[OSM-talk] [tagging] Feature Proposal - RFC - incline up down

[Morten Kjeldgaard]

John Smith wrote:
> --- On Sat, 22/8/09, Morten Kjeldgaard <mok at bioxray.au.dk> wrote:
> 
>> Not really. There's a lot of elevation data available in the GPS traces, and since roads where incline=* is relevant are drawn along GPS traces, it's a matter of exploiting that data value. I'm aware that the GPS elevation data isn't terribly accurate on an absolute scale, but when determining inclines we will be making elevation differences which will decrease the error significantly.
> 
> I doubt most GPS traces would be useful for this kind of thing, most vertical GPS data is +/- 10m under the best possible conditions, usually I seem to be +/- 20m most of the time, and it's not a stable 20m diff it jumps about just like GPS positions do.
> 
> You'd need a GPS with an altimeter, or a stand alone altimeter, to do this kind of elevation differences to get accuracy better than up/down.

Right, but when making differences the error will become much less 
significant. As you know, the GPS device reliably tracks your relative 
movements, although the absolute position might be in error.

When subtracting two positions from each other, the absolute positioning 
error will disappear.

In addition, for many traces there will be multiple measurements, which 
will give a much better determination of the gradient.

Thirdly, time is on our side, because as tech develops, GPS devices become 
much more accurate... think Galileo etc.

Cheers,
Morten