Garmins - How accurate?

17 messages
21/02/2011 at 19:43
Interessted to hear about people thoughts on the accuracy of both the HRM and the distance. I find it bizarre that every 10k I have ever run is 'out' on my 305.
21/02/2011 at 21:02

Various questions need to be addressed.... Are you certain the distance is accurately measured if so by whom and was it certified.
How far out is your garmin and does it not tell you how accurate it is? -- my 310xt displays its degree of error.

Are we talking have a mile out of are we talking a couple of feet

Edited: 21/02/2011 at 21:03
21/02/2011 at 21:34

There are two issues.  The first is how far you actually ran.  Even on an accurately measured course there is a small margin of error.  They always err too long.  Then they measure the shortest racing line but it's unlikely that you will run that line.

   

Secondly, the way the Garmin measures distance is by checking your location periodically.  It then works out the distance from the last time it checked your location.  It does this in a straight line and if you didn't run a straight line betwen those points is will be inaccurate.  The more frequently it checks your location the more accurate it will be.  If it loses the satellite signal then it simply takes a straight line from where the signal was lost to where it is regained.

    

That's why your Garmin is unlikely to ever show exactly 10k on a 10k race.

22/02/2011 at 09:18

Every race will be a slightly different length and officially measured races alway add on a certain percentage to ensure they are not short (so in effect every race you do should be long on your Garmin if its officially measured). Actually distances can vary quite significantly to the extent where certain races are known to be good or bad for PB potential given their actual length. As an example I ran 2 10ks in the space of a few days last year both on officially measured courses. My Garmin recorded my pace as identical for each run but my finishing time differed by about 40 seconds.

As has been said the more twists and turns and changes of direction the less accurate the Garmin readings will be. As an aside there is usually a setting which you can change which alters how often the Garmin takes readings. You can set it to take them more often which gives greater accuracy but uses up more of the memory.

22/02/2011 at 10:05
I'd say they are about as accurate as any non-elite athlete requires.
22/02/2011 at 10:13

Mine (405) is usually pretty damn close to the measured course distance.

On the other hand, my OH's 305 always says more distance has been covered.

I certainly consider my 405 to as accurate as I need for training, etc. 

Edited: 22/02/2011 at 10:14
22/02/2011 at 14:13
Just wondering how anyone could comment on the accuracy of the HRM without access to hi-tech specialist medical equipment?
22/02/2011 at 14:17
Kicked-It wrote (see)
Just wondering how anyone could comment on the accuracy of the HRM without access to hi-tech specialist medical equipment?

Maybe he ran with his finger on his pulse and counted his heart beats all the way round?
23/02/2011 at 08:57
Mr Viper wrote (see)

As an aside there is usually a setting which you can change which alters how often the Garmin takes readings. You can set it to take them more often which gives greater accuracy but uses up more of the memory.


Could you please indicate how could we view/edit the settings
23/02/2011 at 09:03
MandM wrote (see)
Mr Viper wrote (see)

As an aside there is usually a setting which you can change which alters how often the Garmin takes readings. You can set it to take them more often which gives greater accuracy but uses up more of the memory.


Could you please indicate how could we view/edit the settings

I can't remember off the top of my head, but I'll have a play with my Garmin tonight and try and work it out.
27/02/2011 at 06:49
Any chance of an update?
27/02/2011 at 11:09
i find my 305 quite accurate, not done any races with it yet as recently stolen it off hubby while he is off on exercise. but i measure my sunday long runs with the car before i run it and they match very well with the distance on my speedo, obviously a little shorter due to lines the car has to take but close. think last week it was only .1 mile shorter. i to would like to know how to change settings. will have a play later and see if i can find them.
27/02/2011 at 11:21

I agree that its quite accurate but thats probably true for straight line runs, i believe the deviation comes when you have a lot of twist and turn in your run, i was down by 400 yards on a 20 Miler, but i knew that cos the run involved lot of of twist & turn and some loops!

I guess no harm knowing that you could improve distance logging and get greater accuracy.

HM4
27/02/2011 at 16:02
I've got a 205 and find it very accurate however I do have it set to pick up satelites very frequently.
01/03/2011 at 21:43
I think I can imagine how the Garmins, and other similar devices, do their distance estimates (it's my day job). Without going into the nerdy detail, the GPS signal is usually measured every second or so, and between those points the unit will probably use dead reckoning. if the race has sharp bends, especially 180 degree turns, it will tend to overshoot on the corners, giving you an extra bit of distance.

It's worse in built up areas or forests when the signal can get lost for brief periods, and the Garmin will actually smooth out any twists and turns for the same reason, actually giving you a shorter overall distance.

You can even get strange reflections off buildings which mess the whole thing up. During the last London Marathon my 201 got confused by Canary Wharf and added a whole mile on. Then again, the race felt longer than 26.2 miles so it may have been right.
02/03/2011 at 08:43
Garmin say that the margin for error with their units is 2% that's getting up towards half a mile on a marathon and on my 305 I've found that to be the case.  More interestingly I've found whilst running side by side with 205 and 405 users that we are not getting the same results, sometimes a variation of 0.2 or more on 10-15mile routes.  Obviously this isn't down to buildings running line etc etc.  I also found the difference between units was not consistent on the same routes.  However I've seen a 405 that is consistently accurate/agrees with measured courses.  I conclude from this that units are variable and Garmins margin of error of 2% is about right.  You need to know your own Garmin if you want to pace a race with it.  Unfortunately my 305 costs me about 4 mins on a marathon.
02/03/2011 at 11:55

My 305 measures pretty much the same as when I used Mapometer.com.   Im not sure how accurate the HRM is though.  When I set off and I am on a warm up lap it seems to make my HR around 237 which just cant be correct.  It does settle after a while and seems to give a more realistic reading.


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