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GPS Distance - Garmin vs. MB vs. BT
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 2007-02-12 3:38 PM |
Expert  832    Framingham, MA |  Subject: GPS Distance - Garmin vs. MB vs. BTYesterday I tried my Forerunner 305 outside for the first time, and when I got home I was excited to load up the route in my BT log. But, since I needed a GPX file I synched it to MotionBased first. Looking at the distance and time, though, there seemed to be some noticeable differences:
Has anyone else noticed this much of a difference? And I apologize if this has been covered elsewhere, I had trouble sorting through all the posts about routes. BTW, I've attached a zipped copy of the GPX file in question. Many thanks! Edited by swadland 2007-02-12 3:43 PM Attachments ---------------- MotionBasedExport.zip (51KB - 34 downloads) |
| 2007-02-12 4:04 PM in reply to: #688443 |
| , Texas | Subject: RE: GPS Distance - Garmin vs. MB vs. BT Here's my best guess: Forerunner/Training Center - Distance based on actual received sat data during the run. I think only Garmin knows the actual algorithm for this. MotionBased - Distance based on the GPS tracks stored by the Forerunner. MB also uses their own track point "correction" algorithm. Details of MB's calculations are here. BT - Distance based on the GPS tracks stored by the Forerunner. I *think* some tracks are being thrown out, though, for increased s/w effeciency. Also, I'm not sure if BT is doing 3-D (ie including elevation) distances or not? |
| 2007-02-12 4:34 PM in reply to: #688477 |
Champion 11641  Fairport, NY | Subject: RE: GPS Distance - Garmin vs. MB vs. BT Camy's right on all points here, but we don't do 3D distance calculations. Like the MB article states, this isn't a problem that's easily solved to everyone's liking. The Route Tracker is shorter due to some points being removed from uploaded routes for efficiency, but i can see that this needs to be updated. I think i'll change the algorithm to only throw out a point if it meets the 'this far from the previous point' criteria AND the point 2 points prior is not more 2x the distance criteria. The shorter distance is due to corner points being removed. It looks like heading information is sometimes stored in these files, so I'll see if that can be used as a good predictor of when to NOT remove a point. If the distance from the previous point is under x meters but the heading changes significantly enough it should obviously stay.
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| 2007-02-12 8:25 PM in reply to: #688443 |
Expert 832 Framingham, MA | Subject: RE: GPS Distance - Garmin vs. MB vs. BT I'm not sure if this sheds any more light on the situation, but I have my Forerunner set to record data every second, since I sync it with the computer on a regular basis. So, the "GPS for Dummies" question is, how far did I actually run? I measured it today in my car just to get another opinion, and it came out to 2.7 miles. |
| 2007-02-13 2:10 PM in reply to: #688443 |
| , Texas | Subject: RE: GPS Distance - Garmin vs. MB vs. BT I'd recommend against the 1 every second recording for running. Using it will drop a trackpoint every 4 ft or so, which is considerably less than the accuracy of the device. This will flood trackpoints and probably do more harm than good from an overall tracking perspective (not to mention the pain it creates for post-processors like the BT route). I'd imagine the one second recording is there for something like downhill skiing where you're covering alot of distance in a short amount of time. For the most accurate distance, I'd go with the Forerunner and I believe it uses the same algorithm to calculate distance regardless of trackpoint recording. It might also be worth it to turn on Gravity in MB in case some of your elevation gain/loss is errant. |
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