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

[Lambertus]

Robert Vollmert wrote:
> Not sure how common they are, but there's some GPS units with built-in  
> barometer (Garmin Etrex Vista HCX at least). The data they provide  
> should be good enough to be useful.
> 
Those sensors need calibration against a known point and can give a huge 
offset when uncalibrated. I have such a GPS (GPSmap 60CSx) and never 
calibrate it as will most others presumably. As a cyclist all I'm 
interested in is the relative altitude differences not the precise 
actual altitude as opposed to, say, skydivers. Measuring relative 
altitudes is quite accurate (+/- few meters) though. The data is useless 
when you want to use it as a primary source, but it should be alright if 
you want to use it to determine offsets when compared with a known 
point. This requires you to process the GPX nodes trace-by-trace so you 
can determine the offset from which the trace started.

I appose to using altitude data on nodes. E.g. one would not be able to 
move a node when that data is added without having to correct/remove the 
  height data as well. Also adding altitude data invites to add 
-otherwise useless- nodes everywhere. I prefer to have altitude data 
kept apart from OSM database. Perhaps our own SRTM database 
corrected/enhanced using our traces.