[OSM-dev] Altitude data & (cycle) route profiles

Karl Newman siliconfiend at gmail.com
Mon Mar 31 18:42:02 BST 2008

On Mon, Mar 31, 2008 at 11:28 AM, Andy Robinson (blackadder) <
blackadderajr at googlemail.com> wrote:

> >From: MilesTogoe [mailto:miles.togoe at gmail.com] wrote:
> >Sent: 31 March 2008 3:40 PM
> >To: Andy Robinson (blackadder)
> >Cc: 'Sjors Provoost'; dev at 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.

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