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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 '13 at 0:55
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 This page has been superseded by RFC-2", and RFC-2 doesn't include that bit about z being in units of meters.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 '13 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 '13 at 7:20
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 says "An OPTIONAL third position element SHALL be the height in meters above the WGS 84 reference ellipsoid." – John Wiseman Dec 2 '15 at 22:20

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 '15 at 22:41
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 This page has been superseded by RFC-2".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 ( mention the units for altitude/elevation. – John Wiseman Aug 5 '15 at 18:37

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