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Old 28 April 2004, 16:36   #1
Country: UK
Join Date: Apr 2004
Posts: 18
GPS Speed

How accurate is the speed given on a GPS set in relation to actual speed?

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Old 28 April 2004, 16:58   #2
Country: UK - England
Town: Weymouth
Boat name: Splitz
Make: Ring
Length: 6m +
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"Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on."

link to my pasted bit

Incredibly accurate compared to boat speedo. Don't forget tide though as GPS will be measuring SOG (speed over ground) not through the water. If you are after max speed try to do as tide is turning or do 2 runs.

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Old 28 April 2004, 17:00   #3
Country: UK - England
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Very accurate BUT

Your GPS speed is averaged of a period of time. If you are zig-zagging around, say 3 zigs and 2 zags from a to b, depending on the averaging, the GPS may just calculate your speed between a & b in a straight line because it does not know you have been to a1,2 & 3 and b1 & 2 in between. Your indicated speed will be less than your actual speed.
GPS speed is Speed over the Ground (SOG). If you are traveling with a following current your GPS will read your speed + the speed of the current. If into a current SOG + Speed - Current.
On the satalite page you will see something loke DOP which gives an indication of the current accuracy. If there is poor coverage, speed will be less accurate because the GPS is not quite so sure of where it is and where it was.
All in all, it will be much more accurate than the speedo in your car.
Mark H
"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools" Douglas Adams
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Old 29 April 2004, 04:41   #4
Country: UK - England
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depends if the US are pissing about with it. I know they have signed a thingy saying that they wont but my GPS regesterd 35Knots in astern on just after september the 11, Now call me sceptical but this is quite a coensidence. Only lasted for about 30 min though. o and i had good sat coverage. The model of you GPS will affect the accuracy. I have a Garman 12 and a magelan 315 and the magelan refreshes much quicker. Might be software but I expect it is the processes (duel or tri band) don't know though.
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Old 29 April 2004, 06:19   #5
Country: UK - England
Town: Cambridge
Boat name: DIZZY
Make: Aphrodite 101
Length: 5m +
Engine: Nanni
Join Date: Jan 2004
Posts: 137
When enquiring about fitting a speed transducer to my Garmin 188 I was advised not to bother as the GPS is accurate to one tenth of a knot.
As stated before the tidal stream is included in the GPS reading so you need to take this into account.

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Old 29 April 2004, 14:21   #6
Country: UK - England
Town: Penzance
Boat name: Dark Angel
Make: Ribcraft
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Join Date: Oct 2003
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For acurate speed readings check that the 'smoothing' setting is set low around 5 secs or lower this will prevent any problem with course change.


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