Ah, of course, thanks @Dataliberate!

It would be good to be explicit about the units for elevation:
https://schema.org/elevation

Metres v. fractions of a kilometre?
And I am sure Americans would try using Feet...

The WGS 84 does seem to explicitly state values in meters.

All CRS that I know seem to measure elevation in Meters. So I think a clarification to that effect is warranted.
@njh could you please edit the issue title appropriately?

If you supply it as a text, then things like 1,000m & 3,280ft would be understood by search engines I believe.

I doubt that very much, but Schema always allows Text.
So an example like "3280 ft" could be given, with a warning that the value may not be interpreted appropriately. I've removed the comma and added a space for easier parsing, eg https://ci.mines-stetienne.fr/lindt/index.html will actually interpret this correctly, but only in Jena and given an appropriate datatype, eg
"3280 ft"^^cdt:ucum

Recommend description of elevation property should be enhanced thus:

The elevation of a location (WGS 84). Values are of the form ' '. E.g., '1,000 m', '3,200 ft'. Numbers alone should be assumed to be a value in metres.

Thanks for this improvement!

For reference, this is what we eventually put into the (very informal) w3c wgs84 basic geo vocab:

<rdf:Property rdf:about="http://www.w3.org/2003/01/geo/wgs84_pos#alt">
 <rdfs:domain rdf:resource="http://www.w3.org/2003/01/geo/wgs84_pos#SpatialThing" />
 <rdfs:label>altitude</rdfs:label>
 <rdfs:comment>The WGS84 altitude of a SpatialThing (decimal meters above
 the local reference ellipsoid).</rdfs:comment>
</rdf:Property>

See https://www.w3.org/2003/01/geo/ http://www.w3.org/2003/01/geo/wgs84_pos