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  1. #1
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    iphone GPS, I know I didn't climb 8,737' today, but my iphone says I did...

    I've never thought that a phone was all that accurate for measuring vertical feet, but this is way off, the ride in question, according to other GPS devices is anywhere form 4,200' to 4,500' of climbing, but my "GPS kit" app said 8,737 at the end of the ride, WTF?

    Here is a screen cap, The mileage is about right, but why is the vert so far off? Is it just the app I'm using, or is it the phone itself? Or is it poor GPS signal? Thoughts?


  2. #2
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    your iphone doesn't have an altimeter, does it?

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    Nope, I don't think any phone or gps does.

    The highest and lowest elevations are about right, fwiw.

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    a lot of dedicated GPS receivers have barometric altimeters. and yeah, no phone does. you really can't trust GPS-calculated altitude. which is why so many websites will correct data that does not come from a barometric altimeter.

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    Quote Originally Posted by NateHawk View Post
    a lot of dedicated GPS receivers have barometric altimeters. and yeah, no phone does. you really can't trust GPS-calculated altitude. which is why so many websites will correct data that does not come from a barometric altimeter.
    Do you know if some apps correct for altitude when the track is finished?

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    Garmin Connect has corrections and I think Strava does, too. I only know them because their websites are compatible with any gpx file. No clue about other apps

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    It's mostly due to the gps antenna in smartphones which is not ideal for gps signal reception. Quality gps antennas are bulky and can't be placed inside a smartphone, so manufactures settle for tiny internal antennas with poor signal reception.

    Also battery power in a phone is precious, smartphone gps chipsets try to minimize power usage in various ways. That will sometimes produce data dropouts in the gps log.

  8. #8
    try anything on a bike
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    the total ascand total desc are totals. Its adding up how many vertical feet your traveled through out the ride, not just range. So if you go ascend 500ft, then descend 300, then climb 100, then descend 200, it will say you have 600 asc, and 500 desc. So it may not actually be that far off if you are riding hilly terrain for 30+ miles.

    Hope that makes sense.

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    Quote Originally Posted by blue_biker View Post
    the total ascand total desc are totals. Its adding up how many vertical feet your traveled through out the ride, not just range. So if you go ascend 500ft, then descend 300, then climb 100, then descend 200, it will say you have 600 asc, and 500 desc. So it may not actually be that far off if you are riding hilly terrain for 30+ miles.

    Hope that makes sense.
    I think he gets that. he seems more concerned about the discrepancies between his phone app and data recorded from other devices.

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    try anything on a bike
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    Quote Originally Posted by NateHawk View Post
    I think he gets that. he seems more concerned about the discrepancies between his phone app and data recorded from other devices.
    ahh...my bad then.

  11. #11
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    Quote Originally Posted by NateHawk View Post
    I think he gets that. he seems more concerned about the discrepancies between his phone app and data recorded from other devices.
    Yep.

    Having an inaccurate GPS isn't the end of the world but it would be nice to have it be accurate vs. inaccurate.


    I'll give strava a try, although it looks like our trails got snowed in for winter today :-(

    Thanks for the info :-)

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    We see this happen quite often in our riding group. I have a garmin 500 (altimeter) and others use smartphone/strava and another uses a garmin with no altimeter.
    Sometimes our cumulative altitude climbed is bang on. However most of the time the devices are all somewhat different. When viewing through the respective apps though the results tend to normalize (garmin connect). Google earth also tends to be off a fair bit as I don't believe it does correction. But I may be wrong on this.
    I've always found this topic interesting. I work in IT and the fellow across the hall is our GIS manager and he was saying that in our area he actually provides a terrain model with accurate elevation point data but he can't tell if the consumers (google etc) are actually using the terrain model as well as the map data. I suspect that the terrain model is used for elevation correction however I may be wrong on this.

    Natehawk - does this make any sense to you. The thought in our group is that my garmin is the most accurate as I've got the altimeter and have some preset elevation points at known locations.

