I put my Garmin 920 XT and GT 31 under my car seat yesterday and went for a drive. I turned them both on and off under the seat. I'll let the pictures do the talking.
Garmin 920 XT
GT 31
Maybe the Garmin is ok for under arm.
The picture tells me that the Garmin can record a track, but it doesn't tell me that it can accuratey record the speed along that track. Post the number of satellites and the HDOP data for both devices.
Had the watch been on immediately before the test and the GT-31 not been on for quite some time?
This does not test what we are concerned with. It only seems to test the time it takes for the GPS to download the almanac and get a positional fix.
Not a valid test. Kinda pointless anyway as it does not have SDOP data.
Its a great test.......really highlights the strength of signal especially to be able to find a signal under all that arse cheek, thats incredible! !!!
It reminds of early south park when Carmen had a satellite coming out of his arse.....anywho i digress. ....
But seriously, that is a very strong signal.....
Impressive if it is a completely even test. But I'm guessing sailquick is correct on this one.
I think some of the garmins do have the ability to record signal quality as its in their programming guide " var accuracy; // Accuracy - good, usable, poor, not available" Of course what it is exactly is a bit vague and some one would need to change the code in the app the garmins use for windsurfing.
Aus4 sent me the two files for comparison. There is only a 38 second section when the GT-31 was able to produce valid numbers. The graph below shows the speeds in kts. To my eye the GT-31 reflects more accurately the speed of the car, cars don't change speed as rapidly as the red line.
The interesting point though is that throughout the 38 seconds the GT-31 had a lock on 5 satellites and an SDOP of 2, earlier in the track it only had a lock on 3 satellites so was not able to calculate a speed.
When I sail with a GT-31 I always get at least 8 satellites and normally 9 or 10. This produces an SDOP of 1 or less. I would not consider posting results that used only 5 satellites with SDOP reading around 2.
The trouble with the 920XT is that none of this "quality" data is available so the user would just believe the numbers. Even though as shown below it is producing poor results.
Select to expand quotetbwonder said..
To my eye the GT-31 reflects more accurately the speed of the car, cars don't change speed as rapidly as the red line.
The change in speed would be called acceleration?? As far as I can see on the graph, it appears the GT31 (blue line) shows the highest gradient (acceleration) or about the same as 920XT (towards then end of the graph).
Actually the highest rate of acceleration is between the first two samples of the 920XT.
Below is a graph of the accelerations sorted from High to Low (left to right).
The GT-31 has produced smoother data.
Select to expand quotetbwonder said...
Actually the highest rate of acceleration is between the first two samples of the 920XT.
Below is a graph of the accelerations sorted from High to Low (left to right).
The GT-31 has produced smoother data.
Correct me if im wrong but I would expect acceleration values to be simmilar at the start and end of this 38s data for the GT31 (similar speed gradient). In you graph it is near 0 at the end...
Anyway even with the SDOP data who says the GT31 is more accurate (closer to the real speed)? A third device may be needed to valid all this?
Maybe AUS4 car is turbo charged and who knows how he drives to say 'smooth acceleration' is more accurate?
Probably off topic anyway, I think AUS4 was just showing underarm grip and data capture.
As I stated the acceleration values have been sorted from high to low (The horizontal axis is not time).
Agreed off topic. I agree the 920XT is probably capable of producing data when worn on the wrist.
But is that data accurate?
Oops misread your original post, but without time stamp the data is useless.
This exactly what some have pointed out, your max speed on both devices may be very very close but it doesn't mean they were achieved on the same run, exact same time.
This post was just ment to show that the 920XT and probably some other GPS watches have a good enough reception to be used under arm.
If people want to make more out of it, that's your problem.
And just for the record I used two GT 31s and a 920XT. I did 3 runs in medium to heavy traffic including 4 sets of traffic lights.
The 920XT recorded pretty much the same on all three runs. 1 GT 31 recorded about 1/3 of a run and nothing on the other 2 runs. The second GT 31 recorded nothing on all three runs.
Thanks for the testing Aus4, not many of us can do side-by-side testing of gps units.
Another test might be to let the units get a fix next time and do the same drive with them on the dash with a clear view of the sky and compare the speeds?
Dunno - I just like to sail 
Select to expand quoteAUS4 said..
This post was just ment to show that the 920XT and probably some other GPS watches have a good enough reception to be used under arm.
If people want to make more out of it, that's your problem.
And just for the record I used two GT 31s and a 920XT. I did 3 runs in medium to heavy traffic including 4 sets of traffic lights.
The 920XT recorded pretty much the same on all three runs. 1 GT 31 recorded about 1/3 of a run and nothing on the other 2 runs. The second GT 31 recorded nothing on all three runs.
Rick, you are beating a dead horse with a broken stick. Your so called 'test' shows absolutely nothing of value or relevance.
Except to say once again, that if you use the Garmin, you may as well pull the figures out of your ar$e.
You obviously have two perfectly good GT-31. Post from them and use your Garmin watch for on the water feedback and to tell the time. It should be OK for that.
Select to expand quoteshear tip said..
Thanks for the testing Aus4, not many of us can do side-by-side testing of gps units.
Another test might be to let the units get a fix next time and do the same drive with them on the dash with a clear view of the sky and compare the speeds?
Dunno - I just like to sail
Hi Shear Tip,
I posted these comparisons back in feb/march but was accused by the DEAD HORSE that I must have had fiddled with the default settings.
Uploaded with GPSAR Pro 5.11
Garmin 920XT 31.98 30.34 14.54 19.85 24.04 45.588
GT 31 31.65 30.39 14.56 19.84 24.09 45.873
Canmore 102 31.56 30.38 14.57 20.02 24.08 45.812
Uploaded with KA72
Garmin 920XT 31.979 30.226 14.947 19.074 24.044 45.588
GT31 31.344 30.01 14.98 19.012 23.993 45.548
Canmore 102 31.276 30.106 14.929 19.203 23.944 45.4577
Three GT 31s same software and settings
Uploaded with GPSAR Pro
GT 31 (1) 32.20 30.11 14.35 19.96 24.02 51.746
GT 31 (2) 32.24 30.07 14.35 19.96 24.01 51.763
GT 31 (3) 32.29 30.08 14.34 19.96 24.02 51.661
Uploaded with KA 72
GT 31 (1) 32.054 29.914 14.843 19.664 23.719 51.15
GT 31 (2) 32.122 29.927 14.845 19.689 23.762 51.169
GT 31 (3) 32.132 29.929 14.844 19.694 23.764 51.136
Select to expand quoteDavMen said..
Thanks to your Garmin 920 XT, we now know where you live 
He lives by the lake?
Select to expand quoteMobydisc said..DavMen said..
Thanks to your Garmin 920 XT, we now know where you live
He lives by the lake?
Select to expand quoteDavMen said..
Thanks to your Garmin 920 XT, we now know where you live
Ha Ha, not where I live. 
Select to expand quotepowersloshin said..
Speed Fines in the mail
Probably did the 34-35 knots bit in the carpark after some drifting/ circle work.( just to get some decent alphas of course) G force had Aus4 plastered against the driver's side door, arse cheek was elevated, signal got through to GT31. I rest my case.
