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

[Rogier Wolff]

On Tue, Sep 16, 2008 at 03:50:57PM +0100, Robert (Jamie) Munro wrote:
> I also notice that sometimes if I my GPS starts off reading 20m to the
> north of reality, the whole trace is 20m north of reality for a long
> time, but the actual shape of the trace matches the road. I wonder if
> vertical traces are the same - perhaps a GPS will consistently read 10m
> above reality, and if we can correct for that where a trace overlaps
> another trace or SRTM data, we can use it to fill in an SRTM void that
> the trace continues into.

Many GPSes will claim a good fix if they drop from 4 sats to 3
sats. Still 3 sats is not good enough to get a height fix. So the
assumption is that you will stay at the same height. However, if that
assumption is wrong, the position will be off too! The "shape" of the
trace will, for us humans look very much the same as the correct
trace. However, the the offset depends on the difference between
"assumed height" and "real height". If the assumption that the height
doesn't change is correct, but the assumed height is incorrect, then
the trace will be offset as you say, but if the terrain is not flat,
then the trace will shift depending on the height. Even when this is
the case human pattern matching will very quickly recognize that it
looks very similar to the actual trace because recognizable points
like corners line up.

(There is a hill in switzerland, where the GPS in my fathers car
usually has only 3 sats reception. Moreover the road goes down
reasonably steep. So if you drive there, the "moving car" icon on the
map will move offroad as if you're driving straight down the hill
through the field. The displayed track actually crosses the road
further on after the hairpin bend I'll be driving a minute later....)


	Roger. 

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