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    Yep, makes perfect sense. The sites that do elevation corrections tend to use terrain models. What they use I couldn't tell you. I haven't seen a single one offer that information.

    If you use Topofusion, you can actually compare a couple different ones, but you still don't get the kind of info I'd like about the models, which would include the datum of the elevation data used and the model of the Earth (geoid/ellipsoid) used. I think if that information was provided we might get a little better idea of whose corrections might be better.

    And yeah, with the edge 500 auto calibrating to those points you have saved you most likely collect the best elevation data. Most of the elevation datasets only have 30m resolution which is pretty low. 10m is becoming available but not terribly widely. I know PA offers such statewide data for free, however. Still, when considering that some of the best satellite imagery available for free is less than 1m resoluion (TX has 0.5m sat imagery available for free...the files are huge but free), 10m is still low compared to the altimeter available on your GPS which can take a measurement every second

  14. #14
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    My Strava tracks are a bit "generous" but not crazy. They fit to some kind of known elevation data. I think that the generosity of their result has a lot to do with inaccuracy of the points at times. If I zoom into a track, I can see it wiggling in a way that I know my trails don't. If a trail is on a sidehill, that adds some elevation gain and loss.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

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    Quote Originally Posted by NateHawk View Post
    Yep, makes perfect sense. The sites that do elevation corrections tend to use terrain models. What they use I couldn't tell you. I haven't seen a single one offer that information.

    If you use Topofusion, you can actually compare a couple different ones, but you still don't get the kind of info I'd like about the models, which would include the datum of the elevation data used and the model of the Earth (geoid/ellipsoid) used. I think if that information was provided we might get a little better idea of whose corrections might be better.

    And yeah, with the edge 500 auto calibrating to those points you have saved you most likely collect the best elevation data. Most of the elevation datasets only have 30m resolution which is pretty low. 10m is becoming available but not terribly widely. I know PA offers such statewide data for free, however. Still, when considering that some of the best satellite imagery available for free is less than 1m resoluion (TX has 0.5m sat imagery available for free...the files are huge but free), 10m is still low compared to the altimeter available on your GPS which can take a measurement every second
    Thanks for the reply Natehawk! Just curious - do you work in the GIS industry?

  16. #16
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    GPS Kit

    The screenshot posted is from GPS Kit, which is also what I use (great app!)

    Here is something interesting I've noticed with elevation on GPS kit:
    When doing a ride that is strictly up, then down, the total elevations shown in GPS kit match the elevation differences shown on my green trails maps, exactly.

    Also, I've stopped many times mid-ride and compared the "current" elevation to my green trails maps, and found that GPS kit is accurate to the tolerances shown (usually +/-20').

    despite all this, as noted, for "rolly" rides the total elevation numbers get way out of wack.
    With the accuracy noted above in mind, it makes sense that the algorithm used to add up the cumulative total elev's is the issue, rather than the accuracy of elevation readings.

    FWIW, I've compared my GPS Kit #'s to a garmin 200 on the same rides, our numbers were not radically different. I think unless you have a barometric altimeter, total elevation readings are going to be "subject to interpretation".

    I look at it similar to digital tire pressure guages- having a tool that gives you consistent ratings is more important than the actual accuracy of the ratings.

    I've used GPS Kit to navigate a bunch of solo, remote rides, and it has not let me down once. Couple of tips;
    • remember to cache your maps in advance!
    • You can trace jpg/hard copy maps into google earth, then export to GPS kit, to create trail system "overlays"
    • With the background maps cached, you can turn wifi, bluetooth, 3G, notifications, push/pull and all other apps off to maximize battery life.

  17. #17
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    Quote Originally Posted by bikeriderguy View Post
    Thanks for the reply Natehawk! Just curious - do you work in the GIS industry?
    Only inasmuch as I use GIS software to display spatial components of biological data. Not technically the GIS industry, but close.

