[Tagging] Representing "altimetric quotas" in OSM

Greg Troxel gdt at lexort.com
Tue Jan 17 22:21:06 UTC 2017

"Nelson A. de Oliveira" <naoliv at gmail.com> writes:

> I really don't know if the proper term is "altimetric quota" in
> English, sorry.
> What we have are some places where the elevation at some points were
> measured. It's similar to man_made=survey_point but without any
> physical objects or marks there; somebody just measured the elevation
> at that point and charted it.

In English and US surveying practice, we have two things:

  "benchmark": This is a physical monument (brass disk usually, for <
  100 year old marks).  It has an established orthometric height from
  leveling.  The horizontal coordinates are not necessarily known
  precisely, just as the elevation of horizontal controls is not
  necessarily known precisely.  These are the vertical control network
  making up our NGVD29 and NAVD88 datums.  Definitely a survey_point.

  "spot elevation":  This is a notation on a topographic map giving the
  elevation at some point.  There is not necessarily a mark on the
  ground.   These probably do not belong in OSM, but if they are
  recorded in survey databases, it seems reasonable.

Now, control points are likely to be 3D, and use GPS, and have
ellipsoidal coordinates and an elevation derived from ellipsoidal height
and a geoid model.

For measured spot elevations that have some sense of officialness, I
would be tempted to call them man_made=survey_point, perhaps with
survey_poinrt=none :-)

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