On Tue, 29 Dec 2009, Robin Paulson wrote: > the srtm contour data used in some osm renders is great, partic for > cyclists and walkers. i've realised that it's not as high > accuracy/precision as it might be though. i notice that most GPS > devices also record the elevation within their tracklogs, and that it > might be useful to extract this data, and use it to provide more data > for calculating the altitude of a given point. > > any ideas? >
There are many sources of error in SRTM and GPS and barometric height measures. First of all Height compared to what? Sea Level? Sea Level is as arbitrarily defined as any tag in OSM - its what somebody else says "Sea Level" is. Next - there are plenty of fake elevations in SRTM data. I can show you places where the creek is higher than the ground according to SRTM. On the flat plain the only trees grow next to the creek and are an obvious feature - so the satellite measure picks out the trees as a rise in ground level. Sometimes the contour elevation is found to be a big shed. Most of the time the Garmin Etrex Hcx offers the same height as the SRTM data on the imported map, but seeing where and when it makes errors is the key to understanding the errors. A fast bicycle descent wrecks the height calculation because the processor is too slow. It defaults to barometric measure then but this is affected by more than height. _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
