mick
Member
My boat speed as measured by GPS differs by about 25% from that measured by log. Which is likely to be more accurate? Thanks in advance.
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GPS v Log
- Thread starter mick
- Start date
tcm
...
Check the GPS is measuring nautical miles per hour - but then it will be 100% accurate when it works, and little paddly things won't.
Note that the gps measures speed over the ground, whereas the log measuresspeed thru the water - which might be a higher number if going agint tide or lower if going with tide.
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Note that the gps measures speed over the ground, whereas the log measuresspeed thru the water - which might be a higher number if going agint tide or lower if going with tide.
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S
Skyva_2
Guest
The GPS measures speed over ground (SOG), and will usually be accurate.
The boat log measures speed thru the water. If its a paddlewheel it may be affected by weed, barnacles, location, etc, etc. To get SOG as measured by GPS you need to add or subtract the effect of tide. Tide does not affect speed thru the water.
Is your error consistently 25 % plus or 25 % minus?
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The boat log measures speed thru the water. If its a paddlewheel it may be affected by weed, barnacles, location, etc, etc. To get SOG as measured by GPS you need to add or subtract the effect of tide. Tide does not affect speed thru the water.
Is your error consistently 25 % plus or 25 % minus?
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charles_reed
Active member
As already pointed out your GPS measures speed over ground and your log speed through water.
However, it is almost impossible to get a paddle-whell log to be accurate over the whole speed range of a sailing yacht. If it's accurate at the top it will grieviously underread at the bottom and if you calibrate it to read .
So in a word the GPS is more accurate.
I calibrate my log to be accurate in the mid-range 5 - 7 knots.
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However, it is almost impossible to get a paddle-whell log to be accurate over the whole speed range of a sailing yacht. If it's accurate at the top it will grieviously underread at the bottom and if you calibrate it to read .
So in a word the GPS is more accurate.
I calibrate my log to be accurate in the mid-range 5 - 7 knots.
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charles_reed
Active member
As already pointed out your GPS measures speed over ground and your log speed through water.
However, it is almost impossible to get a paddle-wheel log to be accurate over the whole speed range of a sailing yacht. If it's accurate at the top it will grieviously underread at the bottom and if you calibrate it to read .
So in a word the GPS is more accurate.
I calibrate my log to be accurate in the mid-range 5 - 7 knots.
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However, it is almost impossible to get a paddle-wheel log to be accurate over the whole speed range of a sailing yacht. If it's accurate at the top it will grieviously underread at the bottom and if you calibrate it to read .
So in a word the GPS is more accurate.
I calibrate my log to be accurate in the mid-range 5 - 7 knots.
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Birdseye
Well-known member
depends a bit on which gps and which log. i have an old garmin 45 handheld, and this cannot use waas (ie dgps) and averages speed over a fairly short time base. so it fluctuates considerably. i also have a new garmin 76 which uses waas and can have the time base adjusted - this gives very accurate speeds.
as already pointed out, your gps measures speed over the ground whilst your log is through the water (ie ignores tides) . and it needs zeroing for your boat. my new log was 25% out as it came out of the box, so i adjusted it by comparing the distances it recorded with the distances travelled in a no tide situation.
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as already pointed out, your gps measures speed over the ground whilst your log is through the water (ie ignores tides) . and it needs zeroing for your boat. my new log was 25% out as it came out of the box, so i adjusted it by comparing the distances it recorded with the distances travelled in a no tide situation.
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MainlySteam
New member
Hi Birdseye. I assume that you are in the USA because WAAS does not operate in Europe yet as far as I am aware as the satellites are not operational (I believe that Europe will be served by Inmarsat AOR-E (Atlantic Ocean Region-East) and IOR (Indian Ocean Region) and the European Space Agency satellite, ARTEMIS). The European system EGNOS is not due for operation until 2004 (well that was the last date I heard) and that the current test satellites broadcast do not use signals which I understand, but stand to be corrected on, disables the receiver. Also WAAS is not dGPS. If anyone has any information that changes this I would be interested in it.
Not just being pedantic, but if you are in Europe there must be other explanations of your increased accuracy, and I wonder what they are.
John
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Not just being pedantic, but if you are in Europe there must be other explanations of your increased accuracy, and I wonder what they are.
John
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oldharry
Well-known member
get rather the same effect when I tried the GPs against the car speedo - the mileometer on the car underreads more than 12%, although the speedo is only 2 -3 mph under at 70. Which means my car with just under 70k on the clock has actually done over 80k.... "clocking? - honest guv, no ones touched the speedo"
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AndrewB
Well-known member
Car Speed/distance
Unusual. All of the five most recent cars I have owned and tested, both with GPS and relative to kilometer posts, have overread by 8-9% on the speedometer, but the odometer has been spot on. So when the speedo reads 65 it's actually doing 60mph. I've assumed this is industry standard - it makes the car seem faster than it really is, and keeps drivers within speed limits, so I guess everyone's happy. At the same time the distance run is correct. The maximum error legally permitted is 10%.
