Discussion: Differences in Garmins

I have been running with my mate and we both have garmins, me the 310xt and him the 110. His consistently shows that we run further than what my watch does, over 19km it was 600m further. Does anyone know why this is?

I don't know the technical specs or operating modes of the 310xt and 110, but my guess is that they use different sampling strategy and different interpolation algorithms. E.g. one might use 1 second interval sampling and the other might use "smart recording" (i.e. sample when you change direction, change pace etc) together with some algorithm to connect these "smart" samples into a continuous track.

You might be able to download the tracks and compare them. I had an older GPS that seemed like it used heading and speed data to extrapolate position in the absence of signal---it would show me overshooting way beyond what I remembered. This might be just the effect of an inferior receiver. I'd guess that an inferior receiver would miss little jinks, and tend to show less distance than a better receiver if it uses only valid position data and doesn't try to fill in with speed and heading extrapolations. On the other hand, a poor receiver might have more wander as it loses and gains signal, so it might actually add distance.

One common test is to run around a track (a real track or some fixed path) multiple times. That way you can see what sort of errors the GPSs are reporting. You might even put both GPSs on the same arm of one person and see how the tracks differ. (Real tracks (i.e., 400m athletic tracks) are good because the smooth curves give gradual changes to the receiver and yet still show lots of error.) It might be good to also find some fixed path with very sharp bends, like a switchbacked trail or something. Sort of the other end of the spectrum from a 400m track.