March 3rd, 2007


More on GPS.

After I asked about GPS devices, the very lovely ewtikins kindly lent me her Garmin eTrex so I could try it out. So today I have been happily trundling about London collecting GPS data. I put some of it on the OpenStreetMap public domain list of Tube stations. But I have some questions! And I'm sure someone here can help.

What I'm wondering about is accuracy.

First of all, I've been marking 3–5 waypoints at each place I want to find data for, moving around slightly (~20–100cm) in between each one, the idea being that I might be able to get a slightly more accurate fix in some spots than in others (and the definition of "outside a specific Tube station/pub/etc" is quite fuzzy anyway). Is this a good strategy?

Secondly, the eTrex gives me lat/long to 5 decimal places (I haven't got the cable, so all I can do is read the figures off the display; I don't know if having the cable would give me more decimal places). I don't know if this is rounded or truncated. What I've been doing with my data is rounding it to 4 d.p. and then taking the mode. Is that the best way to do it? doop suggested I instead take the mean and then round that to 4 d.p. Would that be better? Or am I barking up completely the wrong tree anyway?

Finally, in the UK, a difference of 0.0001 in lat or long corresponds to about 10m (unless I've got my sums wrong). This is roughly the same as the tracking accuracy reported by the eTrex itself (5m is the best I've had; the worst has been about 25m; it's generally about 9m). Given that, does it actually make sense to give more than 4 decimal places anyway?

Any advice welcome.