Default units for Place Object

[bobwyman]

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Please Describe the Issue:

The Place Object allows the provision of a unit, which presumably modifies the interpretation of its altitude field. However, this relationship is not specified in the text and no default unit is provided.

I suggest the need for a Place primer topic stating that unit modifies altitude. If producers provide an altitude, then unit is a required field and must be provided. If no altitude is provided, unit must not appear. However, if altitude is provided without a unit, readers may assume the default value meters.

It should also be stated that altitude should be interpreted as distance above, or below, mean sea level on Earth rather than distance relative to the surface at the given latitude/longitude or any other point. (Note: This assumption, while appropriate for altitudes on Earth, would not apply on non-Earth bodies that typically have no "sea-level.")

Finally, I suggest that it be stated that latitude, longitude, and altitude, as defined in the current version of the vocabulary, apply only to places which are on the planet Earth. And, it would be useful to clarify that the coordinate reference system for latitude and longitude is EPSG:4326; the EPSG code for WGS84.

Note: I am preparing a FEP to define an extension defining a new crs field that would provide for specification of the coordination reference system to be used in interpreting these values. This extension would allow the use of alternative CRS's on Earth as well as extend the use of Place to the description of locations on non-Earth bodies.

[evanp]

I think this makes a lot of sense. We created https://www.w3.org/wiki/Activity_Streams/Primer/Place_type in issue triage; please review and update as necessary.