| Rfc | 5870 |
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| Title | A Uniform Resource Identifier for Geographic Locations ('geo' URI) |
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| Author | A. Mayrhofer, C. Spanring |
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| Date | June 2010 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) A. Mayrhofer
Request for Comments: 5870 IPCom
Category: Standards Track C. Spanring
ISSN: 2070-1721 June 2010
A Uniform Resource Identifier for Geographic Locations ('geo' URI)
Abstract
This document specifies 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. The default coordinate reference system
used is the World Geodetic System 1984 (WGS-84).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5870.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. IANA Registration of the 'geo' URI Scheme . . . . . . . . . . 6
3.1. URI Scheme Name . . . . . . . . . . . . . . . . . . . . . 6
3.2. Status . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. URI Scheme Syntax . . . . . . . . . . . . . . . . . . . . 6
3.4. URI Scheme Semantics . . . . . . . . . . . . . . . . . . . 7
3.4.1. Coordinate Reference System Identification . . . . . . 7
3.4.2. Component Description for WGS-84 . . . . . . . . . . . 8
3.4.3. Location Uncertainty . . . . . . . . . . . . . . . . . 8
3.4.4. URI Comparison . . . . . . . . . . . . . . . . . . . . 9
3.4.5. Interpretation of Undefined Altitude . . . . . . . . . 10
3.5. Encoding Considerations . . . . . . . . . . . . . . . . . 10
3.6. Applications/Protocols That Use This URI Scheme . . . . . 11
3.7. Interoperability Considerations . . . . . . . . . . . . . 11
3.8. Security Considerations . . . . . . . . . . . . . . . . . 11
3.9. Contact . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.10. Author/Change Controller . . . . . . . . . . . . . . . . . 12
3.11. References . . . . . . . . . . . . . . . . . . . . . . . . 12
4. 'geo' URI Parameters Registry . . . . . . . . . . . . . . . . 12
5. URI Operations . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Use Cases and Examples . . . . . . . . . . . . . . . . . . . . 13
6.1. Plain 'geo' URI Example . . . . . . . . . . . . . . . . . 13
6.2. Hyperlink . . . . . . . . . . . . . . . . . . . . . . . . 14
6.3. 'geo' URI in 2-Dimensional Barcode . . . . . . . . . . . . 15
6.4. Comparison Examples . . . . . . . . . . . . . . . . . . . 15
7. GML Mappings . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. 2D GML 'Point' . . . . . . . . . . . . . . . . . . . . . . 17
7.2. 3D GML 'Point' . . . . . . . . . . . . . . . . . . . . . . 17
7.3. GML 'Circle' . . . . . . . . . . . . . . . . . . . . . . . 17
7.4. GML 'Sphere' . . . . . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8.1. 'geo' URI Scheme . . . . . . . . . . . . . . . . . . . . . 18
8.2. URI Parameter Registry . . . . . . . . . . . . . . . . . . 19
8.2.1. Registry Contents . . . . . . . . . . . . . . . . . . 19
8.2.2. Registration Policy . . . . . . . . . . . . . . . . . 19
8.3. Sub-Registry for 'crs' Parameter . . . . . . . . . . . . . 20
8.3.1. Registry Contents . . . . . . . . . . . . . . . . . . 20
8.3.2. Registration Policy . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9.1. Invalid Locations . . . . . . . . . . . . . . . . . . . . 21
9.2. Location Privacy . . . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . . 22
1. Introduction
An increasing number of Internet protocols and data formats are
extended by specifications for adding spatial (geographic) location.
In most cases, latitude as well as longitude of simple points are
added as new attributes to existing data structures. However, all
those methods are very specific to a certain data format or protocol,
and don't provide a protocol-independent, compact, and generic way to
refer to a physical geographic location.
