I keep reading posts that say the Garmin eTrex/Foretrex GPS's are much less accurate than the GT-11/GT-31, but I have not seen any figures quoted.

What is the generally accepted level of inaccuracy of the Garmin units? Are we talking several knots, or only fractions of a knot?

Has anyone done any "side-by-side" tests?

Thanks,
Harrow.

Hi Harrow. There has been a lot of side by side testing done now over a couple of years and the results are pretty clear. The differences are in the variation between units. The variation run to run in the Garmin using trackpoints can be up to over half a knot. The differences in the latest GT-31 can be as little as 0.02 of a knot. When rankings or event results are decided by less than a tenth of a knot, this is very significant. Using Garmins it would be possible to have ten competitors within half a knot and the actual finishing order could be entirely random or the reverse given the margin of error.

Here is an excellent reference if you want to delve a bit more into statistics: http://www.gps-speed.com/tests.html

There is a whole lot of discussions about Foretrex accuracy, on GPSSS. eg:

www.gps-speedsurfing.com/default.aspx?mnu=forum&forum=1&val=2604
www.gps-speedsurfing.com/default.aspx?mnu=forum&forum=1&val=4231
www.gps-speedsurfing.com/default.aspx?mnu=forum&forum=1&val=5925

Some of the first tests done where static testing to determine what things like positional drift (ie: sit your GPS outside for a while, then look at the track...). Some tests were done to determine how big the error was with under differing atmospheric conditions; numerous devices were placed side-by-side, then taken for a long drive, and so on.

Comparisons have been done between video-timing, navi gt-11 and a few other devices. In particular, lots of side-by-side track comparison was done to determine the probability of error on a per-point basis, and also to determine a subtraction-value to apply to each gps.

The worst-case value for a Foretrex, can be about 2kn; on a good day, it could be less than 0.1kn.

Thanks sailquik/mathew,

That is exactly the sort of info I was after. I really want a unit with external batteries for extended cycling/camping/hiking holiday trips, so for a mug like me who is just curious if he is managing 24 or 28 kts, the eTrex 'H' sounds good value at $127. Also, it now has a new high sensitivity antenna, so perhaps this will add to it's accuracy.

Regards,
Harrow.

Select to expand quote

Harrow said...

Thanks sailquik/mathew,

That is exactly the sort of info I was after. I really want a unit with external batteries for extended cycling/camping/hiking holiday trips, so for a mug like me who is just curious if he is managing 24 or 28 kts, the eTrex 'H' sounds good value at $127. Also, it now has a new high sensitivity antenna, so perhaps this will add to it's accuracy.

Regards,
Harrow.

If you really want a Garmin gps and you intend to use it for sailing as well, the best choices are the "Edge" or the 'Legend Color'. Both will record one second trackpoints without the large grid effect of the Foretrex/Geko so they have far less error, but neither (afaik) can record and save the 'Doppler' info which gives by far the best speed accuracy. The GT-31 is definitely in a different class as far as accuracy is concerned and has a heap of very innovative features that nothing else has at all like the speed genie.
I can get more than a whole day of sailing from the battery of a GT-11/GT-31 and there are ways to prolong it by writing less frequently to the memory if hiking or biking. The Original GT-11 was specifically aimed at bike riders and is still a very good unit for this purpose.(As is of course the GT-31).

The Navi also allows car charger and laptop/computer charging so if you are on a cycling trip you could charge at an internet cafe.

Don't forget the Navi with it's SD card can record months of data at 1 second intervals... a Garmin just hours unless you increase the interval time significantly.

Maybe a solar USB charger would be the go? Not sure how long it takes to charge up a device with something like this but it might help.

http://tinyurl.com/6jz92v

Just read its description, looks quite good as it has an inbuilt battery and you can charge the battery up at home and then charge up your USB device. Not sure how smart it is and whether it would overload the Navi's battery.

Ah, I get it all now. USB gives +5 volts supply, which is used to charge all sorts of things.

I already have a cigarette socket in-car USB charger for my HP Pocket PC that gives a +5 volts USB supply, so no doubt this can be used to charge the GT-31 on a trip.

Only one question now....with the rechargeable battery being built-in...how long until it dies iPod style? Anyone successfully changed over the internal battery?

Not sure if the Navis have been around long enough for them to suffer from dead batteries. If you get three years out of a gadget like that then thats probably good going. The GT-11 I bought from Wind Surf and Snow came with a cigarette lighter charger too.

