GPS accuracy

14 messages
06/04/2011 at 17:21
So i dont usually have to take my iphone with me these days since i bought Garmin 305, but today i was waiting on a call and running with a friend who has HTC we figured we could do a side by side test.

we uploaded the files to an identical web page and look at the difference in resolutions.

http://i52.tinypic.com/11brlp0.jpg


06/04/2011 at 17:25
Sorry, what am I looking for...?
06/04/2011 at 17:40
Peter Collins wrote (see)
Sorry, what am I looking for...?


I'm guessing we are looking at the frequency of data points along the route showing that the Garmin is recording about 58 data points compared to about 14 on Runkeeper on the iPhone and 12 on Runkeeper on the HTC.

I also guess the logical conclusion is that the more data points along the route the more accurate the plotting of the route and consequently the more accurate the measure of distance travelled.

Does that sound right or am I missing something and that logic is flawed ? It is interesting to see it so clearly represented on these images but not too surprising.

As a matter of interest what distance did each device record for the total run ? 

cougie    pirate
06/04/2011 at 17:53
I'd think that the garmin should take the most points as thTs what it's designed for. I use my iPhone and it's good enough for me. Yes - distances recorded would be interesting.
06/04/2011 at 18:08
Sorry peter i should have explained better, weevil is correct,

distances on the entire run were

6.69mi - garmin
6.45mi - iPhone
6.42mi - HTC

Ive used my iphone for close to 18 months for tracking and to be honest it was close enough when running 5 - 10k.
Once i started doing half marathon distances i found that the "close enough" was sometimes half a mile out, but it was only a 5 min fix in the runkeeper "edit route" page
on the silverstone half marathon where the distances are very accurate my iphone was only recording 12.4 miles as i crossed the finish line, whilst my friends HTC was on 14.9 miles for some reason, my other running partners garmin watch was 13.12 miles and that was the reason i invested, especially now they are so cheap.
Edited: 06/04/2011 at 18:09
06/04/2011 at 18:52
There seem to be divided opinions on the Forums about GPS logging of runs.

People seem to fall into one of 3 categories

1. Don't log their runs and don't see the point
2. Log their runs for fun but aren't too worried if it is off.
3. Log their runs religiously as part of a training regime and want detailed accurate info

And don't even start on the Footpod vs GPS accuracy debate :-]

Now that Nike have a product out there we can look forward to lots of postings discussing the relative accuracy of Nike vs Garmin !

Still this should be useful information for anybody looking to buy a GPS device and wondering what the relative merits of the dedicated device are.

Thanks :-]


07/04/2011 at 09:57

I use a Garmin 405, and it seems to be very accurate against measured race distances.  As Cougie says, that's what it's designed for.

I just like to know how far I've run on training runs, and I use it for speed sessions as it beeps to tell me when I have to speed up/can slow down 

You can probably get an app for that, but then I don't have an iPhone

07/04/2011 at 12:31

Yes, I've found the same, I now run only with the Garmin and then upload the data onto runkeeper.

 It's up to the application to request current GPS position from the iPhone but the more requests you make the lower the battery life. When Runkeeper first launched the battery life was less than a couple of hours with it running so I imagine they've got some smart filtering that varies requests by current speed to get a trade off of accuracy vs battery life.

07/04/2011 at 12:42

The Garmin GPS algorithm is tried and tested. I've used one of their Etrex units for years. It works out how fast you are moving and how often you change velocity and plots the points appropriately. ie if you speed up and slow down it is picking up your speed changes and if you are weaving side to side it is picking up your direction changes. If you run in a straight line quickly the points will be more spread out.

All GarminGPS does is plot points (and some heart rate stuff?).

The iPhone and the HTC are limited on memory and battery life and also have to do lots of other functions. I suspect that they limit the number of points appropriately.

seren nos    pirate
07/04/2011 at 12:49
but the garmins are never 100% accurate .so you have to use common sense...................for a couple of races i have worn one on each arm and they do get out of sync.sometimes significantly.......at 1 20 miler recently my left arm ran .2 of a mile further than my right arm
07/04/2011 at 12:57
@TimR - the iPhone and HTC have far more memory than any dedicated GPS device. My HTC has 16GB to dedicate to GPS data, for example.

@seren nos - no GPS devices are 100% accurate - none. Not even those that the military use.
07/04/2011 at 13:01
"Not even those that the military use"

the cause of collateral damage on their target seeking missiles??

"bugger - missed - flaming Garmin...."

07/04/2011 at 13:07
seren nos wrote (see)
but the garmins are never 100% accurate .so you have to use common sense...................for a couple of races i have worn one on each arm and they do get out of sync.sometimes significantly.......at 1 20 miler recently my left arm ran .2 of a mile further than my right arm


Seren is quite right - my Squeeze and I run together sometimes, and his Garmin 305 would consistently record a longer distance than mine, and also on race distances (where mine is usually within 0.02 of a mile to the race distance).

He's just bought a new one (can't remember which number), so it'll be interesting to see if that is more accurate.

07/04/2011 at 14:16
seren nos wrote (see)
but the garmins are never 100% accurate .so you have to use common sense...................for a couple of races i have worn one on each arm and they do get out of sync.sometimes significantly.......at 1 20 miler recently my left arm ran .2 of a mile further than my right arm

The GPS is accurate to within 10m positional accuracy. I don't know how it measures the actual distance you have travelled. So the points you plot are accurate but it will depend on what you did between the points on your actual distance.

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