Location-aware applications and location-based services are fast
emerging on the Internet. Most web search engines use geographic
information, and a vivid open source mapping community has brought an
enormous momentum into location aware technology. A wide range of
tools and data sets that formerly were accessible to professionals
only recently have become available to a wider audience.
The 'geo' URI scheme is another step in that direction and aims to
facilitate, support, and standardize the problem of location
identification in geospatial services and applications. Accessing
information about a particular location or triggering further
services shouldn't be any harder than clicking on a 'mailto:' link
and writing an email straight away.
According to [RFC3986], a Uniform Resource Identifier (URI) is "a
compact sequence of characters that identifies an abstract or
physical resource". The 'geo' URI scheme defined in this document
identifies geographic locations (physical resources) in a coordinate
reference system (CRS), which is, by default, the World Geodetic
System 1984 (WGS-84) [WGS84]. The scheme provides the textual
representation of the location's spatial coordinates in either two or
three dimensions (latitude, longitude, and optionally altitude for
the default CRS of WGS-84). An example of such a 'geo' URI follows:
geo:13.4125,103.8667
Such URIs are independent from a specific protocol, application, or
data format, and can be used in any other protocol or data format
that supports inclusion of arbitrary URIs.
For the sake of usability, the definition of the URI scheme is
strictly focused on the simplest, but also most common representation
of a spatial location -- a single point in a well known CRS. The
provision of more complex geometries or locations described by civic
addresses is out of scope of this document.
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 definition of 'crs' values
beyond the default of "wgs84" is therefore out of scope of this
document.
This specification discourages use of alternate CRSes in use cases
where comparison is an important function.
Note: The choice of WGS-84 as the default CRS is based on the
widespread availability of Global Positioning System (GPS) devices,
which use the WGS-84 reference system. It is anticipated that such
devices will serve as one of the primary data sources for authoring
'geo' URIs, hence the adoption of the native GPS reference system for
the URI scheme. Also, many other data formats for representing
geographic locations use the WGS-84 reference system, which makes
transposing from and to such data formats less error prone (no re-
projection involved). It is also believed that the burden of
potentially required spatial transformations should be put on the
author rather then the consumer of 'geo' URI instances.
Because of their similar structure, 'geo' URI instances can also be
mapped from and to certain ISO 6709 [ISO.6709.2008] string
representations of geographic point locations.
2. Terminology
Geographic locations in this document are defined using WGS-84 (World
Geodetic System 1984), which is equivalent to the International
Association of Oil & Gas Producers (OGP) Surveying and Positioning
Committee EPSG (European Petroleum Survey Group) codes 4326 (2
dimensions) and 4979 (3 dimensions). This document does not assign
responsibilities for coordinate transformations from and to other
Spatial Reference Systems.
A 2-dimensional WGS-84 coordinate value is represented here as a
comma-delimited latitude/longitude pair, measured in decimal degrees
(un-projected). A 3-dimensional WGS-84 coordinate value is
represented here by appending a comma-delimited altitude value in
meters to such pairs.
Latitudes range from -90 to 90 and longitudes range from -180 to 180.
Coordinates in the Southern and Western hemispheres as well as
altitudes below the WGS-84 reference geoid (depths) are signed
negative with a leading dash.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. IANA Registration of the 'geo' URI Scheme
This section contains the fields required for the URI scheme
registration, following the guidelines in Section 5.4 of [RFC4395].
