Geo URI scheme
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The geo URI scheme is a
Uniform Resource Identifier A Uniform Resource Identifier (URI) is a unique sequence of characters that identifies a logical or physical resource used by web technologies. URIs may be used to identify anything, including real-world objects, such as people and places, conc ...
(URI) scheme defined by the
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...
'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-dimensional
coordinate reference system A spatial reference system (SRS) or coordinate reference system (CRS) is a framework used to precisely measure locations on the surface of the Earth as coordinates. It is thus the application of the abstract mathematics of coordinate systems and ...
in a compact, simple, human-readable, and
protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technolog ...
-independent way.
The current revision of the
vCard vCard, also known as VCF (Virtual Contact File), is a file format standard for electronic business cards. vCards can be attached to e-mail messages, sent via Multimedia Messaging Service (MMS), on the World Wide Web, instant messaging, NFC ...
specification supports geo URIs in a vCard's "GEO" property, and the GeoSMS standard uses geo URIs for
geotagging Geotagging, or GeoTagging, is the process of adding geographical identification metadata to various media such as a geotagged photograph or video, websites, SMS messages, QR Codes or RSS feeds and is a form of geospatial metadata. This data u ...
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'' (which had implemented
ICBM address ICBM (intercontinental ballistic missile) address or missile address is hacker slang for one's longitude and latitude (preferably to seconds-of-arc accuracy) when placed in a signature or another publicly available file. Origin The form that u ...
es).


Example

A simple geo URI might look like: :geo:37.786971,-122.399677 where the two numerical values represent
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
and
longitude Longitude (, ) is a geographic coordinate that specifies the east–west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter l ...
respectively, and are separated by a
comma The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline ...
. They are coordinates of a horizontal grid (2D). If a third comma-separated value is present, it represents
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
; 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 a
semicolon The semicolon or semi-colon is a symbol commonly used as orthographic punctuation. In the English language, a semicolon is most commonly used to link (in a single sentence) two independent clauses that are closely related in thought. When a ...
, 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, as
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScri ...
: :Wikimedia Headquarters so that a geo URI-aware
user agent In computing, a user agent is any software, acting on behalf of a user, which "retrieves, renders and facilitates end-user interaction with Web content". A user agent is therefore a special kind of software agent. Some prominent examples of us ...
such as a
web browser A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used on ...
could launch the user's chosen mapping service; or it could be used in an
Atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
feed or other
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...
file.


Coordinate reference systems

The values of the coordinates only make sense when a
coordinate reference system A spatial reference system (SRS) or coordinate reference system (CRS) is a framework used to precisely measure locations on the surface of the Earth as coordinates. It is thus the application of the abstract mathematics of coordinate systems and ...
(CRS) is specified. The default CRS is the World Geodetic System 1984 (WGS-84), 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 The Moon is Earth's only natural satellite. It is the List of natural satellites, fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth ( ...
or
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
. The syntax and semantic of the CRS parameter, separated by a semicolon, is described at section 8.3 of RFC 5870. Examples: * The
Washington Monument The Washington Monument is an obelisk shaped building within the National Mall in Washington, D.C., built to commemorate George Washington, once commander-in-chief of the Continental Army (1775–1784) in the American Revolutionary War and the ...
's location expressed with UTM-zone 18N and it
standard 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, the crs and the u parameters must come first. If both are used, the crs must precede the u. All parameters are
case-insensitive In computers, case sensitivity defines whether uppercase and lowercase letters are treated as distinct (case-sensitive) or equivalent (case-insensitive). For instance, when users interested in learning about dogs search an e-book, "dog" and "Dog" a ...
, 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 /code>, where coord-c is optional. The semantic of coord-c for
WGS-84 The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also descr ...
is
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
(specifically the "ground
elevation The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...
", relative to the current
geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
Earth Gravitational Model The Earth Gravitational Models (EGM) are a series of geopotential models of the Earth published by the National Geospatial-Intelligence Agency (NGA). They are used as the geoid reference in the World Geodetic System. The NGA provides the models ...
– 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 There are several kinds of mean in mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. ...
. 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 Numerical analysis is the study of algorithms that use numerical approximation (as opposed to symbolic computation, symbolic manipulations) for the problems of mathematical analysis (as distinguished from discrete mathematics). It is the study of ...
, 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 th
RFC 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's ''uncertainty'' mathematical properties: # 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. # fixed measure unit: the RFC obligate the use of
meters The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its p ...
as ''uncertainty'' measure units, even when coordinates (CRS) use other (like default that is decimal degrees). It is a semantic and a conversion problem: the # 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 like ISO 6709:2008). #* adopting standard error model: the model of the most common descriptive statistics modeling. #* It is imposed, is independent of selection process of ''uncertainty'' description, there are no other choices. # 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 An ant colony is a population of a single ant species capable to maintain its complete lifecycle. Ant colonies are eusocial, communal, and efficiently organized and are very much like those found in other social Hymenoptera, though the vario ...
to illustrate: * the colony is a 3D object at the (exactly) the Terrain 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 a
GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
location 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 as
Android OS Android is a mobile operating system based on a modified version of the Linux kernel and other open-source software, designed primarily for touchscreen mobile devices such as smartphones and tablets. Android is developed by a consortium of deve ...
, have adopted extensions to the "geo" URI scheme: * z: Zoom level for
Web Mercator projection Web Mercator, Google Web Mercator, Spherical Mercator, WGS 84 Web Mercator or WGS 84/Pseudo-Mercator is a variant of the Mercator map projection and is the de facto standard for Web mapping applications. It rose to prominence when Google Maps ado ...
scaling. 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. 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 The Wikimedia Foundation, Inc., or Wikimedia for short and abbreviated as WMF, is an American 501(c)(3) nonprofit organization headquartered in San Francisco, California and registered as a charitable foundation under local laws. Best kno ...
office, one should not use geo:37.78918,-122.40335 but geo:0,0?q=37.78918,-122.40335.


See also

*
LOC record In the Domain Name System, a LOC record (experimental ) is a means for expressing geographic location information for a domain name. It contains WGS84 Latitude, Longitude and Altitude (ellipsoidal height) information together with host/subnet phys ...


References


External links


RFC5870

Geo URI
website {{URI scheme Internet protocols Geocodes Geographic information systems URI schemes