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geo URI scheme

The geo URI scheme is a Uniform Resource Identifier (URI) scheme defined by the Internet Engineering Task Force's RFC 5870 (published 8 June 2010)[1] 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 in a compact, simple, human-readable, and protocol-independent way.[1]

The current revision of the vCard specification[2] 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,[3] 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[4] (which had implemented ICBM addresses).

Example edit

A simple geo URI might look like:

geo:25.245470718844146,51.45400942457904

where the two numerical values represent latitude and longitude respectively,[1] and are separated by a comma.[1] They are coordinates of a horizontal grid (2D). If a third comma-separated value is present, it represents altitude;[1] 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.[1]

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.[1] 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:

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

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 edit

The values of the coordinates only make sense when a coordinate reference system (CRS) is specified. 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 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.[1] 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.[1] All parameters are case-insensitive,[1] 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

The use of the lowercase representation of parameter names (crs u and mapcolors) is preferred.

Semantics and usual interpretations edit

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 edit

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

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 in meters (specifically the "ground elevation", relative to the current geoidEarth Gravitational Model – attached to WGS84),[5] 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'".[6]

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.

The use of a geoid stands in contrast to GeoJSON, which uses direct ellipsoid height.[7]

Uncertainty edit

 
Facets of the uncertainty. According to ISO 5725-1: accuracy is the proximity of measurement results to the true value; precision is the degree to which repeated (or reproducible) measurements under unchanged conditions show the same results.

Remembering the example above,

geo:37.786971,-122.399677;u=35

The u=35 part informs the uncertainty. As will be showed, geometrically the uncertainty is a disc of radius u in turn of the point of the geo URI.

Geo URI is not about exact abstract positions, strictly it is a 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 number. The statistical meaning is implicit, come from the references of the RFC: the only normative reference with something about uncertainty is the 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:

  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 of meters 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
  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 like ISO 6709:2008).
  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 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 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.[8] 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 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 edit

Some vendors, such as Android OS, have adopted extensions to the "geo" URI scheme:[9][10]

  • z: Zoom level for Web Mercator projection 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.

Google Maps adopts 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 the Wikimedia Foundation office, one should not use geo:37.78918,-122.40335 but geo:0,0?q=37.78918,-122.40335.

See also edit

References edit

  1. ^ a b c d e f g h i j k l "RFC 5870 - A Uniform Resource Identifier for Geographic Locations (geo URI)". Internet Engineering Task Force. 2010-06-08. Retrieved 9 June 2010.
  2. ^ "RFC 6350 - vCard Format Specification". Internet Engineering Task Force. 2011-08-11. Retrieved 19 Jun 2012.
  3. ^ "Android Intents List". Retrieved 2012-06-19.
  4. ^ . Geourl.org. Archived from the original on 2013-12-03. Retrieved 2011-12-24. GeoURL is a location-to-URL reverse directory. This will allow you to find URLs by their proximity to a given location. Find your neighbor's blog, perhaps, or the web page of the restaurants near you. GeoURL is listing 9,601,000 sites. Add yourself to the database.
  5. ^ Section 2 of RFC 5870.
  6. ^ Section 3.4.5 of RFC 5870.
  7. ^ Section 4, RFC 7946 – The GeoJSON Format.
  8. ^ Using RFC 5491, that expressed that "... in theory, the area or volume represents a coverage in which the user has a relatively high probability of being found, and the point is a convenient means of defining the centroid for the area or volume" we can use also the concept home range of the ants or the ant's queen, to define the anthill.
  9. ^ "Google Maps Intents for Android | Maps URLs". Google Developers.
  10. ^ "Common Intents (Maps)". Android Developers.