3.1. URI Scheme Name
geo
3.2. Status
permanent
3.3. URI Scheme Syntax
The syntax of the 'geo' URI scheme is specified below in Augmented
Backus-Naur Form (ABNF) [RFC5234]:
geo-URI = geo-scheme ":" geo-path
geo-scheme = "geo"
geo-path = coordinates p
coordinates = coord-a "," coord-b [ "," coord-c ]
coord-a = num
coord-b = num
coord-c = num
p = [ crsp ] [ uncp ] *parameter
crsp = ";crs=" crslabel
crslabel = "wgs84" / labeltext
uncp = ";u=" uval
uval = pnum
parameter = ";" pname [ "=" pvalue ]
pname = labeltext
pvalue = 1*paramchar
paramchar = p-unreserved / unreserved / pct-encoded
labeltext = 1*( alphanum / "-" )
pnum = 1*DIGIT [ "." 1*DIGIT ]
num = [ "-" ] pnum
unreserved = alphanum / mark
mark = "-" / "_" / "." / "!" / "~" / "*" /
"'" / "(" / ")"
pct-encoded = "%" HEXDIG HEXDIG
p-unreserved = "[" / "]" / ":" / "&" / "+" / "$"
alphanum = ALPHA / DIGIT
Parameter names are case insensitive, but use of the lowercase
representation is preferred. Case sensitivity of non-numeric
parameter values MUST be described in the specification of the
respective parameter. For the 'crs' parameter, values are case
insensitive, and lowercase is preferred.
Both 'crs' and 'u' parameters MUST NOT appear more than once each.
The 'crs' and 'u' parameters MUST be given before any other
parameters that may be defined in future extensions. The 'crs'
parameter MUST be given first if both 'crs' and 'u' are used. The
definition of other parameters, and <crslabel> values beyond the
default value of "wgs84" is out of the scope of this document.
Section 8.2 discusses the IANA registration of such additional
parameters and values.
The value of "-0" for <num> is allowed and is identical to "0".
In case the URI identifies a location in the default CRS of WGS-84,
the <coordinates> sub-components are further restricted as follows:
coord-a = latitude
coord-b = longitude
coord-c = altitude
latitude = [ "-" ] 1*2DIGIT [ "." 1*DIGIT ]
longitude = [ "-" ] 1*3DIGIT [ "." 1*DIGIT ]
altitude = [ "-" ] 1*DIGIT [ "." 1*DIGIT ]
3.4. URI Scheme Semantics
Data contained in a 'geo' URI identifies a physical resource: a
spatial location identified by the geographic coordinates and the CRS
encoded in the URI.
3.4.1. Coordinate Reference System Identification
The semantics of <coordinates> depends on the CRS of the URI. The
CRS itself is identified by the optional 'crs' parameter. A URI
instance uses the default WGS-84 CRS if the 'crs' parameter is either
missing or contains the value of 'wgs84'. Other <crslabel> values
are currently not defined, but may be specified by future documents.
Interpretation of coordinates in the wrong CRS produces invalid
location information. Consumers of 'geo' URIs therefore MUST NOT
ignore the 'crs' parameter if given, and MUST NOT interpret the
<coordinates> sub-components without considering and understanding
the 'crs' parameter value.
The following component description refers to the use of the default
CRS (WGS-84) only. Future documents specifying other 'crs' parameter
values MUST provide similar descriptions for the <coordinates> sub-
components in the described CRS.
3.4.2. Component Description for WGS-84
The <latitude>, <longitude>, and <altitude> components as specified
in the URI scheme syntax (Section 3.3) are to be used as follows:
o <latitude> MUST contain the latitude of the identified location in
decimal degrees in the reference system WGS-84.
o <longitude> MUST contain the longitude of the identified location
in decimal degrees in the reference system WGS-84.
o If present, the OPTIONAL <altitude> MUST contain the altitude of
the identified location in meters in the reference system WGS-84.
If the altitude of the location is unknown, <altitude> (and the comma
before) MUST NOT be present in the URI. Specifically, unknown
altitude MUST NOT be represented by setting <altitude> to "0" (or any
other arbitrary value).
The <longitude> of coordinate values reflecting the poles (<latitude>
set to -90 or 90 degrees) SHOULD be set to "0", although consumers of
'geo' URIs MUST accept such URIs with any longitude value from -180
to 180.
'geo' URIs with longitude values outside the range of -180 to 180
decimal degrees or with latitude values outside the range of -90 to
90 degrees MUST be considered invalid.
