Simple question. (I think) GTC / GP3S / GT31 related.

GTC requirements for Distance are in Km. If the GT31 is setup for speeds based on knots, does that mean the GT31 is calculating its distance / speeds etc based on nautical miles.

Example.

On GP3S UK units are set to show distance in Miles, when I change the settings to show distance in Km, it would appear the calculation to do this is using distance based on miles and not nautical miles.

ie. Mark Bailey session (14/03/2010)

Distance=16miles, change GP3S units to km = 25.74km

However 16 nautical miles = 29.63km

I'm not quite sure which value to post on GTC ..... ok it's only a small difference on the lower numbers, but once larger numbers are acheived .. that could make a difference.

cheers
H

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OPH said...

Simple question. (I think) GTC / GP3S / GT31 related.

GTC requirements for Distance are in Km. If the GT31 is setup for speeds based on knots, does that mean the GT31 is calculating its distance / speeds etc based on nautical miles.

Example.

On GP3S UK units are set to show distance in Miles, when I change the settings to show distance in Km, it would appear the calculation to do this is using distance based on miles and not nautical miles.

ie. Mark Bailey session (14/03/2010)

Distance=16miles, change GP3S units to km = 25.74km

However 16 nautical miles = 29.63km

I'm not quite sure which value to post on GTC ..... ok it's only a small difference on the lower numbers, but once larger numbers are acheived .. that could make a difference.

cheers
H

an interesting question Howard. I calculate my distance off Realspeeds calculation not the GPS one. See if it changes when you use your programs to do the distance.

there are different miles

a mile is roughly 1.6kms
and a nautical mile is 1.852 km's
is your discrep here?

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kato said...

OPH said...

Simple question. (I think) GTC / GP3S / GT31 related.

GTC requirements for Distance are in Km. If the GT31 is setup for speeds based on knots, does that mean the GT31 is calculating its distance / speeds etc based on nautical miles.

Example.

On GP3S UK units are set to show distance in Miles, when I change the settings to show distance in Km, it would appear the calculation to do this is using distance based on miles and not nautical miles.

ie. Mark Bailey session (14/03/2010)

Distance=16miles, change GP3S units to km = 25.74km

However 16 nautical miles = 29.63km

I'm not quite sure which value to post on GTC ..... ok it's only a small difference on the lower numbers, but once larger numbers are acheived .. that could make a difference.

cheers
H

an interesting question Howard. I calculate my distance off Realspeeds calculation not the GPS one. See if it changes when you use your programs to do the distance.

Thanks for the prompt reply Craig.

As we are in the early days of GTC, I'm just copying the uploaded figures off the guys' GP3S uploads, so do not have to hand the data off their Navi's. Once our season is under way proper, the guys will fill in their own GTC data.

I'll experiment with some of my tracks though.

Can we assume that the programs , Realspeed , GPS Results , GPSAR, KA72, calculate distance based on Nautical Miles, prior to conversion to KM.

It would appear that GP3S, when switching units for distance between Miles and KM, that the Miles is not Nautical Miles, but land based Miles.

As you state ... if using session data straight off the GT31 in one of the programs the distance figure should be correct .. be it KM or Miles.

If, as I have done, is copy the session data off the GP3S session upload, and all the UK guys have the distance units set to Miles, which in effect is Nautical Miles, then when changing the units setting for distance to KM it converts the distance to KM based on land miles not nautical miles.


sorry thats a bit long winded. I only came across the issue whilst double checking the data.

I use this tool for converting NM to KM

http://www.speedsailing.com/Calculator.htm

I thought I'd see what the result would be switching the units for distance to KM in GP3S.


Perhaps I need to raise this with GP3S crew?

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OPH said...

>>>>>

Perhaps I need to raise this with GP3S crew?

I'd think so, if they are using land miles, that's most confusing.
Don't the Dutch use Kilometres on the road?????

But if you're sure that's what they're doing, and they're happy doing it, then it's just a matter of using the right conversion.

But now you've bought distance up, there's an other small anomaly.

doppler distance versus trackpoint distance.
Real speed shows both of these, and if there's no trackpoint "spike" doppler is usually greater. And this discrepancy seems to get greater, the higher the minimum speed on the GT is set.

I think most of us in Aus, with realspeed use the Doppler read out, (I've set my min speed to 2kts).
I don't think the other programs give this option.

Can't speak for how the other software works, but www.ka72.com/ performs all its calculations internally in m/s and displays results in whatever unit it is set to display, so in the free site, distance is always returned in km (whereas other speeds are in knots.) All GT-31 distances are in doppler.

Not to blow my own trumpet too hard, but the site now allows you to post direct to GPSTC from your upload. That would tend to reduce the potential for human error.

The discrepancy you mentioned is almost certainly due to the difference between nautical miles and land miles. I suspect the GP3S UK units are in miles as a nod to the archaic UK road system (speaking as an expat, I am allowed to knock it.) Are board displacements in litres or pints?

hmmmm....

