I (Herby) have used a GT-31 since May 2008. Previous to then I used a Garmin Etrex Venture.
When I sail I wear the GT-31 on my arm and strap the Venture to my mast where it is easy to view Max Speed and Distance Traveled etc.
The only data I post is from the GT-31.
A few days ago Lyn acquired a GT-31 and uses hers the same as mine except her Garmin is an Etrex (yellow model before the H series).
She immediately noticed a fairly large discrepancy between the analyzed results from the two GPS models. On the last 3 days the 2 sec result from her Garmin averages 1.5 knots above the GT-31.
Today I analyzed the data from both of my GPS units and the Garmin 2 sec result was 1.68 knots above the GT-31
If today, we had posted the results from our Garmins we would have both achieved PBs for the 2 Sec categories.
I think Lyn feels a bit cheated and wonders should she continue to use the GT-31.
Which is accurate one?? what explanation can I offer her.

The short non tech explaination. Use the Navi as it has better accuracy (Doppler) vs Garmin and its filtering software and trackpoints. Garmins are great for reading while sailing.

Herby,
I probably think you'll find there are some people who feel cheated by other people using anything but a GT-31.

Hi Herby,

I think you joined the GTC after we had the big debate about which to use...

Lots of very smart people have figured out that the garmins read 1 - 1.5 knots higher than they actually travel. So the GT-31 is much more accurate.


Regardless, it makes sense to always use the one device to measure your speed. So if you set your previous PB's with a GT-31, then you should use the same device to claim a new PB.

Data processing should always be done as a science, with disinterested observers. Using a different calculation method to give yourself a better number is very much frowned upon, and will only hurt yourself.

I am sick of this racist Garmin crap.
I have a Garmin Fortrex 201 which always reads less than my GT 31 and GT 11.
Where is that picture of Spotty's 50 knot at Sandy pt where his Garmin reads less than his GT 31 ??.
I have lost count of how many repairs, warranties of GT 31 I have read about and seen.

This sounds just like the age old debate about Dyno figures on car forums. GPS devices are great tools for measuring your own progress but as soon as people in different places start comparing results there will always be discrepancies. There has to be some kind of +/- accuracy on these devices, so you can't really compare results down to fractions of a knot unless everyone is using the same device on the same day at the same location.

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Squid Lips said...

This sounds just like the age old debate about Dyno figures on car forums. GPS devices are great tools for measuring your own progress but as soon as people in different places start comparing results there will always be discrepancies. There has to be some kind of +/- accuracy on these devices, so you can't really compare results down to fractions of a knot unless everyone is using the same device on the same day at the same location.

Actually, you can compare results down to a fraction of a knot -> as Nebbian said, some smart people have spent many hours determining the worst-case accuracy of quite a few brands of GPS's -> that thats why we use the GT-31's (and previously the GT-11's).

And the whole point of GPS's is so that everyone doesn't have to sail at the same location, using some kind of (impossibly hard to deploy) timing gear.