the course and heading. Tom
Tim, yep GPS shows where you have been
and my mate is always telling me where to go, sometimes its a heading and sometimes its a biblical place.
and my mate is always telling me where to go, sometimes its a heading and sometimes its a biblical place.
GPS Reciever provides only Position Data
Any additional data such as SOG or COURSE to a waypoint are only mathematically derived data based on historical database(time and position). There is no such thing as heading data on a GPS. It can calculate COURSE to a mathematical waypoint, and COURSE data based on where you HAVE BEEN. Nothing more. Read your GPS manual: I don't think that you will find any mention of HEADING anywhere. When used in conjunction with your compass, it will provide you with the data to get from one point to another in the shortest span of time. Period. Variations between course and compass heading are the NORM and expected unless you are motoring in an indoor swimming pool. Position data has essentially the same the accuracy whether you are moving or standing still. Jon McClain
Any additional data such as SOG or COURSE to a waypoint are only mathematically derived data based on historical database(time and position). There is no such thing as heading data on a GPS. It can calculate COURSE to a mathematical waypoint, and COURSE data based on where you HAVE BEEN. Nothing more. Read your GPS manual: I don't think that you will find any mention of HEADING anywhere. When used in conjunction with your compass, it will provide you with the data to get from one point to another in the shortest span of time. Period. Variations between course and compass heading are the NORM and expected unless you are motoring in an indoor swimming pool. Position data has essentially the same the accuracy whether you are moving or standing still. Jon McClain
Garmin Map76S and Map76CS
Contains electronic compass that will give a heading even when the user is standing still, no movement required. Must be held level to get reading.
Contains electronic compass that will give a heading even when the user is standing still, no movement required. Must be held level to get reading.
GPS Position
As I understand the situation, the boat relaying her position to the Coast Guard was aground, therefore not moving. The GPS position given by the skipper would have been accurate and allowed the Coast Guard to find the grounded vessel easily. Assuming the GPS was functioning correctly. There was no mention of heading, track or course given in the original post. I think this thread was revealing in terms of how many folks mentioned GPS accuracy in terms of heading. It can only indicate your position and track over the earth, which is always a sum of vectors when moving on the water or in the air. GPS track data is quite accurate too. With WAAS, there is about 20 feet of inaccuracy which is quite good enough for recreational sailing.
As I understand the situation, the boat relaying her position to the Coast Guard was aground, therefore not moving. The GPS position given by the skipper would have been accurate and allowed the Coast Guard to find the grounded vessel easily. Assuming the GPS was functioning correctly. There was no mention of heading, track or course given in the original post. I think this thread was revealing in terms of how many folks mentioned GPS accuracy in terms of heading. It can only indicate your position and track over the earth, which is always a sum of vectors when moving on the water or in the air. GPS track data is quite accurate too. With WAAS, there is about 20 feet of inaccuracy which is quite good enough for recreational sailing.
extreme example
To exemplify the prior points, you could have your boat pointing south, but actually moving north, either because it is in reverse, or tide, current or wind are blowng you backwards. Your magnetic compass would show you pointing south, but the gps would show you moving north, independent of which way you were pointing. My gps has a feature which looks like the card is spinning, if I stop moving. This prevents me from getting an accurate heading, and cues me to the fact that that function is not accurate.
To exemplify the prior points, you could have your boat pointing south, but actually moving north, either because it is in reverse, or tide, current or wind are blowng you backwards. Your magnetic compass would show you pointing south, but the gps would show you moving north, independent of which way you were pointing. My gps has a feature which looks like the card is spinning, if I stop moving. This prevents me from getting an accurate heading, and cues me to the fact that that function is not accurate.
Electronic compass
John, that feature on the Garmin Map76 models is actually an electronic compass. The GPS signal cannot determine the direction a receiver is facing. I would be very carefull using an electronic compass. They must be calibrated very carefully to be accurate. Tim R.
John, that feature on the Garmin Map76 models is actually an electronic compass. The GPS signal cannot determine the direction a receiver is facing. I would be very carefull using an electronic compass. They must be calibrated very carefully to be accurate. Tim R.
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