The battery probably can be changed at a hobbie shop but it involves splitting your Navi apart which probably wont do the waterproofing much good. The original is stuck to the inside back of the case with double sided tape. Wouldn't want to split the alu battery covering trying to get it off. Li-ion batteries burn explosively if punctured or shorted.

I find I can get several sessions on a charge. There are probably a 1000 charges in the battery. That's a lot of use.

Select to expand quote

yoyo said...

The battery probably can be changed at a hobbie shop but it involves splitting your Navi apart which probably wont do the waterproofing much good. The original is stuck to the inside back of the case with double sided tape. Wouldn't want to split the alu battery covering trying to get it off. Li-ion batteries burn explosively if punctured or shorted.

I find I can get several sessions on a charge. There are probably a 1000 charges in the battery. That's a lot of use.

Yoyo, I am curious now....may I ask why you have such intimate knowledge of the inside of your Navi?

Judging from several of my laptop and camera Li-ion batteries, battery life starts to decline steadily after 3 years. Anything after that is a bonus. This is regardless of whether you have recharged it 100 or 1000 times.

How much is a GT-11 gps complete with waterproof bag?

Select to expand quote

Itch said...

How much is a GT-11 gps complete with waterproof bag?

Got mine direct from Boards Crazy (Simon) last season, came with GT-11 (now replaced with the GT-31) AquaPac card and card ready kit for around $250 delivered all they way to WA. First GT-11 failed the screen readout inside 6 months, Simon posted me a replacement two days later Replacement hasn't given any issues

AL green at Caloundra city sailboards still has some GT11 for $150.

Ive had my Navi almost 2 years now and no Probs touch wood

$150, I'd grab one for sure

Yesterday my Garmin Venture gave a result .74 kn faster than my GT-31.
Don't know how to prove it but I believe the GT-31 is the most accurate because it uses Dopler.
Herby always posts his Dopler results, but because Lyn uses a Garmin, well her results are Trackpoint
The GT-31 is much lighter (weightwise) than the Garmin.

Herby

Select to expand quote

Herbylyn said...


Yesterday my Garmin Venture gave a result .74 kn faster than my GT-31.
Don't know how to prove it but I believe the GT-31 is the most accurate because it uses Dopler.
Herby always posts his Dopler results, but because Lyn uses a Garmin, well her results are Trackpoint
The GT-31 is much lighter (weightwise) than the Garmin.

Herby

Is that Garmin 1 second trackpoints? With the foretrex and Geko the trackpoints results can often be of this order of difference compared with Doppler, partly because of the grid effect (the distance calculated includes all the zig zags and is therefore longer than it should be) and partly because of atmospheric interference. In the 1 second Garmins that save the trackpoints with higher precision (Edge and Legend C and possibly a few others) the differences are usually not quite as great.

Anyone know the going price for a GT31? - and how do they differ to the GT11?

In reply to Sailquick.....It is a Garmin Venture and set to 2 sec intervals.

Unfortunately the Venture will only save 2048 tracks and then starts to wrap which is pretty useless for recording Distance Traveled. If I set it at 1 sec then it will only last for half the present distance. It is handy for me as I keep it on mast with Big numbers and keep an eye on the display. I also carry a GT-31.

Herby. The display speeds of all the various gps units are actually derived from Doppler calculations and tend to be very accurate. Unfortunately, most GPS units are not set up to record this data for later use as we can with the GT-11/31. I used to use my Foretrex the same way but now I have a couple of GT-11/31's I use one of them on my forearm for continuous feedback instead.

Thanks Sailquick, At least I am now better informed and I understand it a little better. What a boon the little GPS has become to the windsurf community.

Select to expand quote

pedro e said...

Anyone know the going price for a GT31? - and how do they differ to the GT11?

No knowledge about prices but the GT-31 has a different model GPS chip. It has a Sirf3 chip v's the GT-11 which had a Sirf2 chip. The new chip is much more sensitive and is able to pick up and use very weak signals. This was brought home to me recently at the Al Green event when I had a GT-11 and GT-31 in the same Aquapac. Late in the afternoon there were only 6 satellites in the sky and two of these were very low in the horizon. The GT-11 could not pick them up and had only 3-4 signals which makes the tracks invalid. The GT-31 was able to pick up all 6 satellites and produce data that was valid and useful. I was glad I had it because a couple of my best runs were done during this period and they would not have counted in my average if I was relying only on the GT-11. I must say that this was the first time I had this problem with the GT-11 for such an extended period but it only takes loss of sats for a few seconds to invalidate a run that could have been a new PB.

Select to expand quote

sailquik said...

