Geo URI scheme


The geo URI scheme is a Uniform Resource Identifier scheme defined by the Internet Engineering Task Force's RFC 5870 as:
a Uniform Resource Identifier for geographic locations using the 'geo' scheme name. 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 current revision of the vCard specification supports geo URIs in a vCard's "GEO" property, and the GeoSMS standard uses geo URIs for geotagging SMS messages. Android based devices support geo URIs, although that implementation is based on a draft revision of the specification, and supports a different set of URI parameters and query strings.
A geo URI is not to be confused with the former website of GeoURL.

Example

A simple geo URI might look like:
where the two numerical values represent latitude and longitude respectively, and are separated by a comma. They are coordinates of a horizontal grid. If a third comma-separated value is present, it represents altitude; so, coordinates of a 3D grid. Coordinates in the Southern and Western hemispheres as well as altitudes below the coordinate reference system are signed negative with a leading dash.
The geo URI also allows for an optional "uncertainty" value, separated by a semicolon, representing the uncertainty of the location in meters, and is described using the "u" URI parameter. A geo URI with an uncertainty parameter looks as follows:
A geo URI may, for example, be included on a web page, as HTML:
so that a geo URI-aware user agent such as a web browser could launch the user's chosen mapping service; or it could be used in an Atom feed or other XML file.

Coordinate reference systems

The values of the coordinates only make sense when a coordinate reference system is specified. The default CRS is the World Geodetic System 1984, and it is not recommended to use any other:
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 syntax and semantic of the CRS parameter, separated by a semicolon, is described at section 8.3 of RFC 5870. Examples:
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:
Being in doubt, remember that use of the lowercase representation of parameter names 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 coodinates as coordinates = coord-a "," coord-b , where coord-c is optional. The semantic of coord-c for WGS-84 is altitude, and the concept is extend for other coordinates.
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, 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 relative to the geoid. 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 as the approximate physical position of an object in the Earth's surface.
The RFC 5870 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 number. The statistical meaning is implicit, come from the references of the RFC: the only normative reference with something about uncertainty is the . The main informative reference, ISO 6709:2008, not use the term "uncertainty", but use the terms "accuracy" and "precision", which are uncertainty facets and can be interpreted in accordance with ISO 5725-1.
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:
  1. uncertainty is symmetric: the RFC is explicit, and we can understand it as valid simplification hypothesis. "The single uncertainty value is applied to all dimensions given in the URI". Results in a spherical volume around the point.
    By RFC 5491 "locations are expressed as a point and an area or volume of uncertainty around the point".
  2. * Using RFC 5491, we can suppose that "It is RECOMMENDED that uncertainty is expressed at a confidence of 95% or higher". Therefore, the uncertainty is two standard deviations, 2σ, and it is the radius of the disk that represents uncertainty geometrically.
  3. fixed measure unit: the RFC obligate the use of meters as uncertainty measure units, even when coordinates use other. It is a semantic and a conversion problem: the
  4. Gaussian error model: RFC say nothing, we interpreting the phrases "amount of uncertainty in the location" and "the uncertainty with which the identified location of the subject is known", all in the context of the normative reference, RFC 5491.
  5. * adopting standard error model: the model of the most common descriptive statistics modeling.
  6. * It is imposed, is independent of selection process of uncertainty description, there are no other choices.
  7. total uncertainty: it is only one parameter representing "all uncertainty", the uncertainty in the spatial measure and uncertainty about object definition or object's center. It is a sum of random variables. There is no simplification hypothesis defined to reduce it to a one-variable model.
Imagining the location of an ant colony to illustrate:
The total uncertainty is the sum of GPS error and object-definition error. The latitude and longitude GPS errors need to be simplified and converted into meters. If the errors were inferred from a different model, they need to be converted to the Gaussian model.

Unofficial extensions

Some vendors, such as Android OS, have adopted extensions to the "geo" URI scheme:
Android adopts an unconventional approach to parsing the points: it does not show a map pin at the point given normally, and a map pin will show up only when given as the query. In other words, to show a pin at the Wikimedia Foundation office, one should not use geo:37.78918,-122.40335 but geo:0,0?q=37.78918,-122.40335.