I 've just checked my data on GP3S and Steve Thorps' session from the 26th Feb @ Southend.

Taking Steve's session.

Distance = 25 Miles

Now the question is ..... is that 25 nautical miles or 25 road miles. As our GT31s are set to NM I presume that figure that has been computed through GPS Results is based on Nautical Miles.

25 Nautical Miles = 46.3 Km
25 Road Miles = 40.2 Km

Steve's GTC data for that session = 40.45 Km


Still trying to work this out ..... Now checking Pete Youngs' (The Matrix - Portland Pirates) Session from the 20th March

GP3S Distance posted = 43 Miles
GTC Posted Distance = 69.58 Km

43 Nautical Miles = 79.63 Km
43 Road Miles = 69.18

GP3S Unit conversion from Miles to Km

43 Miles converted to Km = 70 Km


hmmm??????

How did you post to GP3S?

Automatically?

If so then the distances posted are probably correct in km.

Happy to double check any tracks if you want to send them to info at gpsteamchallenge dot com dot au.

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nebbian said...

How did you post to GP3S?

Automatically?

If so then the distances posted are probably correct in km.

Happy to double check any tracks if you want to send them to info at gpsteamchallenge dot com dot au.

Spot on Nebs ..... yep tracks are auto uploaded typically from GPS Results.

Gps results - "best speed list" displays the sessions distance in km, which equates to GP3S session distance when distance is set to km under the settings tab.

After rounding up or down its difficult to say what km is being converted to when miles is selected under the settings tab, miles or nm.

I need to check my tracks on GP3S for sessions over 50km, where any rounding up or down from the auto upload, of the distance sailed will be negligable.

At first glance though it would appear that km are being converted to road miles not nautical miles on GP3S.

Thanks to all for your feedback and help.

@ Nebs,
Whilst we are on the subject, following on from Mike's comment, is there a requirement in GTC for min GPS speed setting for logging data. I ca'nt help but think of the number of speed spots around the world, especially when sailing broad to super broad that involve as much walking as sailing, usually back up wind, therefore total distance logged = 50/50 sailing / walking. hmmm.

@Dylan, thanks for the info, we had a look as KA72 a few months ago .... sounds like a re-visit is required ..... not had a need to use the GPS for nearly 6 months!

OPH,
As Dylan mentioned, his KA72 auto-upload eliminates potential for user error in transferring data manually. Best thing since sliced bread in my opinion and maybe the GPSTC hierarchy could look at making it de rigueur for logging of all sessions. Certainly put everyone on the same level playing field and would allow anyone to download other members tracks for analysis.

PS - I reckon if you have to walk back to your start point during a session then including 'non-sailed' distance isn't an issue. Seriously at most you'll only do it a maximum of 10 times at a maximum of 1km each time. Any further and you'd have some serious chafing issues to deal with (well I would anyway).

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sausage said...

OPH,
As Dylan mentioned, his KA72 auto-upload eliminates potential for user error in transferring data manually. Best thing since sliced bread in my opinion and maybe the GPSTC hierarchy could look at making it de rigueur for logging of all sessions. Certainly put everyone on the same level playing field and would allow anyone to download other members tracks for analysis.

PS - I reckon if you have to walk back to your start point during a session then including 'non-sailed' distance isn't an issue. Seriously at most you'll only do it a maximum of 10 times at a maximum of 1km each time. Any further and you'd have some serious chafing issues to deal with (well I would anyway).

Thanks Brian, yep I agree KA72 with the auto upload is the solution for eliminating user error.

There has been talk recently of a single upload to GP3S and GTC ....

I'd be happy to use KA72 so long as I can upload the teams data on their behalf.

Distance tho mate would be an issue if GTC is won and lost on 10km of walked session. - highly unlikely .... but ...

The Dutch crews are taking the challange to beat SRM with real vigour, Europe has n't fired up after it's prolonged winter.... when it does ... well time will only tell .. but I expect this will be a very close gtc comp this year.

If you want to increase your distance by walking your kit instead of sailing it, then good luck to you!

It's more a handicap than an advantage...

In the scheme of things it really doesn't matter. I always claim distance walked because I couldn't be @rsed finding the bits that I was walking, as opposed to slogging...

Just use the total distance from when you turned it on to when you stopped. And have fun!!

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nebbian said...

If you want to increase your distance by walking your kit instead of sailing it, then good luck to you!

It's more a handicap than an advantage...

In the scheme of things it really doesn't matter. I always claim distance walked because I couldn't be @rsed finding the bits that I was walking, as opposed to slogging...

Just use the total distance from when you turned it on to when you stopped. And have fun!!

Ok mate thats fine.

Some of the Independant forecasters are pointing to the hottest summer on record
over here ..... lol, 4 months frozen out in winter and 4 months of hot calm weather over summer ... will we ever get to sail speed this year?