geo URI scheme

Thegeo URI schemeis aUniform Resource Identifier(URI) scheme defined by theInternet Engineering Task Force's RFC 5870 (published 8 June 2010)as:

a Uniform Resource Identifier (URI) for geographic locations using the 'geo'scheme name.A 'geo' URI identifies a physical location in a two- or three-dimensionalcoordinate reference systemin a compact, simple, human-readable, andprotocol-independent way.

The current revision of thevCardspecificationsupports geo URIs in a vCard's "GEO" property, and theGeoSMSstandard uses geo URIs forgeotaggingSMS messages.Androidbased 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 ofGeoURL(which had implementedICBM addresses).

Example

A simple geo URI might look like:

geo:25.245470718844146,51.45400942457904

where the two numerical values representlatitudeandlongituderespectively,and are separated by acomma.They are coordinates of ahorizontal grid(2D). If a third comma-separated value is present, it representsaltitude;so, coordinates of a 3D grid. Coordinates in the Southern and Western hemispheres as well as altitudes below the coordinate reference system (depths) are signed negative with a leading dash.

The geo URI also allows for an optional "uncertainty" value, separated by asemicolon,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:

geo:37.786971,-122.399677;u=35

A geo URI may, for example, be included on a web page, asHTML:

<a href= "geo:37.786971,-122.399677;u=35">Wikimedia Headquarters</a>

so that a geo URI-awareuser agentsuch as aweb browsercould launch the user's chosen mapping service; or it could be used in anAtomfeed or otherXMLfile.

Coordinate reference systems

The values of the coordinates only make sense when acoordinate reference system(CRS) is specified. The default CRS is theWorld Geodetic System 1984(WGS-84),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).

The only justified use of other CRS today is, perhaps, to preserve projection inlarge-scale maps,as localUTM,or for non-terrestrial coordinates such as those onthe MoonorMars.The syntax and semantic of the CRS parameter, separated by a semicolon, is described at section 8.3 of RFC 5870. Examples:

• TheWashington Monument's location expressed withUTM-zone 18Nand itsstandard ID:

  geo:323482,4306480;crs=EPSG:32618;u=20

• 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, thecrsand theuparameters must come first. If both are used, thecrsmust precede theu.All parameters arecase-insensitive,so, imagining a future new parametermapcolors,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

The use of the lowercase representation of parameter names (crsuandmapcolors) 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

1. Ocean
2.Reference ellipsoid
3. Local plumb line
4. Continent
5. Geoid

The syntax of the Geo UI defines coordinates ascoordinates = coord-a "," coord-b [ "," coord-c ],wherecoord-cis optional. The semantic ofcoord-cforWGS-84isaltitudein meters (specifically the "groundelevation",relative to the currentgeoid–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 themean 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.

The use of a geoid stands in contrast toGeoJSON,which uses direct ellipsoid height.

Uncertainty

Facets of the uncertainty. According toISO 5725-1:accuracyis the proximity of measurement results to the true value;precisionis the degree to whichrepeated(orreproducible) measurements under unchanged conditions show the same results.

Remembering the example above,

geo:37.786971,-122.399677;u=35

Theu=35part informs the uncertainty. As will be showed, geometrically the uncertainty is a disc of radiusuin turn of the point of the geo URI.

Geo URI is not about exact abstract positions, strictly it is alocation 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-statisticalnumerical analysis,theGeoURI uncertaintyis acondition number.The statistical meaning is implicit, come from the references of the RFC: the only normative reference with something aboutuncertaintyis theRFC 5491 (section 5).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 (illustrated).

Putting all together, adopting these clues, the usual statistical assumptions, and the explicit definitions of the RFC, we obtain the Geo URI'suncertaintymathematical 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"(section 3.4.3). Results in a spherical volume around the point (or a disk by 2D projection).
   By RFC 5491 "locations are expressed as a point (...) and an area or volume of uncertainty around the point".
   • 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.
2. fixed measure unit: the RFC obligate the use ofmetersasuncertaintymeasure units, even when coordinates (CRS) use other (like default that is decimal degrees). It is a semantic and a conversion problem: the
3. 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 (and the informative references likeISO 6709:2008).
   • adoptingstandard error model:the model of the most commondescriptive statistics modeling.
   • It is imposed, is independent ofselection processofuncertaintydescription, there are no other choices.
4. 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 asum of random variables.There is no simplification hypothesis defined to reduce it to a one-variable model.

Imagining the location of anant colonyto illustrate:

• the colony is a 3D object at the (exactly) theTerrain surface,so at precise altitude (approximated to a zero uncertainty measure).
• the 3D object has some consensual definition, but it is not precise, so, its uncertainty can not be neglected. This lack of precision can be about the fact that the anthill is hidden under the ground (it is an "estimated object" ), or the formal definition of its delimitation, etc.This kind of uncertainty has no correlation with the location (e.g. GPS) uncertainty measure.
  • the disk representing the anthill (as uncertainty of the object) is modeled as 2σ to be a 95% of confidence area.
• the point is aGPSlocation measure, that is, the "center" of the projection of the 3D object in the 2D surface.

The total uncertainty is the sum of GPS error and object-definition error. The latitude and longitude GPS errors need to be simplified (to a disk) 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 asAndroid OS,have adopted extensions to the "geo" URI scheme:

• z:Zoom level forWeb Mercator projectionscaling. The value is an integer from 1 to 21.
• q:Perform a search for the keyword given around the point. If the location is given as "0,0", search around the current position. A parenthetical can be used to indicate the label to show on the map.

Google Mapsadopts an unconventional approach to displaying the points: it shows the map for, but does not display a map pin, when a location is given in the standard way. A pin only shows up when given as the query. In other words, to show a pin at theWikimedia Foundationoffice, one should not usegeo:37.78918,-122.40335butgeo:0,0?q=37.78918,-122.40335.

See also

• LOC record

External links

• RFC5870
• Geo URIwebsite