[Tagging] RFC ele:regional

[Greg Troxel]

Volker Schmidt <voschix at gmail.com> writes:

> I am not an expert, but it looks as if the Wiki page Key:ele
> <https://wiki.openstreetmap.org/wiki/Key:ele> is not up-to-date.
> I thought that WGS84 uses the EGM96 Geoid, named "WGS84 EGM96 Geoid".
> Hence there should be no difference between WGS84 and EGM96 elevations.

To be somewhat pedantic, EGM96 is a function that takes lat/lon as input
and produces a value which is the offset of the ellipsoid and  the geoid
(equal-gravity surface).   So "EGM96 elevation" is a big awkward.   But,
one uses WGS84 ellipsoidal height and EGM96 to get "WGS84 altitude".

I believe EGM96 was replaced with EGM08:
  https://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_wgs84.html

But this is basically an update with more precision, so it's very much
the same thing.

> Also it would be helpful, f you could give examples of local elevation
> systems which would need explicit tagging.
> When I see an elevation value on the ground I do not see any reference to
> the reference system, so I cannot know, as a mapper, what reference system
> is at the base of the informaton that I find on the ground. In that respect
> the proposal is not at all clear from a practical perspective.

Completely agreed.  It is a good guess that signs you see are in your
national vertical datum.  But some (most?) places have multiple datums,
and it seems very likely that values people have known have been copied
forward on signs for who knows how long, and there's no way to tell
which one is meant.    This is true on the US for things like mountains
and "highest point on the masschusetts turnpike" signs -- which lacks a
datum.  The wikipedia article doesn't address this point :-)