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Elevation data - which to trust?

CubicCubic Posts: 594
edited July 2009 in Road beginners
Hi,

I did a 50mile ride around Surrey last weekend. My Garmin 705 tells me that I did 3685ft of ascent during the ride, but if I plot the same route on MapMyRide it says I only did 2280ft. And BikeRouteToaster says that I did 2776ft.

Anyone know which system is the most accurate, or are they all going to be very approximate? If so what is the best way to work out the total ascent?

Thanks.

Posts

  • roger_merrimanroger_merriman Posts: 6,165
    they are likely to be all rather approximate, to be honest.
  • Do you have long-travel front shocks? That might explain the difference - the Garmin is going up and down more than the rest of the bke.

    More seriously though, GPS elevation is probably the least accurate. Try BikeHike and take the average of the 3 maps.
  • I thought the 705 had a Barometric Altimeter?? I would have thought it would have been pretty accurate (but I am no expert)
  • amaferangaamaferanga Posts: 6,789
    I use the ascent/descent figure from my Garmin as a guide. Though the absolute figure may be wrong (but no more wrong than bikehike, bikely, etc.) I thinks its useful for comparing my rides.
    More problems but still living....
  • AGNIAGNI Posts: 140
    The Edge 705 has a barometric altimeter so it reads your altitude from the atmospheric pressure. Question is how accurate is this due to changes in pressure with general weather conditions changing?

    Last weekend I went over a lot of hilly terrain that I had not rode over before so I put up the elevation trace of the ride so I could see what inclines were coming up next. As it turns out, it wasn't entirely accurate
    Still suffering with wind
  • bobtbuilderbobtbuilder Posts: 1,537
    I don't rate the altitude function on the Garmin at all.

    Although 99% of my rides start & finish from my house, the Garmin always registers a different figure of ascent to descent. On a recent 50 mile ride, the Garmin said that I had ascended 400ft more than I had descended. As the total ascent was only 4,500 feet, that's almost 10% inaccuracy.
  • GeorgeShawGeorgeShaw Posts: 764
    This thread is repeated regularly. Any altitude measurements will vary due to a number of things. For the raw data, then the main variable is the sample rate, especially on rolling roads. But then each software package will use different calculations and roundings. I can put the raw data from my Garmin into 4 different software packages and get 4 radically different results.
  • nasahapleynasahapley Posts: 717
    I don't rate the altitude function on the Garmin at all.

    Me neither. I tend to figure out total ascent figures for my fell-runs with an OS map as if you're just doing three or four big climbs with no other undulations it's pretty easy to do that way. On the occasions when I've borrowed a Garmin the ascent figures have always been very optimistic - up to 30% over. This isn't having a pop at Garmins, they're obviously useful for other stuff, it's just that measuring altitude whether by barometric pressure or triangulation is bloody hard to do. Where I've taken the time to plot a route on bikehike it tends to come out pretty close, but it's a pain to do it that way!
  • themightywthemightyw Posts: 409
    AGNI wrote:
    Last weekend I went over a lot of hilly terrain that I had not rode over before so I put up the elevation trace of the ride so I could see what inclines were coming up next. As it turns out, it wasn't entirely accurate

    Wow I didn't know the 705 could do that... assume it was a course you'd downloaded that already had elevation data on it? Sorry that this is slightly off topic, but am doing an unknown ride in a few weeks and this would be really useful.
  • AGNIAGNI Posts: 140
    themightyw wrote:
    AGNI wrote:
    Last weekend I went over a lot of hilly terrain that I had not rode over before so I put up the elevation trace of the ride so I could see what inclines were coming up next. As it turns out, it wasn't entirely accurate

    Wow I didn't know the 705 could do that... assume it was a course you'd downloaded that already had elevation data on it? Sorry that this is slightly off topic, but am doing an unknown ride in a few weeks and this would be really useful.

    The route was previously calculated from bikeroutetoaster so I presume it got the elevation data from there. When you have 705 working, you can go into a variety of views with the mode button. When in the different views, flick the joystick up or down for further views and one will take you to the elevation data. There are two sets of elevation screens. One shows the elevation you done and the other shows the elevation trace on and coming up.

    Best thing to do is sit down and have a good play with it (no pun intended)
    Still suffering with wind
  • amaferangaamaferanga Posts: 6,789
    nasahapley wrote:
    I don't rate the altitude function on the Garmin at all.

    Me neither. I tend to figure out total ascent figures for my fell-runs with an OS map as if you're just doing three or four big climbs with no other undulations it's pretty easy to do that way. On the occasions when I've borrowed a Garmin the ascent figures have always been very optimistic - up to 30% over. This isn't having a pop at Garmins, they're obviously useful for other stuff, it's just that measuring altitude whether by barometric pressure or triangulation is bloody hard to do. Where I've taken the time to plot a route on bikehike it tends to come out pretty close, but it's a pain to do it that way!

    I actually find that the bikehike figure is consistently higher (maybe 10 - 20%) than what I measure on my Garmin Legend HCx. The ascent figure for bikhike is also usually significantly higher than that of bikely.
    More problems but still living....
  • doyler78doyler78 Posts: 1,951
    Just on those elevation profiles on the sites what does it take to register anything over 4% on mapmyride? None of my routes ever show anything over this yet there are sections which are 10-20%.

    Bikehike seems to be the one that gives me the closest elevation totals to what my Polar CS600 reports so I use it most of the time now for planning a route.
  • da gooseda goose Posts: 284
    Got a mavic computer altimeter very accurate...for sure barometric pressure changes any kit but prior to buying used a proper mountain climbing jobbo almost idetical results....no gps though (am i bothered?)
  • Do you have long-travel front shocks? That might explain the difference - the Garmin is going up and down more than the rest of the bke.

    More seriously though, GPS elevation is probably the least accurate. Try BikeHike and take the average of the 3 maps.

    That's probably the wrong way round. both bikehike and mapmyride use elevation data most likley taken from the NGIA US DTED level 1 data: which has elevation data on about a hundred metre grid - fine for flying, not quite so good for riding. It tends to underestimate, especially if you go up/down a lot in short distances. - hence the lack of elevations over 4% ish.

    GPS altitude is ok, if you've got a lot of satellites, and coupled with a barometric altimiter, is probably pretty good. it records data points a lot more often, and as long as it doesn't look crazy (like you're going backwards or a negative gradient) i'd go with it as the more accurate one.
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