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Unusual. All of the five most recent cars I have owned and tested, both with GPS and relative to kilometer posts, have overread by 8-9% on the speedometer, but the odometer has been spot on. So when the speedo reads 65 it's actually doing 60mph. I've assumed this is industry standard - it makes the car seem faster than it really is, and keeps drivers within speed limits, so I guess everyone's happy. At the same time the distance run is correct. The maximum error legally permitted is 10%.
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richardandtracy
New member
Re: Car Speed/distance
There's a reason for the over-read on the speedo.
The speedo, whether electronic or mechanical display, uses magnetic induced eddy current drag in an aluminium plate acting against a spring or transducer. The magnets will always loose power as time goes on, so the magnets are made over strong to begin with, leading to a speed over-read when the vehicle is new. After 25 years, you may find it under reads by a certain amount (though with modern rare earth magnets the loss is slower & less significant).
I imagine one day opto-electronic transducers will take over, which will always read accurately, but we aren't there yet.
Regards
Richard.
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There's a reason for the over-read on the speedo.
The speedo, whether electronic or mechanical display, uses magnetic induced eddy current drag in an aluminium plate acting against a spring or transducer. The magnets will always loose power as time goes on, so the magnets are made over strong to begin with, leading to a speed over-read when the vehicle is new. After 25 years, you may find it under reads by a certain amount (though with modern rare earth magnets the loss is slower & less significant).
I imagine one day opto-electronic transducers will take over, which will always read accurately, but we aren't there yet.
Regards
Richard.
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dickh
New member
Re: Car Speed/distance
Many years ago, during my apprenticeship at AC-Delco, I worked in the speedo repair shop foe a while and we used to try and calibrate the speedos to read accurately at 30mph and 70mph. If they were accurate at 30mph(which is what we did) they usually under read at 70mph - apart from the Police speedos which we made as accurate as possible at 30 & 70mph.
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I'd rather be sailing...
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Many years ago, during my apprenticeship at AC-Delco, I worked in the speedo repair shop foe a while and we used to try and calibrate the speedos to read accurately at 30mph and 70mph. If they were accurate at 30mph(which is what we did) they usually under read at 70mph - apart from the Police speedos which we made as accurate as possible at 30 & 70mph.
<hr width=100% size=1>dickh
I'd rather be sailing...
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B
bob_tyler
Guest
Re: Car Speed/distance
On a car, if the tyres are worn the diameter of the wheels will be slightly less. The wheels will therefore turn faster for the given speed over the ground and the speedo will read higher.
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On a car, if the tyres are worn the diameter of the wheels will be slightly less. The wheels will therefore turn faster for the given speed over the ground and the speedo will read higher.
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seahorse
New member
Re: GPS v Log, some numbers.
This last weekend I sailed a return trip to a harbour 25 miles away. My Garmin45 indicated boat speed varying between 2 &3 knots whilst my log needle sulked in the corner. On the return trip un a reach readings were 2.8 up to 7.4 on a reach, the log indicated 3 kts. Track milage from the GPS was 56 mls whilst the log indicated 21 mls!!
No wonder so many ships got wrecked using traditional nav.
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This last weekend I sailed a return trip to a harbour 25 miles away. My Garmin45 indicated boat speed varying between 2 &3 knots whilst my log needle sulked in the corner. On the return trip un a reach readings were 2.8 up to 7.4 on a reach, the log indicated 3 kts. Track milage from the GPS was 56 mls whilst the log indicated 21 mls!!
No wonder so many ships got wrecked using traditional nav.
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Birdseye
Well-known member
Hi MainlySteam. Sorry, but you're wrong! The European equivalent of waas has been trialled and the signals have been left switched on, so a good 50% of the time here in S Wales we can get differential gps on waas enabled gps equipment. And it is differential - its just that the correction signal is coming from a satellite rather than a ground station.
To be honest, I think that the greatest accuracy increase is coming from the ability to increase the time base over which the speed is worked out. The old garmin 45 was an 8 channel set and seemed to average over about2 secs. So even when the new set is working without waas signals (or whatever the european name is) its still way better.
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To be honest, I think that the greatest accuracy increase is coming from the ability to increase the time base over which the speed is worked out. The old garmin 45 was an 8 channel set and seemed to average over about2 secs. So even when the new set is working without waas signals (or whatever the european name is) its still way better.
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MainlySteam
New member
Hi Birdseye - I think we are actually in almost complete agreement.