External links edit

  • RFC5870
  • Geo URI website

scheme, uniform, resource, identifier, scheme, defined, internet, engineering, task, force, 5870, published, june, 2010, uniform, resource, identifier, geographic, locations, using, scheme, name, identifies, physical, location, three, dimensional, coordinate, . The geo URI scheme is a Uniform Resource Identifier URI scheme defined by the Internet Engineering Task Force s RFC 5870 published 8 June 2010 1 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 in a compact simple human readable and protocol independent way 1 The current revision of the vCard specification 2 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 3 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 4 which had implemented ICBM addresses Contents 1 Example 2 Coordinate reference systems 3 Semantics and usual interpretations 3 1 Altitude 3 2 Uncertainty 4 Unofficial extensions 5 See also 6 References 7 External linksExample editA simple geo URI might look like a rel nofollow class external free href geo 25 245470718844146 51 45400942457904 geo 25 245470718844146 51 45400942457904 a where the two numerical values represent latitude and longitude respectively 1 and are separated by a comma 1 They are coordinates of a horizontal grid 2D If a third comma separated value is present it represents altitude 1 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 1 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 1 A geo URI with an uncertainty parameter looks as follows a rel nofollow class external free href geo 37 786971 122 399677 u 35 geo 37 786971 122 399677 u 35 a A geo URI may for example be included on a web page as HTML lt a href a rel nofollow class external free href geo 37 786971 122 399677 u 35 geo 37 786971 122 399677 u 35 a gt Wikimedia Headquarters lt a gt 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 editThe values of the coordinates only make sense when a coordinate reference system CRS is specified 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 syntax and semantic of the CRS parameter separated by a semicolon is described at section 8 3 of RFC 5870 Examples The Washington Monument s location expressed with UTM zone 18N and its standard ID a rel nofollow class external free href geo 323482 4306480 crs EPSG 32618 u 20 geo 323482 4306480 crs EPSG 32618 u 20 a A geo URI for a hypothetical lunar CRS created in 2011 might be a rel nofollow class external free href geo 37 786971 122 399677 crs Moon 2011 u 35 geo 37 786971 122 399677 crs Moon 2011 u 35 a The order in which the semicolon separated parameters occur is partially significant 1 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 1 All parameters are case insensitive 1 so imagining a future new parameter mapcolors it can be ignored by simpler applications and the above example is exactly equivalent to a rel nofollow class external free href geo 323482 4306480 CRS epsg 32718 U 20 mapcolors for daltonic geo 323482 4306480 CRS epsg 32718 U 20 mapcolors for daltonic a The use of the lowercase representation of parameter names crs u and mapcolors is preferred Semantics and usual interpretations editThe 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 edit nbsp 1 Ocean 2 Reference ellipsoid 3 Local plumb line 4 Continent 5 GeoidThe 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 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 lt altitude gt MAY assume that the URI refers to the respective location on Earth s physical surface However an lt altitude gt value of 0 MUST NOT be mistaken to refer to ground elevation 6 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 The use of a geoid stands in contrast to GeoJSON which uses direct ellipsoid height 7 Uncertainty edit This section may be too technical for most readers to understand Please help improve it to make it understandable to non experts without removing the technical details May 2019 Learn how and when to remove this template message nbsp Facets of the uncertainty According to ISO 5725 1 accuracy is the proximity of measurement results to the true value precision is the degree to which repeated or reproducible measurements under unchanged conditions show the same results Remembering the example above geo 37 786971 122 399677 u 35The u 35 part informs the uncertainty As will be showed geometrically the uncertainty is a disc of radius u in turn of the point of the geo URI Geo URI is not about exact abstract positions strictly it is a 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 number The statistical meaning is implicit come from the references of the RFC the only normative reference with something about uncertainty is the 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 2s and it is the radius of the disk that represents uncertainty geometrically fixed measure unit the RFC obligate the use of meters 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 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 8 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 2s to be a 95 of confidence area the point is a GPS 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 editSome vendors such as Android OS have adopted extensions to the geo URI scheme 9 10 z Zoom level for Web Mercator projection 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 Google Maps adopts 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 the Wikimedia Foundation office one should not use a rel nofollow class external free href geo 37 78918 122 40335 geo 37 78918 122 40335 a but a rel nofollow class external free href geo 0 0 q 37 78918 122 40335 geo 0 0 q 37 78918 122 40335 a See also editLOC recordReferences edit a b c d e f g h i j k l RFC 5870 A Uniform Resource Identifier for Geographic Locations geo URI Internet Engineering Task Force 2010 06 08 Retrieved 9 June 2010 RFC 6350 vCard Format Specification Internet Engineering Task Force 2011 08 11 Retrieved 19 Jun 2012 Android Intents List Retrieved 2012 06 19 GeoURL 2 0 The GeoURL ICBM Address Server Geourl org Archived from the original on 2013 12 03 Retrieved 2011 12 24 GeoURL is a location to URL reverse directory This will allow you to find URLs by their proximity to a given location Find your neighbor s blog perhaps or the web page of the restaurants near you GeoURL is listing 9 601 000 sites Add yourself to the database Section 2 of RFC 5870 Section 3 4 5 of RFC 5870 Section 4 RFC 7946 The GeoJSON Format Using RFC 5491 that expressed that in theory the area or volume represents a coverage in which the user has a relatively high probability of being found and the point is a convenient means of defining the centroid for the area or volume we can use also the concept home range of the ants or the ant s queen to define the anthill Google Maps Intents for Android Maps URLs Google Developers Common Intents Maps Android Developers External links editRFC5870 Geo URI website Retrieved from https en wikipedia org w index php title Geo URI scheme amp oldid 1184127189, wikipedia, wiki, book, books, library,

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