[OSM-dev] gpx elevation data for given bounding box?

[Karl Newman]

On Sat, Sep 13, 2008 at 2:06 PM, Igor Brejc <igor.brejc at gmail.com> wrote:

> Tom Hughes wrote:
> > Stefan de Konink wrote:
> >
> >
> >> The reason seems to be that some people *think* they are not accurate.
> >>
> >
> > I think the experiment we conducted at the 4th birthday party showed
> > pretty conclusively that it's not accurate. We were only about 12 foot
> > above sea level (as measured by looking over the wall at the Thames) but
> > the various GPS units present reported values from about 0 to about 200
> > foot or so. They also varied radically over time despite the fact that
> > we were sat still.
> >
> > Tom
> >
> >
> Some units (Garmin eTrex Vista, for example), use barometric altimeter
> as the only source for the elevation data. Which means that it's
> reliable only if calibrated regularly (every few hours, if weather
> changes). Otherwise the data is more or less useless for mapping
> purposes - errors could be in hundreds of meters. So you cannot rely on
> such GPX elevation logs.
>
> Igor
>
>
Garmin units with barometric altimeters use an autocalibration method where
the GPS altitude is used as an input to correct for pressure changes.
According to some users' tests, if set up properly initially (i.e.,
calibrate it to a known elevation or wait about half an hour before starting
out), it works quite well, maintaining accuracy within 5 to 10 meters the
entire day. See here for one such testimonial (not the best documentation of
an experiment, but it's what I could find quickly):
http://forums.groundspeak.com/GC/index.php?showtopic=187738

Karl
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