3.4.3. Location Uncertainty
The 'u' ("uncertainty") parameter indicates the amount of uncertainty
in the location as a value in meters. Where a 'geo' URI is used to
identify the location of a particular object, <uval> indicates the
uncertainty with which the identified location of the subject is
known.
The 'u' parameter is optional and it can appear only once. If it is
not specified, this indicates that uncertainty is unknown or
unspecified. If the intent is to indicate a specific point in space,
<uval> MAY be set to zero. Zero uncertainty and absent uncertainty
are never the same thing.
The single uncertainty value is applied to all dimensions given in
the URI.
Note: The number of digits of the values in <coordinates> MUST NOT be
interpreted as an indication to the level of uncertainty.
3.4.4. URI Comparison
Comparison of URIs intends to determine whether two URI strings are
equivalent and identify the same resource (rather than comparing the
resources themselves). Therefore, a comparison of two 'geo' URIs
does not compare spatial objects, but only the strings (URIs)
identifying those objects.
The term "mathematically identical" used below specifies that some
components of the URI MUST be compared as normalized numbers rather
than strings to account for the variety in string representations of
identical numbers (for example, the strings "43.10" and "43.1" are
different, but represent the same number).
Two 'geo' URIs are equal only if they fulfill all of the following
general comparison rules:
o Both URIs use the same CRS, which means that either both have the
'crs' parameter omitted, or both have the same <crslabel> value,
or one has the 'crs' parameter omitted while the other URI
specifies the default CRS explicitly with a <crslabel> value of
"wgs84".
o Their <coord-a>, <coord-b>, <coord-c> and 'u' values are
mathematically identical (including absent <uval> meaning
undefined 'u' value).
o Their sets of other parameters are equal, with comparison
operations applied on each parameter as described in its
respective specification.
Parameter order is not significant for URI comparison.
Since new parameters may be registered over time, legacy
implementations of the 'geo' URI might encounter unknown parameters.
In such cases, the following rules apply:
o Two 'geo' URIs with unknown parameters are equivalent only if the
same set of unknown parameter names appears in each URI, and their
values are bitwise identical after percent-decoding.
o Otherwise, the comparison operation for the respective URIs is
undefined (since the legacy implementation cannot be aware of the
comparison rules for those parameters).
Designers of future extension parameters should take this into
account when choosing the comparison rules for new parameters.
A URI with an undefined (missing) <coord-c> (altitude) value MUST NOT
be considered equal to a URI containing a <coord-c>, even if the
remaining <coord-a>, <coord-b>, and 'u' values are equivalent.
For the default CRS of WGS-84, the following comparison rules apply
additionally:
o Where <latitude> of a 'geo' URI is set to either 90 or -90
degrees, <longitude> MUST be ignored in comparison operations
("poles case").
o A <longitude> of 180 degrees MUST be considered equal to
<longitude> of -180 degrees for the purpose of URI comparison
("date line" case).
3.4.5. Interpretation of Undefined Altitude
A consumer of a 'geo' URI in the WGS-84 CRS with undefined <altitude>
MAY assume that the URI refers to the respective location on Earth's
physical surface at the given latitude and longitude.
However, as defined above, altitudes are relative to the WGS-84
reference geoid rather than Earth's surface. Hence, an <altitude>
value of 0 MUST NOT be mistaken to refer to "ground elevation".
3.5. Encoding Considerations
The <coordinates> path component of the 'geo' URI (see Section 3.3)
uses a comma (",") as the delimiter for subcomponents. This
delimiter MUST NOT be percent-encoded.
It is RECOMMENDED that for readability the contents of <coord-a>,
<coord-b>, and <coord-c> as well as <crslabel> and <uval> are never
percent-encoded.
