/ Hey does my gps watch work out tunnel lengths?

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Bjartur i Sumarhus on 26 Oct 2012
Prob a daft question but my run today involved running through a foot tunnel to greenwich park. Obviously it lost gps but I didn't check when i came up the otherside if it figured out the distance once it refound the sat connection. So can I trust the distance?

(I want it to be worng because otherwise it was a pathetic time ;-)
rallymania - on 26 Oct 2012
In reply to Game of Conkers:

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
tony on 26 Oct 2012
In reply to Game of Conkers:
> Prob a daft question but my run today involved running through a foot tunnel to greenwich park. Obviously it lost gps but I didn't check when i came up the otherside if it figured out the distance once it refound the sat connection. So can I trust the distance?

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.
tom_in_edinburgh - on 26 Oct 2012
In reply to Game of Conkers:

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.
Bjartur i Sumarhus on 26 Oct 2012
In reply to tony: YEs, good point. When I upoad it to Garmin connect i will have a look at the route on the map
tony on 26 Oct 2012
In reply to Game of Conkers:

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.
the real slim shady - on 26 Oct 2012
In reply to Game of Conkers: if your tunnel was a curve the watch would only measure the straight line distance between the start and end of the tunnel
Neil Henson - on 26 Oct 2012
In reply to Game of Conkers: A series of tunnels on the Paris Marathon route created a large descrepancy between what I knew I had ran (from the mile markers) and what my watch was telling me. Quite annoying when you are tired and trying to work out how far you have left to go.
DancingOnRock - on 26 Oct 2012
In reply to derek the dragon:
> (In reply to Game of Conkers) if your tunnel was a curve the watch would only measure the straight line distance between the start and end of the tunnel

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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