geo URI scheme

A 'geo' URI identifies a physical location in a two- or three-dimensional coordinate reference system in a compact, simple, human-readable, and protocol-independent way.

The default CRS is the World Geodetic System 1984 (WGS-84),[1] and it is not recommended to use any other: The optional 'crs' URI parameter described below may be used by future specifications to define the use of CRSes other than WGS-84.

This is primarily intended to cope with the case of another CRS replacing WGS-84 as the predominantly used one, rather than allowing the arbitrary use of thousands of CRSes for the URI (which would clearly affect interoperability).

[1] The only justified use of other CRS today is, perhaps, to preserve projection in large-scale maps, as local UTM, or for non-terrestrial coordinates such as those on the Moon or Mars.

The Geo URI scheme semantics, expressed in the section 3.4 of the RFC 5870, is not explicit about some mathematical assumptions, so it is open to interpretation.

The semantic of coord-c for WGS-84 is altitude in meters (specifically the "ground elevation", relative to the current geoid – Earth Gravitational Model – attached to WGS84),[5] and the concept is extended for other coordinates (of non-default CRS).

The RFC explains that "... undefined MAY assume that the URI refers to the respective location on Earth's physical surface."

Putting all together, adopting these clues, the usual statistical assumptions, and the explicit definitions of the RFC, we obtain the Geo URI's uncertainty mathematical properties: Imagining the location of an ant colony to illustrate: The total uncertainty is the sum of GPS error and object-definition error.