/ Hey does my gps watch work out tunnel lengths?
(I want it to be worng because otherwise it was a pathetic time ;-)
you enter tunnel, GPS signal drops off, it stops tracking you and starts listening for new signals
you pop out the tunnel, the watch starts reaquiring signals and fixing you location and then starts tracking you again... this can take anywhere from a few seconds to a minute or so depending on several factors. the whole time you are moving away from the end of the tunnel
so, no it's probably not super accurate
Probably not. Depending on the gps, try uploading the route and then view it on Google Earth. You'll get to see the route that was recorded. It probably won't included the bit where the satellite signal was lost.
Not sure about a watch but fancier GPS software will extrapolate your position based on assuming you continue at your previous speed and direction so they can be OK if they can't see the satellite for a short period.
I remember doing the Oban half a couple of years and being a bit disconcerted by the discrepancy between the watch distance and the mile markers. When I uploading the route onto Google Earth, it was clear that the signal had been degraded at one point, when we were running close to the base of some cliffs, which was enough to skew the measurements.
I don't think it does. The running watches record every few seconds, unlike the general purpose GPS units that use an algorithm based on speed and change in direction to decide when to create a time stamp.
If it doesn't get a position, it doesn't record and then it can't calculate the distance that it has missed recording.
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