[Tagging] Feature proposal - RFC - ele with units

[Greg Troxel]

Minh Nguyen <minh at nguyen.cincinnati.oh.us> writes:

> And now for something unrelated to route relations. I've written up a
> proposal that would:
>
> * Tweak the definition of the ele=* key to accept values in feet, as
> long as the unit is specified, just like height=*, maxheight=*, etc. A
> bare number without a unit symbol would continue to be in meters.

That seems ok.  You said "feet" so presuambly that is "international
feet" rather than "US survey feet".  Although, the difference is so
slight, and vastly smaller than the differences in datum and the likely
errors that it does not matter.

For the record, I think ele in osm is "WGS84 Orthometric Height", which
means more precisely:

  Start with ellipsoidal height in the currently-active realization of
  WGS84 which is WGS84(G2139).

  Appply the geoid model associated with the currently-active WGS84
  realization, which is EGM2008.

which leads to something that feels like "height above sea level", and
is close but not exactly equal to height expressed in national datums
like the US NAVD88 and the European EVRF, but is close enough for
people that are not able to measure height to 25 cm.

(If you are someone who thinks you can measure height to 25cm or
thereabouts, please reach out in private mail; I'd like to compare
notes.)

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