Geo URI scheme

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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:

Contents

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

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

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 (crsu 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

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

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. Accuracy and precision.svg
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 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, have adopted extensions to the "geo" URI scheme: [9] [10]

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

Related Research Articles

<span class="mw-page-title-main">Geodesy</span> Science of measuring the shape, orientation, and gravity of Earth

Geodesy or geodetics is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems. Geodesy is an earth science and many consider the study of Earth's shape and gravity to be central to that science. It is also a discipline of applied mathematics.

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth. For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time, which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.

A Uniform Resource Identifier (URI), formerly Universal Resource Identifier, is a unique sequence of characters that identifies an abstract or physical resource, such as resources on a webpage, mail address, phone number, books, real-world objects such as people and places, concepts. URIs are used to identify anything described using the Resource Description Framework (RDF), for example, concepts that are part of an ontology defined using the Web Ontology Language (OWL), and people who are described using the Friend of a Friend vocabulary would each have an individual URI.

<span class="mw-page-title-main">Geographic coordinate system</span> System to specify locations on Earth

A geographic coordinate system (GCS) is a spherical or geodetic coordinate system for measuring and communicating positions directly on Earth as latitude and longitude. It is the simplest, oldest and most widely used of the various spatial reference systems that are in use, and forms the basis for most others. Although latitude and longitude form a coordinate tuple like a cartesian coordinate system, the geographic coordinate system is not cartesian because the measurements are angles and are not on a planar surface.

<span class="mw-page-title-main">Earth radius</span> Distance from the Earth surface to a point near its center

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<span class="mw-page-title-main">Geoid</span> Ocean shape without winds and tides

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 through the continents. According to Gauss, who first described it, it is the "mathematical figure of the Earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of Earth. It can be known only through extensive gravitational measurements and calculations. Despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.

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<span class="mw-page-title-main">World Geodetic System</span> Geodetic reference system

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<span class="mw-page-title-main">ISO 6709</span> International standard for representation of geographic location

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<span class="mw-page-title-main">Earth ellipsoid</span> Geometric figure which approximates the Earths shape

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<span class="mw-page-title-main">Web Mercator projection</span> Mercator variant map projection

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Under Chinese law, the use of geographic information in the People's Republic of China is restricted to entities that have special authorization from the administrative department for surveying and mapping under the State Council. Consequences of the restriction include fines for unauthorized surveys, lack of geotagging information on many cameras when the GPS chip detects a location within China, and incorrect alignment of street maps with satellite maps in various applications.

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 Spanring, Christian; Mayrhofer, Alexander (2010-06-08). "RFC 5870 - A Uniform Resource Identifier for Geographic Locations (geo URI)". Internet Engineering Task Force. Retrieved 9 June 2010.
  2. ↑ Perreault, Simon (2011-08-11). "RFC 6350 - vCard Format Specification". Internet Engineering Task Force. Retrieved 19 Jun 2012.
  3. ↑ "Android Intents List" . Retrieved 2012-06-19.
  4. ↑ "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.
  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.
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