[Tagging] Feature proposal - RFC - Documenting feet as an an optional elevation unit

[Greg Troxel]

As someone not happy about the deprecation of mailinglists, a few brief
comments here:

  First, I think this proposal is fine, as documenting widespread
  practice.  Regardless of my further comments, I think it's solidly
  progress to adopt it.

  While yonur comments about survey feet are valid, modern elevations
  (NAVD88) are as far as I can tell actually in meters, and when
  expressed in feet, in international feet.  Elevations are small enough
  that 2 ppm is less than the errors in the values.

  I would expect the proposal to give an example.  It seems that one
  would have a tag
    ele=6288 ft
  for Mount Washington (showing my East Coast bias).
  
  It would be good to explicitly state that in keeping with convention,
  ft means international feet, perhaps with a parenthetical comment that
  if someone meant US Survey Feet they would have written ftUS.   Maybe
  this is already documented.

  There is a much more serious problem in that few seem to understand
  that elevation is only meaningful relative to a vertical datum.  OSM
  documents WGS84.  Even fewer understand that this is a mess because
  WGS84 is an ensemble containing a low-accuracy member
  (WGS84(TRANSIT)), and that the only reasonable interpretation is that
  data should be expressed in the most recent realization.

  Further, WGS84's first height definition is ellipsoidal height, and
  that simply is not elevation.  Obviously elevation should be in "WGS84
  Orthometric Height", which is what GPS receivers provide as elevation.
  But elevations are not published in WGS Orthometric Height; they are
  published in a national or regional datum which is pretty close, as
  all datums at least roughly target a similar origin.

  Practically, people type in numbers from a sign, and this sign was
  probably copied from some earlier sign, and may be in some ancient
  datum, and may have been erroneous.   This proposal has no bearing on
  that, and that's ok.