[Talk-us] Incorrect Summit Elevations - Colorado

[Greg Troxel]

  It appears that there is a systematic error in the summit elevations
  in OSM, at least in Colorado.  For example, Longs Peak is listed in
  OSM as having an elevation of 4340 meters (14,239 ft).  The topo map
  has it as 14,251 ft.  I have noticed the same type of issue with
  nearby peaks.  I suspect this has to do with the GNIS import.  The
  elevations in the GNIS are from the National Elevation Dataset (NED),
  not spot (e.g. summit) elevations.  The NED gives the (presumably
  average) elevation for a 3x3 arc second area.  According to the
  metadata for the GNIS:

I don't think you are going to get accurate summit elevations from a
DEM.

  Precise and official elevations are very important for hikers and
  mountineers, or at least we like to think they are.

Agreed, but the concept of elevation is highly tricky.  I think I
understand it, having read several geodesy textbooks...

  I believe that the National Geodetic Survey holds data on the official
  elevations for summits within the U.S., but I need to look into that.
  I am happy to start working on fixing this issue, but I didn't want to
  commence if someone else is already working on.

I would expect them to have elevations for control points, and that many
summits have a control point near the top.


The basic issue is the concept of "vertical datum".  In the US you will
find data in two such datums: NGVD29 and NAVD88.  I think a lot of the
old data is in NGVD29, and modern practice is to use NAVD88.  Then
there's WGS84, which really gives you "ellipsoidal height" and tries to
convert to "orthometric height" via a "geoid model".  This is hard
because the earth is not only not flat, but also the "equipotential
surface" (of constant gravity) is not a pure ellipsoid.

See the height entries here:

  http://www.ngs.noaa.gov/faq.shtml


So the first question is:

  what is the definition of the meaning of altitudes in OSM, and where
  is it documented.
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