Originally Posted by
rsiesta
I have seen several threads about inconsistent distance measurements when using gps on both runs and rides. I am using mapmyride and found an anomaly with a route that has a turn around. I am curious of anyone that has distance errors are on a route that turns around and retraces steps and if anyone can suggest a way to fix this problem.
Ron
Dear Ron
There’s a lot of talk about technicalities of GPS accuracy which I am finding a little superfluous.
I use MapMyRun on iPhone SE and a VDO bike computer, and I was finding quite large discrepancies. My first thought was - is the circumference in my bike computer correct? My second thought was - is the tyre pressure consistent on every ride? I figured that regardless of the accuracy of MapMyRun and/or phone-based GPS, I’m likely to get discrepancies from those two variables.
I believe calibration as well as those two variables were the cause of the differences. So I set about trying to calibrate my bike computer with the phone GPS app measurements.
Firstly - I inflated my tyres to my usual pressures. And I measured my actual wheel circumference, using a pencil, a tape measure and rolling my bike with me on it on a flat surface. The physical measurement was not quite the same as the “translation table” stated for my wheel specification.
Secondly - I used more care about inflating to same/similar pressures before each ride.
Then, I started to record the distance given for each ride by the bike computer and the phone app in a spreadsheet. And tracked a dozen or rides in this way. The differences were anywhere 2.6% to 5.9% between the two, but showed(proved) that my bike computer was consistently over-stating the distances by those percentages.
I then reduced my wheel circumference setting on my bike computer by 4%; 4% was the “average” difference between the two measurements over those dozen rides.
I started a new spreadsheet and have been tracking about another ten or so rides. So far, with the amended bike circumference setting in my bike computer, the differences between distances recorded by my phone and bike computer is down to 0.74%!!! Yes, a variance of less than 1%.
By my calculation, if I further reduce the wheel circumference setting on my bike computer by 0.74%, I think the two measurements for all future rides would be absolutely negligible.
The only times that the distance measurements between the two devices vary by “a lot”*** are -
1. on routes where I’ve had to stop many times for lights, etc.
2. where I have not checked my tyre pressure after a few days.
3. really short rides - below 5km (to get milk, bread etc
).
*** I consider “large” variances that are
markedly different in each spreadsheet, whether they were plus or minus.
I hope the above makes sense and is useful for you in somehow calibrating your phone or navigator GPS for tight turning circles.
For me, I was aiming to get accurate distances and accurate speeds on my bike computer; as I believe that my bike computer after calibration will be way more accurate than any phone GPS.
I am hopeful that if I tweaked my bike computer circumference by the second variance, my next lot of measurements will contain absolutely minuscule differences. (I will try to remember to post a further note here at that stage.)
Please PM me if you would like a link to my Google Sheet or I can send you a copy of the sheet.
(edited for spelling mistakes.)