[Tagging] Elevation and height on vertical features

Colin Smale colin.smale at xs4all.nl
Fri Jan 8 16:47:40 UTC 2016

On 2016-01-08 17:18, Greg Troxel wrote:

> So we just have to fix things that are wrong, and transform heights in
> other datums into WGS84 before entering them.  This is exactly the same
> situation that we encounter for horizonal datums, except that people are
> even less aware of which vertical datum they are using.

How did all the elevation data get into OSM in the first place? GPS is
notoriously bad at determining elevation/altitude. Apparently some
receivers will give you the raw WGS84 altitude, and others will make
some "corrections". If the elevation in OSM was taken from some other
published (and suitably licensed, of course) source, I would probably
expect it to be relative to the traditional local datum and not WGS84.
So which ones are which? Which of the data already in OSM needs fixing,
and which is already correct? 

Maybe we should discard all the elevations currently in OSM, and ask the
user "are you sure your elevations are in WGS84?" before accepting new

> I don't think it makes sense to add datum tags and have heights in other
> datums.  That just pushes the work onto the data consumer and adds
> confusion.

How can being more explicit about the datum lead to added confusion? We
allow explicit units for maxspeed etc, so why not allow an analogous
concept here? 

is interesting reading material 
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