Forums > Windsurfing   Gps and Speed talk

GPS distance vs Realspeed

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Created by lao shi > 9 months ago, 15 Sep 2008
lao shi
SA, 1300 posts
15 Sep 2008 7:31PM
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Hi guys
I know this question has been asked before but when I searched I could not find the answer and the posts were too old to continue.
Is the reason my distance from yesterday differs because of not logging slow speeds? Or is there another answer? Can't have walked 5.5Km
My trip meter (GT11) shows 77.76 Nmile (144Km) yet realspeed shows 138.5.
I know it is a small difference but I feel like I worked for every one of those Km's today.
Which one are most people using to post in the TC?

Pugwash
WA, 7672 posts
15 Sep 2008 6:20PM
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Definately 144km

elmo
WA, 8737 posts
15 Sep 2008 6:43PM
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Mark send me the raw data file.

lao shi
SA, 1300 posts
15 Sep 2008 8:18PM
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Thanks Elmo
Snides beat you to it so will see what he comes up with.

elmo
WA, 8737 posts
15 Sep 2008 6:53PM
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No wucken furies

snides8
QLD, 1730 posts
15 Sep 2008 8:59PM
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hey Mark, just ran it see your email....

lao shi
SA, 1300 posts
15 Sep 2008 8:35PM
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So GPSAR Pro says 139.23. Still a fair discrepancy from the trip meter. Maybe this is one that sailquick or timemachine can answer.

mineral1
WA, 4564 posts
15 Sep 2008 7:52PM
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Lao, just emailed you some data Q

Mineral

nebbian
WA, 6277 posts
15 Sep 2008 8:39PM
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I use "Total Doppler Distance" for all my tracks, don't look at any other ways of calculating it so I don't go through the anguish of not knowing which method to choose.

For the GTC I recommend everyone uses Doppler for all categories except Alphas. This seems to give the most foolproof, reliable data.

lao shi
SA, 1300 posts
15 Sep 2008 11:18PM
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Nebbs, is that different to file and properties? Using this the distance does not seem to be different between doppler and trackpoints. Normally this is what I use.

Snides sent me the rest of the data and the largest difference to the realspeed figures is .04 of a knot on 2 sec (nothing). So clearly the data is being read consistently just curious as to how the display differs. Noticed the difference a few times but never really been bothered by it.

Off topic. As the competition hots up and decimal places make a difference to rankings should we have some guidelines for the coming year? 7/8th on 2 sec separated by.04knots. 5/6th on ave by.05. Maybe only posting to nearest 1/2 Knot in 2sec, ave,alpha?

Just throwing it out there[}:)]

mineral1
WA, 4564 posts
15 Sep 2008 10:22PM
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Mark just sent you email,you will be happy I am sure

bucket
WA, 74 posts
15 Sep 2008 10:26PM
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I'm interested to know the answer to this one... I use the file properties way of finding distance.... not sure if this is the way to do it... ??

nebbian
WA, 6277 posts
15 Sep 2008 10:42PM
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lao shi said...

Nebbs, is that different to file and properties? Using this the distance does not seem to be different between doppler and trackpoints. Normally this is what I use.


Hi Lao Shi,

Yes it's different. Show your divisions, then right click on say your 2 sec division and select "Edit". You want to have it set up like this:



For the 1 hour division, you want to untick "strict" and "One result per run" because otherwise you won't get any results.

For the Alpha division, you need to select "Trackpoints" instead of "Use Dopper if available".


To get the total distance, go to the trackpoints table, and find the bottom result in the "Tot Dop Dist" column. In this example it's 106.027 km:




lao shi
SA, 1300 posts
16 Sep 2008 12:24AM
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Thanks Nebbs
That method gives me 144.03km.Which is the trip meter reading.
Also agrees with minerals analysis.
Thanks for your help guys.

Rindwider
WA, 48 posts
15 Sep 2008 10:59PM
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...you do have to be a little careful using the trip meter if you forgot to reset it from last session . Nebb's method looks fool proof.

mineral1
WA, 4564 posts
15 Sep 2008 11:04PM
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bucket said...

I'm interested to know the answer to this one... I use the file properties way of finding distance.... not sure if this is the way to do it... ??


Just sent you info and its same as what Nebs just posted.

nebbian
WA, 6277 posts
15 Sep 2008 11:22PM
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I just had a look at your file, I get 144.337 km, so you're definitely entitled to use that figure

Only problem with posting a high distance is that the next PB is just that little bit harder to get next time!

(Also checked all your other numbers, they all look good to me )

evets
WA, 685 posts
16 Sep 2008 8:19AM
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Lao I reckon you are entiltled to every last centimetre for Sunday. It was so gusty it was a hard day for distance IMO
I was shattered after sailing just short of 70ks. You and Nebs did a great job on a challenging day.



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"GPS distance vs Realspeed" started by lao shi