I’ve been thinking back to the threads at the start of the year about total miles ridden in 2010 and total height climbed. I was using Motion-X on the iPhone but as it eats battery life I bought the GT600 GPS tracker that was reviewed in the mag last year. Nothing fancy, just a simple GPS receiver that tracks you wherever you go then you can get the routes off the device as GPX or KML.

So I’ve taken some routes from Motion-X and some from the GT600 and uploaded them to the site I’m using to track my progress this year ( http://www.ridewithgps.com ) however when I upload the routes on there, the GPX file will give a total ascent figure of say 3000 feet, but the website tells me that according to the route I did, they think my total ascent was about half. No matter what the GPX says, the site says the ascent/ descent was about half.

So what’s more accurate, the GPX file or the website? The route on the GPX is spot on and accurately shows where I’ve ridden so I’m surprised the ascent/descent figure is so far off!

Some GPS units will calculate altitude based on a barometric reading, GPS data, or a combination of the two. This then gets passed to the software, which might have it’s own elevation data for given points on a map. That can account for the difference, though I don’t know which is the most accurate 🙂

The GPS will record every little up/down it does, the website just counts the contour lines crossed.

More importantly the GPS is possibly only accurate to ~10m, which is fine in the horisontal plane, but if the position varies by 10m between two vertical fixes it records that elevation change. Multiply that over the whole ride (it might be 1000’s of fixes, only needs a few % to be errors) and you get an inacurate climbing total.

On average its somewhere arround 60% for my garmin edge 305, so if it says 1800ft, the actual number is more like 1100ft.

Ditto max speed, but the other way around, I’ve got a wheel sensor for the garmin that will tell me I’ve done 40mph, but mapmyride.com cant see that data so just takes the fastest speed between 2 fixes which is invariably higher, you could stand stationary and it would still say your max speed was 2-3mph as any 2 fixes wont be identical.

I use an eTrex with barometric altimeter. I assume it’s more stable and more reliable than purely GPS based, but I’ve never compared the two. All are going to be affected by +/- errors to some extent. The GPS probably counts every +/- 1 foot or meter in its running total, but the website probably has some kind of moving average to filter out that noise. Stick the same track in 3 websites and you’ll probably get 3 similar but different results.
Cycling is probably the worst though, because you get many more datapoints going uphill than downhill (unless you ride downhill at an uphill pace), so by riding up an mountain and riding back down the same way you can potentially get quite different vertical up/down figures.

Strangely, ridewithgps.com usually disagrees with my Edge 705 the other way – it (ridewgps) claims to be superaccurate on US mapping but always overestimates my climbing by 300-400m.
I trust the GPS – if I sign up for a race with a claimed 2200m of climbing, my GPS will usually record around that figure. Same goes for route guides, downloaded tracks from other riders, etc – the figures usually agree until I put it into the website.

Elevation figures derived from mapping rather than recorded from GPS/barometric will also differ… potentially a lot. GPSVisualizer can do that if the GPX file has no elevation data. Ride along a hillside with a big drop-off to one side and any +/-5m lateral deviation gives a huge elevation error, which all adds up. Probably more of an issue with a plotted route than a recorded track.

Normally I’d trust the website. Saying that, I’m tempted to test out a well known hill, and compare GPS, processed GPX and map contour to see which is closest.

bob, possibly the more expensive edges use barrometers? I’m guessing the barrometers have quite a heavy algorythm filtering the data so that the position doesn’t change with each gust of wind, unlike the GPS where it takes pretty much the raw data (because you can’t be in 2 places at once).

on a garmin forerunner 405.

so like the calories used, i dont pay too much attention to the ascent/descent, sometimes when i get home, its higher/lower than when i left!

I’ve had the same as Bob_summers – the vast majority of the time my GPS (with barometric altimeter) will give a much lower height gain than the figure given when I copy the route across to MemoryMap. I figure that the actual reading is somewhere between the 2 as GPS will tend to over estimate for the reasons given above, where as barometric data will only record general height gain, not the little short sharp climbs & subtle undulations etc (IFAIK anyway)

I have a GT600 and my ascent/descents were coming out ridiculous for rides around the Surrey Hills…10000ft+! However the GPS logger was recording the data accurately enough and it was more just the way the software (In my case the Sports Analyser sw that came with the GT600) was anaylsing the information. I raised it on a thread on this forum and somebody suggested loading the GPX into Google Earth and the ascent/descent came out far more realistically (around 3000ft instead of 10000ft)so I took it up on the @trip/igotu forum. The igotu bods said it was an issue with no easy fix but they have added it to the debug list

Easy.

1 piece of flapjack, or bacon, peanut butter & apricot jam sandwich = 500m of ascent.

End the ride at the same place as you started and the total descent is the same as the ascent.

Garmin dakaota 20

f’rexample (training ride today uploaded from edge 705):

bigjohn, i think your system’s out – i had a banana but the flapjack stayed in the jersey pocket 😉