A geo URI for a hypothetical lunar CRS created in 2011 might be: geo:37.786971,-122.399677;crs=Moon-2011;u=35 The order in which the semicolon-separated parameters occur is partially significant. Whilst the labeltext parameter and future parameters may be given in any order, the crs and the u parameters must come first. If both are used, the crs must precede the u. All parameters are case-insensitive, so, imagining a future new parameter mapcolors, it can be ignored by simpler applications, and the above example is exactly equivalent to: geo:323482,4306480;CRS=epsg:32718;U=20;mapcolors=for_daltonic Being in doubt, remember that use of the lowercase representation of parameter names (crs u and mapcolors) is preferred. Semantics and usual interpretations 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. After ~10 years of its publication, there are some consensual or "most frequently used" assumptions. Altitude The syntax of the Geo UI defines coordinates as coordinates = coord-a "," coord-b [ "," coord-c ], where coord-c is optional. The semantic of coord-c for WGS-84 is altitude (specifically the "ground elevation", relative to the current geoid – Earth Gravitational Model – attached to WGS84), and the concept is extended for other coordinates (of non-default CRS). The RFC explains that "... undefined <altitude> MAY assume that the URI refers to the respective location on Earth's physical surface." However, "... an <altitude> value of 0 MUST NOT be mistaken to refer to 'ground elevation'". In other words, when an altitude is defined, the measurement is done relative to the geoid (#5; black line in the image), a surface defined by Earth's gravity approximating the mean sea level. When it is undefined, the elevation is assumed to be the altitude of the latitude-longitude point, that is its height (or negative depth) relative to the geoid (i.e. "ground elevation"). A point with a measure "altitude=0" is, however, not to be confused with an undefined value: it refers to an altitude of 0 meters above the geoid. Uncertainty Geo URI is not about exact abstract positions, strictly it is an location estimate, and we can interpret it (from RFC 5870 and RFC 5491) as the approximate physical position of an object in the Earth's surface. The RFC 5870 does not formalize the use of the "uncertainty" term. So, in a coarse-statistical or any non-statistical numerical analysis, the GeoURI uncertainty is a condition