GPS Speed

[Capt_Marlinspike]

I have a Raymarine ST60 log which seems to show the correct speed through the water, based on passage times. The Garmin 128 GPS seems to consistently show a speed of about 20% more than the log when sailing at slack tide.
As the ST60 is measuring speed and the GPS is (presumably) measuring velocity I would expect the GPS to under read rather than over read. Anybody understand this?

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[Guest]

The Log speed through water should really not equate to passage times ..... the GPS should in fact.

The Log shows speed through water that is generally moving, and therefore causing error and it cannot indicate SOG except in true still water.

The GPS shows true SOG as calculated between position fixes of the GPS ....... in practice the time intervals are so short that the speed jumps around a bit unless you employ averaging.

The GPS is probably the most accurate speed indicator you have on your boat - but again it is NOT speed through water.


<hr width=100% size=1>Nigel ...
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[oldsaltoz]

G'day Alan,

Your GPS is reading speed over ground (Not speed thru the water), even at slack tide you have other effects such as wind and local currents.

I would go by the gps for location and speed over ground, all logs are effected by a number of variables, tack, location, clear water, turbulence, condition of paddle wheel, salinity/density and the list goes on.

Hope this helps....

<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[Guest]

Forgot to add ...

Despite many 'pub-storys' a lot of boaters tend to repeat things like when leaving mooring / arriving morring etc. What I mean here is the state of tide.

EG : I like to leave my mooring in Marina at 1-2 hours BEFORE High water, returning at about 1 hour before or on High Water. This means that I tend to have similar tidal effects and also tend to cover similar routes across the solent wherever destination is in Solent.

What does this mean ? It means for me to make a statement about Log / GPS etc. is maybe taking it further than really is available. Many of us tend to only look around at instruments when needed or not in close situation and we can take that time !!

I know and understand that many will shoot me down for the above - but how many out there seriously use or need all the instruments etc. they have ? Many of us are coastal hopping, daysailing, tripping across spaces like Solent etc. With the increase of availability and decrease of price - many items such as GPS are becoming common-place on all sorts of boats, cars etc. etc. Even to the GPS watch used by sports people etc.......

Are we losing sight of some of the older things and skills ??

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[Capt_Marlinspike]

I think I probably phrased the question badly. When i said the log seemed to read right from passage times what I meant was that the speed indicated on the log when compared with calculated passage times "allowing for tidal stream". If the speed shown on the GPS was right then I should be arriving much sooner!
I understand the difference between speed over ground and speed through the water. My issue is that the GPS reads more than the log regardless of whether I have a fair or foul tide. When the tide is against me the difference between GPS and log is smaller but the GPS still generally reads more.


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[Guest]

OK .... I now see where you are coming from ..... (forgive the pun !!!)

I also did not mean to insult by explaining SOG / COG / STW etc.

Interesting ............ I thought a GPS of mine was faulty once - so I put it in the car and stuck it in a Mobile phone holkder on the windscreen ..... switched on and drove round with it set to Km/Hr .......... it was spot on !!! even following the increase and decrease of speed with only a short few seconds delay !

Have you tried it in the car ?? It's the quickest and easiest way to do it ..... but of course if its a mounted set with external fixed antennae ....... have you a mate with a hand-held to carry and check your GPS against while on boat ?

Can you get a mates boat to side-saddle with you and compare log readings ?

Have you got measured mile markers along the coast near you or fixed transits etc. that you can accurately measure speed / time against to compare ?

I think you have to actually nail down which and what by using external means of comparison.

Interesting and probably frustrating !!

I know this sounds stupid - but you do have both log and GPS on Nautical Miles / Knots ????? Statute miles and Nautical miles have about 800 ft diff. = about 13-14% ...... coincidence ????



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[bedouin]

That is very difficult to explain. As others have said, your GPS will measure speed over ground totally accurately so in still water with no significant wind, tide or currents the two should measure identically.

It is not unusual on a cross-tide passage to cover 10-20% greater distance over the ground compare to through the water, with even greater differences being possible when going with or against the tide. Without knowing more about the passages on which you have checked the Log vs GPS

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[vyv_cox]

The accuracy of the GPS is normally excellent but I suppose there could be some reason why it was incorrect. Can you verify it against another?

Logs are notoriously inaccurate. Not only can they be influenced by their position in the hull but they are easily fouled and may never have been calibrated when installed. Also, they are often only accurate within a certain range and can be very different outside it.

Are you sure about your passage time observation? The log inputs the speed but also the distance, so it will take the same time to reach a shorter distance going slowly as a longer one going quickly. If you see what I mean.

My money is on the GPS.

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[Capt_Marlinspike]

Looks like the best solution will be to try a measured mile. I think someone told me that there was a measured 1/2 mile in Poole. Anyone know?

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[Guest]

Make sure you know WHAT mile ..... statute or Nautical !!


<hr width=100% size=1>Nigel ...
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[PerL]

When you put it in your car, did you compare with a fix GPS in the car? I've tried my GPS (for cars) in a number of cars and it always show a slower speed than the car. From what I gather, car manufacturers always set their meters to show a higher speed than what is actually true (with about 10%) in order to decrease road accidents and whatnot.

Without getting stuck on that, it seems as if your GPS showed the same as your car, there's something wrong with the GPS. Of course, I could be wrong or there could be country-specific differences.

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[Guest]

As I was told - that practice stopped years ago ......

I can also say that I have been asked as a young lad to drive along at various speeds in obvious steps with a police car behind to check my speedo ..... OK a few years ago - but if the speedo was set wrong - then I would look as though I was cheating !!!!


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[PerL]

Funny, I've tried with two cars that I know to be less than a year old. Same error slight difference in percentage though.

GPS should not be able to consistently tell the wrong speed. Car settings could be country specific, even though that seems strange.

Nowadays, I always log my driving on the GPS, since then at least I know which speed I have held should I get caught speeding ...

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[Paul_H]

Why not use your GPS to calibrate your own measured distance eg. between two buoys or transits.


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[Miker]

I've given up trying to calibrate my log. The best I could do was a measured distance between two buoys, travelling both ways at what I thought was slack tide. Both the handheld and fixed GPS show identical information, and I really only use the log to tell me how much I am gaining or losing to the tide, which usually seems about right, and playing about with the sails. My crew is a compulsive tweaker!
Overall, I suspect that the log is still reading a bit high as the distances seem longer than calculated, but then I doubt if I ever sail in a perfect straight line.

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[powerskipper]

There is a mesured 1/2 mile just round corner from swanage. It on the charts. It is the twin white post on the hill just past light house, good luck

<hr width=100% size=1>Julie

 

[Birdseye]

unlikely to be a gps error, but if you are comparing instantaneous speed readings on the two you will get funny and variable results. best way is to compare the distance recorded by the two over say a mile (problem cannot be a gps clock error). if you still have a difference of gps overreading, then either you have it set on statute miles or your log is underreading.

how do you know there is no tide or current? tidal diamonds are very specific to a location, and the situation can be different only a hundred yards away.

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[brianrunyard]

There's a measured 1/2 mile in Poole harbour between Buoys 68 and 72 in the Wreham Channel, see http://www.phc.co.uk/guide.htm and click on '8'.

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard