Accuracy of the Garmin 500 and Garmin 310XT

Izlude - 2010-08-24 11:09 AM So I routinely use both on my bike rides and was curious to know what people's experiences are in terms of accuaracy with both devices. I like to compare miles for both and notice maybe 1-2 mile difference between the two. I do prefer to upload data from the Garmin 500 to Garmin connect to analyze the data since it is more accuarate with elevation. I know you have to account for the standard error with GPS but I notice the milage for the Garmin 310 XT seems to almost always exactly the same as a program I use for my iphone where the Garmin 500 always seems to add a 1-2 miles. Not sure if this is a big deal, just curious to see if people experience the same thing. I'd figure since they are from the same company they would be very close to each other in terms of accuracy and miles recorded.

tcovert - 2010-08-24 2:48 PM I've found the 310XT to be very accurate so far (so was my 205).  Can't comment on the 500.

You can't really trust elevation or calorie totals from a Garmin unit, but distance is going to generally be pretty close.  For instance, if a marathon or half marathon comes up more than 26.2 or 13.1, it's far more likely (almost to a certainty) a result of running a longer distance due to not taking the tightest tangents on the turns than error on the part of the device.

nhunter344 - 2010-08-24 3:50 PMAnyone know why Garmin cant(wont) correct the elevation data from the 310xt once the data is uploaded to Garmin Connect?

I understand not being able to perform differantial correction on-the-fly or to recieve WAAS updates, but not being able to correct it once uploaded is pretty lame.

Hopefully Im just ignorant on the matter and Garmin does provide correction to its data, but so far, I havent found it.

cbire880 - 2010-08-24 3:34 PM GPS is very good over long distances with stable speeds.  That is why it is so great for aircraft.  On a small scale, the solution is too jumpy for slow speed (10's of mph or less instead of 100's) without some form of augmentation.  Your wheel speed and cadence sensor gives you that augmentation that tells the GPS "No he didn't just change direction and go 40mph in a new direction."  I don't have one, but I would like to see how much smoother my data is with a foot pod while running versus GPS alone.

I suspect you are either seeing the effects of elevation change from not using a wheel speed sensor or the lack of elevation change in the non-bike specific devices.  I have an Edge 705 and a Forerunner 405 for running.  I know the 405 does not account for elevation change in its speed computation.  The 705 and 500 have altimeters and likely give you actual road speed (like a car) instead of the horizontal speed only (like the 405 does).  I do not know the methodologies for the 705 or 310XT.

jsiegs - 2010-08-24 4:05 PM

So my max speed isn't really 43,200 miles per hour??? dang.  GPS for me has been very good at distance (when over a mile or so), average speeds, steady state speeds.  While accelerating, it tends to lag a bit due to (I assume) an algorithm which averages the data to smooth it and reduce the occurance of things like 43,200mph speeds.  There are errors in GPS no doubt, but over a little time and distance, the random errors will cancel each other out and lead to more accurate data.