Activity Streams/Primer/Place type

The Place type describes a logical or physical place. It's described in the places appendix in the Activity Vocabulary, as well as at Place.

Important properties of a Place are:

name for named places, summary for a place without a name ("a spot in California")
latitude - decimal latitude
longitude - decimal longitude
units - this is the unit applied for both radius and altitude. Default is "m" for meters.
accuracy - a floating-point value for the percentage of accuracy
radius - the radius of the area, or the error value (+/- 50 meters, for example)
altitude - altitude of the point in a spherical coordinate system. This is 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.")

The Place object does not have a lot of geographical detail, for example, parent or children in a geographical hierarchy, codes in a standard vocabulary like ISO 3166, or a polygon or multipolygon representation.

The Place object is also primarily structured for places on Earth, and does not have properties for places on other planets or in other coordinate systems. The coordinate reference system for latitude and longitude is EPSG:4326; the EPSG code for WGS84.

The Place type also doesn't have properties for fictional places such as The Shire.

Recommendations for publishers

Use latitude and longitude for point-like places
Use altitude if available for point-like places
Define units explicitly, even if using the default value of "m"
Avoid radius and accuracy as they are not well-defined

Recommendations for consumers

Use latitude and longitude for point-like places
Use altitude if available for point-like places
If the units is not defined, use the default "m"
Ignore radius and accuracy as they are not well-defined