  18. #18
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    Quote Originally Posted by NateHawk View Post
    Only inasmuch as I use GIS software to display spatial components of biological data. Not technically the GIS industry, but close.
    I figured you must use the stuff in day to day life. Thanks again your posts are always appreciated.

  19. #19
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    Stava is about the worse GPS software for the Iphone... WAY too much averaging when it looses sat signal.. which they all do, it's the phone not the software, but the software many times will "ghost" during the brakes and it's how they do that that matters...

    honestly the Best GPS software I've used for the Iphone has been Kinetic Lite .. when it stop it actually marks the lost signal in the track, still averages like the others but lets you know when it's doing it. Everytrail, MotionX, and GPSkit are all decent too... I still use Everytrail the most because I just find the interface the easiest to deal with, and if you get into the advanced settings you can set it up to be reasonably accurate. but then I don't care about the alt, speed and other info, just the basic mapping feature.
    Going to try and bring Trail Tire TV back. go take a look... http://trailtiretv.blogspot.com/

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    Quote Originally Posted by thomllama View Post
    Stava is about the worse GPS software for the Iphone... WAY too much averaging when it looses sat signal.. which they all do, it's the phone not the software, but the software many times will "ghost" during the brakes and it's how they do that that matters...

    honestly the Best GPS software I've used for the Iphone has been Kinetic Lite .. when it stop it actually marks the lost signal in the track, still averages like the others but lets you know when it's doing it. Everytrail, MotionX, and GPSkit are all decent too... I still use Everytrail the most because I just find the interface the easiest to deal with, and if you get into the advanced settings you can set it up to be reasonably accurate. but then I don't care about the alt, speed and other info, just the basic mapping feature.
    so are you saying that the Kinetic Lite program will only display track when it has signal and then show you when it lost it, as opposed to connecting the segments with signal with long straight lines where you lost it?

  21. #21
    Trail Tire TV on blogger
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    Quote Originally Posted by NateHawk View Post
    so are you saying that the Kinetic Lite program will only display track when it has signal and then show you when it lost it, as opposed to connecting the segments with signal with long straight lines where you lost it?
    yup.. that's what it seems to do... It's free try it and see for yourself...

    I will say I like the interface also, all but one spot the map... it's kinda weird and small, and exporting seems to be a bit confusing, only reason I'm not using it as my regular GPS...
    Going to try and bring Trail Tire TV back. go take a look... http://trailtiretv.blogspot.com/

  22. #22
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    Altitude totals go crazy when the gps signal is lost for as little as a single track point. Most logging software inserts a "0" elevation which creates crazy ascent and descent totals. Take a close look at gpx files for zero elevation points. Unless you ride at sea level, those are all bogus, they only confuse gps software and websites that don't look up real elevations.

    All my gps data errors disappeared when I started using an external bluetooth gps receiver with my Galaxy S. The phone's internal gps just isn't sensitive enough for logging continuous track logs.

  23. #23
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    Quote Originally Posted by thomllama View Post
    yup.. that's what it seems to do... It's free try it and see for yourself...

    I will say I like the interface also, all but one spot the map... it's kinda weird and small, and exporting seems to be a bit confusing, only reason I'm not using it as my regular GPS...
    I don't use a smartphone with a GPS. the old Garmin GPS receivers used to just stop recording altogether and then start a new track segment when the signal was reaquired. at some point, Garmin changed things and started connecting those segments with a straight line. something about how the GPS determined a new track was started. there are settings you can adjust now, but I can't make it happen exactly the way it used to.

  24. #24
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    How, in the name of things holy, did you go 68 mph on a bike???

  25. #25
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    Quote Originally Posted by Motor60 View Post
    How, in the name of things holy, did you go 68 mph on a bike???
    didn't stop recording when mounted on a vehicle? Or a very, very steep downhill descent?
    2011 Specialized Rockhopper Expert 29er

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