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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 say 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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Welcome. It is meters: wiki.geojson.org/RFC-001. Make a search for "z" and look at the second result. –  Andre Silva Jun 14 '13 at 0:02
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 This page has been superseded by RFC-2", and RFC-2 doesn't include that bit about z being in units of meters.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
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