It is EGNOS, the European equivalant of WAAS, which is turned on for testing in Europe and I said that was so, but it is not WAAS. They are both satellite based augmentation systems, however they are different systems but interoperable (to the best of my knowledge they do not yet share almanacs though). I stated that EGNOS was transmiting a "do not use" signal and I actually asked for clarification as to whether that inhibited recreational EGNOS capable receivers - apparantly it does not, and thanks for the information on that.
It is my understanding that you will end up with both systems in Europe because, as I said, it is my understanding that both WAAS and EGNOS satellites will cover Europe. The systems are driven by civil aviation - Japan is apparantly also developing its own system (again for civil aviation it must be interoperable with the others) for use in its own Flight Information Region.
I stand by my comment that WAAS and EGNOS are different to dGPS although they all use similar fundamentals, but dGPS is not a satellite based augmentation system; dGPS will remain operating instead of or in parallel to WAAS and EGNOS, if only because of the already installed infrastructure.
I also completely agree with your comments on accuracy.
If I appeared picky, I am sorry and have given myself a smack on the back of my hand. There is not a lot of information out there on the systems and some confusion (I readily acknowledge my knowledge is not expert, it is just that for a work assignment I had the need to look at the coverage of both systems); maybe it would be the basis of an informative article in one of the ybw magazines as to what is happening and how it all works (unless they have already done so and I missed it).
John
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It is EGNOS, the European equivalant of WAAS, which is turned on for testing in Europe and I said that was so, but it is not WAAS. They are both satellite based augmentation systems, however they are different systems but interoperable (to the best of my knowledge they do not yet share almanacs though). I stated that EGNOS was transmiting a "do not use" signal and I actually asked for clarification as to whether that inhibited recreational EGNOS capable receivers - apparantly it does not, and thanks for the information on that.
It is my understanding that you will end up with both systems in Europe because, as I said, it is my understanding that both WAAS and EGNOS satellites will cover Europe. The systems are driven by civil aviation - Japan is apparantly also developing its own system (again for civil aviation it must be interoperable with the others) for use in its own Flight Information Region.
I stand by my comment that WAAS and EGNOS are different to dGPS although they all use similar fundamentals, but dGPS is not a satellite based augmentation system; dGPS will remain operating instead of or in parallel to WAAS and EGNOS, if only because of the already installed infrastructure.
I also completely agree with your comments on accuracy.
If I appeared picky, I am sorry and have given myself a smack on the back of my hand. There is not a lot of information out there on the systems and some confusion (I readily acknowledge my knowledge is not expert, it is just that for a work assignment I had the need to look at the coverage of both systems); maybe it would be the basis of an informative article in one of the ybw magazines as to what is happening and how it all works (unless they have already done so and I missed it).
John
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BrendanS
Well-known member
The do not accept signal is accepted and used by some receivers, but not by others. This was widely discussed on several internet sites a year or so ago. Two of the satellites are in test use, though things are continually being swapped around, so you cannot rely on them.
If you do an internet search, there is a huge amount of information available , it just takes time and patience to search through it all.
<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide
If you do an internet search, there is a huge amount of information available , it just takes time and patience to search through it all.
<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide
charles_reed
Active member
Methinks
the log paddle needed a good clean - or the log had never been calibrated.
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the log paddle needed a good clean - or the log had never been calibrated.
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MainlySteam
New member
Thanks for that feedback Brendan, and also that from Birdseye.
I have a practical interest as we have specified WAAS GPS plotters in a couple of new build vessels whose operating area may be covered by both systems. When the boats are delivered next year I will probably go to see them unloaded and run up, so will try and have a little play around with WAAS and EGNOS if they have coverage by then.
John
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I have a practical interest as we have specified WAAS GPS plotters in a couple of new build vessels whose operating area may be covered by both systems. When the boats are delivered next year I will probably go to see them unloaded and run up, so will try and have a little play around with WAAS and EGNOS if they have coverage by then.
John
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seahorse
New member
Re: Methinks
Me thinks that like most sailors they buy a boat & hope everything works like it's supposed to, a bit like buying a car isn't it?
Like many of the threads on this forum this one seems to have gone off from what i thought was a practical question & seem to have been hijacked away by experts who seem to be interested in scoring points off one another, pity really.
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Me thinks that like most sailors they buy a boat & hope everything works like it's supposed to, a bit like buying a car isn't it?
Like many of the threads on this forum this one seems to have gone off from what i thought was a practical question & seem to have been hijacked away by experts who seem to be interested in scoring points off one another, pity really.
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MainlySteam
New member
Re: Methinks
Seahorse - Sorry if you feel that way. Hope you are not trying to hijack the thread into a discusssion as to whether experts can contribute or not. If so I suggest you start another one.
John
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Seahorse - Sorry if you feel that way. Hope you are not trying to hijack the thread into a discusssion as to whether experts can contribute or not. If so I suggest you start another one.
John
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