Regarding internationalization, the currently specified components do
allow for ASCII characters exclusively, and therefore don't require
internationalization. Future specifications of additional parameters
might allow the introduction of non-ASCII values. Such
specifications MUST describe internationalization considerations for
those parameters and their values, and MUST require percent-encoding
of non-ASCII values.
3.6. Applications/Protocols That Use This URI Scheme
As many other URI scheme definitions, the 'geo' URI provides resource
identification independent of a specific application or protocol.
Examples of potential protocol mappings and use cases can be found in
Section 6.
3.7. Interoperability Considerations
Like other new URI schemes, the 'geo' URI requires support in client
applications. Users of applications that are not aware of the 'geo'
scheme are likely not able to make direct use of the information in
the URI. However, a client can make indirect use by passing around
'geo' URIs, even without understanding the format and semantics of
the scheme. Additionally, the simple structure of 'geo' URIs would
allow even manual dereference by humans.
Clients MUST NOT attempt to dereference 'geo' URIs given in a CRS
that is unknown to the client, because doing so would produce
entirely bogus results.
Authors of 'geo' URIs should carefully check that coordinate
components are set in the right CRS and in the specified order, since
the wrong order of those components (or use of coordinates in a
different CRS without transformation) are commonly observed mistakes
producing completely bogus locations.
The number of digits in the <coordinates> values MUST NOT be
interpreted as an indication of a certain level of accuracy or
uncertainty.
3.8. Security Considerations
See Section 9 of RFC 5870.
3.9. Contact
Alexander Mayrhofer <axelm@ipcom.at>, <http://geouri.org/>
Christian Spanring <christian@spanring.eu>
3.10. Author/Change Controller
The 'geo' URI scheme is registered under the IETF part of the URI
tree. As such, change control is up to the IETF.
3.11. References
RFC 5870
4. 'geo' URI Parameters Registry
This specification creates a new IANA Registry named "'geo' URI
Parameters" registry for the <parameter> component of the URI.
Parameters for the 'geo' URI and values for these parameters MUST be
registered with IANA to prevent namespace collisions and provide
interoperability.
Some parameters accept values that are constrained by a syntax
definition only, while others accept values from a predefined set
only. Some parameters might not accept any values at all ("flag"
type parameters).
The registration of values is REQUIRED for parameters that accept
values from a predefined set.
The specification of a parameter MUST fully explain the syntax,
intended usage, and semantics of the parameter. This ensures
interoperability between independent implementations.
For parameters that are neither restricted to a set of predefined
values nor the "flag" type described above, the syntax of allowed
values MUST be described in the specification, for example by using
ABNF.
Documents defining new parameters (or new values for existing
parameters) MUST register them with IANA, as explained in
Section 8.2.
The 'geo' URI Parameter Registry contains a column named "Value
Restriction" that describes whether or not a parameter accepts a
value, and whether values are restricted to a predefined set. That
column accepts the following values:
o "No value": The parameter does not accept any values and is to be
used as a "flag" only.
o "Predefined": The parameter does accept values from a predefined
set only, as specified in an RFC or other permanent and readily
available public specification.
o "Constrained": The parameter accepts arbitrary values that are
only constrained by a syntax as specified in an RFC or other
permanent and readily available public specification.
Section 8.2.1 contains the initial contents of the Registry.
5. URI Operations
Currently, just one operation on a 'geo' URI is defined - location
dereference: in that operation, a client dereferences the URI by
extracting the geographical coordinates from the URI path component
<geo-path>. Further use of those coordinates (and the uncertainty
value from <uval>) is then up to the application processing the URI,
and might depend on the context of the URI.
An application may then use this location information for various
purposes, for example:
o A web browser could use that information to open a mapping service
of the user's choice, and display a map of the location.
o A navigational device such as a Global Positioning System (GPS)
receiver could offer the user the ability to start navigation to
the location.
Note that the examples and use cases above as well as in the next
section are non-normative, and are provided for information only.