Herby. The display speeds of all the various gps units are actually derived from Doppler calculations and tend to be very accurate. Unfortunately, most GPS units are not set up to record this data for later use as we can with the GT-11/31. I used to use my Foretrex the same way but now I have a couple of GT-11/31's I use one of them on my forearm for continuous feedback instead.

Oh!!!! Now here is a new fact coming out. So, if I do not intend to do any downloading, are you saying that the displayed max speed is just as accurate on the Garmin units??

Select to expand quote

Harrow said...
Oh!!!! Now here is a new fact coming out. So, if I do not intend to do any downloading, are you saying that the displayed max speed is just as accurate on the Garmin units??

Correct! (subject to possible slight differences in filtering). And this has been stated in just about every discussion of GPS's since the Doppler capability was discovered a couple of years ago.

If you don't want to download, then the GT-31 is still far better because it will save the best top speeds in the genie for recall at the end of the session. Not only do you have your best top speed, you have all the rest of the top dozen or so listed as well!

Select to expand quote

sailquik said...

pedro e said...

Anyone know the going price for a GT31? - and how do they differ to the GT11?

No knowledge about prices but the GT-31 has a different model GPS chip. It has a Sirf3 chip v's the GT-11 which had a Sirf2 chip. The new chip is much more sensitive and is able to pick up and use very weak signals. This was brought home to me recently at the Al Green event when I had a GT-11 and GT-31 in the same Aquapac. Late in the afternoon there were only 6 satellites in the sky and two of these were very low in the horizon. The GT-11 could not pick them up and had only 3-4 signals which makes the tracks invalid. The GT-31 was able to pick up all 6 satellites and produce data that was valid and useful. I was glad I had it because a couple of my best runs were done during this period and they would not have counted in my average if I was relying only on the GT-11. I must say that this was the first time I had this problem with the GT-11 for such an extended period but it only takes loss of sats for a few seconds to invalidate a run that could have been a new PB.

great feed back sailquick,i have been debating wether to get a 31 or not for a couple of weeks now and your observation as prompted me to do it,cheers

I cannot agree that if you don,t want to download then your displayed performance will be correct. Neither Lyns Etrex or my Venture displays are ever the same as we get out of Realspeed, or another program GPS Trackmaker. Sometimes the error is above and sometimes below.

Select to expand quote

Herbylyn said...

I cannot agree that if you don,t want to download then your displayed performance will be correct. Neither Lyns Etrex or my Venture displays are ever the same as we get out of Realspeed, or another program GPS Trackmaker. Sometimes the error is above and sometimes below.

Herb, I am guessing that the question is not whether the Garmin displayed speed is the same as what you get out of the GPS speed sailing program, but rather, is the Garmin displayed speed the same as the GT-31 (or other GPS) displayed speed.

Regards,
Harrow.

In my case the Garmin is always different to the GT and when I put them thru RealSpeed the GT is always closer.

Select to expand quote

pedro e said...

Anyone know the going price for a GT31? - and how do they differ to the GT11?

I picked up a GT-31 from RPS on the weekend, they are about $230.00.

Select to expand quote

Herbylyn said...

I cannot agree that if you don,t want to download then your displayed performance will be correct. Neither Lyns Etrex or my Venture displays are ever the same as we get out of Realspeed, or another program GPS Trackmaker. Sometimes the error is above and sometimes below.

True. The display speeds are often slightly different between GT-11/31 and Foretrex. Usually not much different and usually in the same small order of difference that you get between two side by side GT-11's. Remember that these are unsynchronised GPS and you are talking about a peak 1 second speed. It will seldom ever be the same part of that best 1 second (unsynchronised).
The Garmins also round the best speed to 1 decimal place. The GT's display to 2 decimal places. If you take this into account they are usually pretty close.
The speeds you get from Realspeed for the Garmins will be based on Trackpoints. The speed on the display will be calculated from Doppler info. I would expect them to be different.
The Doppler speeds calculated by RealSpeed from the GT-11/31 bin file are almost always extremely close to the max display speed.
If you look at the 10 sec max speeds saved in the Speed Genie from a GT-11/31 they will also usually be extremely close to the RealSpeed output (but rounded to 2 decimal places) but remember that display speeds can't always be relied on as there is none of the error checking/filtering that you get in the analysis programs.

thanks for the feedback on price and differences between units - very useful.

Another question: how fast is this technology improving? I remember back about 8 years ago how primitive digital cameras were compared to today - the technology has leapt ahead and the price has plummeted. Much the same with computers.