On Mon, Mar 31, 2008 at 11:28 AM, Andy Robinson (blackadder) < [EMAIL PROTECTED]> wrote:
> >From: MilesTogoe [mailto:[EMAIL PROTECTED] wrote: > >Sent: 31 March 2008 3:40 PM > >To: Andy Robinson (blackadder) > >Cc: 'Sjors Provoost'; dev@openstreetmap.org; 'Karl Newman' > >Subject: Re: [OSM-dev] Altitude data & (cycle) route profiles > >what is the accuracy on regular GPS elevation ? +- ? Here in mountain > >territory we deal with a lot of roads and trails that may change 100's > >of feet (or meters), thus even a low accuracy of +- 5-10 ft would still > >be a help. And likewise to show off big things like the Chunnel or the > >high bridge in France.... > > For regular GPS receiver picking up just a handful of satellites the > accuracy from GPS alone isn't much better than +/- 50m (or worse) and it > doesn't get much more accurate when you have the maximum set in view. > Don't > ever expect GPS alone to deliver good vertical height data. Horizontal > position can be established much more accurately. This is why some upper > market GPS's have barometric pressure support, but as another has pointed > out these need calibrating each time you go out to be reasonably accurate. > <snip> > > Cheers > > Andy > GPS vertical accuracy is much worse than horizontal accuracy because you can't receive signals from the satellites in the best location to correct your elevation (they're behind the earth from you). Andy is correct that for the best accuracy, the GPS models with barometric altimeters included need calibrating to a known pressure or elevation when you go out. However, the Garmin models do okay because they slowly auto-calibrate to the GPS elevation. Not perfect, but better than nothing. Karl
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