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The GeoJSON spec says "The default CRS is a geographic coordinate reference system, using the WGS84 datum, and with longitude and latitude units of decimal degrees". It doesn't seem to specify what the units of elevation/altitude/z are for points.

Is there a default unit of altitude for GeoJSON? If not, are there common conventions, or a way to specify units?

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  • Interesting that the specification doesn't give any 3d examples.
    – Fezter ♦
    Jun 14, 2013 at 0:55
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    Andre, RFC-001 says "This is historical material. This page has been superseded by RFC-2", and RFC-2 doesn't include that bit about z being in units of meters.
    – John Wiseman
    Jun 14, 2013 at 7:11
  • Not entirely surprising, GeoJSON is designed to be as simple as possible. Things like Z/M values and CRSes are useful, but much less commonly used and covered, though that is likely to change.
    – scw
    Jun 14, 2013 at 7:20
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    As an update, a couple draft specs have come and gone, but the latest draft spec (as of 2015-12-01), draft-ietf-geojson-00, at datatracker.ietf.org/doc/draft-ietf-geojson/?include_text=1 says "An OPTIONAL third position element SHALL be the height in meters above the WGS 84 reference ellipsoid."
    – John Wiseman
    Dec 2, 2015 at 22:20

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The current correct answer (as mentioned by John Wiseman in a comment on the question) is:

An OPTIONAL third-position element SHALL be the height in meters above or below the WGS 84 reference ellipsoid.

This can be found under "4. Coordinate Reference System" in RFC 7946 available at https://datatracker.ietf.org/doc/rfc7946/?include_text=1

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Meters above mean sea level, per the wiki specification doc:

WGS84 is implied and coordinates represent decimal degrees ordered as "longitude, latitude [,elevation]" with z expressed as metres above mean sea level per WGS84.

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  • Not doubting the quote, but does WGS84 actually define MSL? I could have accepted some kind of height-above-ellipsoid...
    – BradHards
    Aug 3, 2015 at 22:41
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    That quote is from RFC-001, which says "This is historical material. This page has been superseded by RFC-2". Neither RFC-2 nor the most recent specification I could find (wiki.geojson.org/GeoJSON_draft_version_6) mention the units for altitude/elevation.
    – John Wiseman
    Aug 5, 2015 at 18:37

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