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The mileage on my old bike computer got its distance from number of revolutions the wheel turned.
Does the distance on a GPS compensate for gradients or is it working on the assumption of a flat surface?
Or is it simply not significant.
Asking because my GPS measured distance over a 120 mile hilly loop is a little shorter than the same ride done with a bike computer - reasonably accounted for by other reasons, but it got me wondering.
Not significant and (I believe) not taken into account. Neither method is sufficiently accurate for it to be a surprise when they disagree, multiple GPS devices will disagree on the same trip anyway.
Both are probably wrong.
Don't know, I'd be surprised if it did as GPS altitude isn't as accurate as position (and barometric is even worse).
GPS's typically under read slightly as they measure the straight line distance between a point recorded at intervals (1s, 4s, 10s, whatever) so it always slightly cuts corners.
More likely your other computer is over reading because you measured the diameter of the wheel incorrectly when riding the flat bit of the tyre on the road causes its diameter to drop significantly, or the tyre was slightly lower pressure.
If you transfer the gpx file to wiggle it will correct height gain to a base map
But in the end I think we have to accept that the question "How long was that ride?" doesn't have a simple, consistent answer
It's a switchable option on some GPS trackers (certainly is on a Garmin Fenix). I've never tried it, but I guess on a hilly route you'd grab a few more k's over the course of a run/ride.
It's probably only worth having if the hardware has a barometer, GPS triangulated elevation data is fairly guff.
Vague memory that you could set a Garmin to measure 2D or 3D distance.
Most accurate way is to pair with a wheel-speed sensor and let it figure things out automatically.
jam bo - Member
Both are probably wrong.
Probably. It's not really important for my use except I was trying to measure up for a projected 200km audax.
The obvious answer is to stick in a few spare km just in case. 🙂
If you do the maths, it is usually not significant. Unless you are constantly cycling up and down very steep hills.
eg for a 200km audax, with an average gradient of 10% over the whole ride. That is only about 1km extra distance.
And it is very unlikely that the average is that much. It might be 10% over short sections, but most of the ride would be much flatter.
It varies is the best answer. The real solution is to plot the GPS to a map that corrects for the gradient.
Altitude by GPS is generally inaccurate when it comes to dropping sats. Barometer goes out with weather changes (worse on a long ride at times)
Most accurate way is to pair with a wheel-speed sensor and let it figure things out automatically.
I had always thought that when paired, like this, Garmins would default to GPS and only use wheel sensor if there as no GPS signal or (at start of ride) if signal hadn't been acquired. Out of curiosity anyone know how Garmins use wheel sensors when available?
Pretty sure it uses the wheel sensor for speed/distance, it definitely stops the annoying auto-pause on tight switchbacks off road.
Can concur that barometric data be pish.
Me n wife with same device 6 ft apart were often showing 200+m difference in height ....with her device having here 50+ m below sea level at some points - when we could see the sea !
50m below sea level pah! When the first Gulf War was on you could find yourself 5Km below sea level when the US military turned the scrambling back on!
GPS Lat and Long require three satellites as a minimum, altitude requires an extra satellite. It also depends where in the sky those satellites are - ideally they are spread around rather than in a cluster
trail_rat - Member
...with her device having here 50+ m below sea level at some points - when we could see the sea !
She must be very short... 🙂
I can't account for the discrepancy between the two, but the baro altitude will write rightly say you're higher or lower than you are, based on the fact that it'll be using a standard pressure setting. So if you're in the middle of a massive high it'll tell you that you're lower than you are, if you're in a low it'll tell you that you're higher than you are. The barometric altitude on a GPS tracker is only useful for measuring the difference in altitude over a ride and will be affected by changes in the weather