6. Use Cases and Examples
6.1. Plain 'geo' URI Example
The following 3-dimensional 'geo' URI example references to the
office location of one of the authors in Vienna, Austria:
geo:48.2010,16.3695,183
Resolution of the URI returns the following information:
o The 'crs' parameter is not given in the URI, which means that the
URI uses the default CRS of WGS-84.
o The URI includes <coord-c>, is hence 3-dimensional, and therefore
uses 'urn:ogc:def:crs:EPSG::4979' as the WGS-84 CRS identifier.
o The <coord-a> value (latitude in WGS-84) is set to '48.2010'
decimal degrees.
o The <coord-b> value (longitude in WGS-84) is set to '16.3695'
decimal degrees.
o The <coord-c> value (altitude in WGS-84) is set to 183 meters.
o Uncertainty is undefined.
A user could type the data extracted from this URI into an electronic
navigation device, or even use it to locate the identified location
on a paper map.
6.2. Hyperlink
'geo' URIs (like any other URI scheme) could also be embedded as
hyperlinks in web pages. A Hyper Text Markup Language (HTML) snippet
with such a hyperlink could look like:
<p>one of Vienna's popular sights is the
<a href='geo:48.198634,16.371648;crs=wgs84;u=40'>Karlskirche</a>.
Resolution of the URI returns the following information:
o The 'crs' is given in the URI and sets the CRS used in the URI to
WGS-84 explicitly.
o The URI does omit <coord-c>, is hence 2-dimensional, and therefore
uses 'urn:ogc:def:crs:EPSG::4326' as the WGS-84 CRS identifier.
o The <coord-a> value (latitude in WGS-84) is set to '48.198634'
decimal degrees.
o The <coord-b> value (longitude in WGS-84) is set to '16.371648'
decimal degrees.
o The <coord-c> (altitude) value is undefined; therefore, the client
MAY assume the identified location to be on Earth's physical
surface.
o The 'u' parameter is included in the URI, setting uncertainty to
40 meters.
A web browser could use this information from the HTML snippet, and
offer the user various options (based on configuration, context), for
example:
o Display a small map thumbnail when the mouse pointer hovers over
the link.
o Switch to a mapping service of the user's choice once the link is
selected.
o Locate nearby resources, for example by comparing the 'geo' URI
with locations extracted from GeoRSS feeds to which the user has
subscribed.
o Convert the coordinates to a format suitable for uploading to a
navigation device.
Note that the URI in this example also makes use of the explicit
specification of the CRS by using the 'crs' parameter.
6.3. 'geo' URI in 2-Dimensional Barcode
Due to it's short length, a 'geo' URI could easily be encoded in
2-dimensional barcodes. Such barcodes could be printed on business
cards, flyers, and paper maps, and subsequently used by mobile
devices, for example as follows:
1. User identifies such a barcode on a flyer and uses the camera on
his mobile phone to photograph and decode the barcode.
2. The mobile phone dereferences the 'geo' URI, and offers the user
the ability to calculate a navigation route to the identified
location.
3. Using the builtin GPS receiver, the user follows the navigation
instructions to reach the location.
6.4. Comparison Examples
This section provides examples of URI comparison. Note that the
unknown parameters 'foo' and 'bar' and unregistered 'crs' values in
this section are used for illustrative purposes only, and their
inclusion in the examples below does not constitute any formal
parameter definition or registration request.
o The two URIs <geo:90,-22.43;crs=WGS84> and <geo:90,46> are equal,
because both use the same CRS, and even though the longitude
values are different, both reflect a location on the north pole
(special "poles" rule for WGS-84 applies - longitude is to be
ignored). Note that the 'crs' parameter values are case
insensitive.
o The URIs <geo:22.300;-118.44> and <geo:22.3;-118.4400> are equal,
because their coordinate components are mathematically identical.
o The set of <geo:66,30;u=6.500;FOo=this%2dthat> and <geo:
66.0,30;u=6.5;foo=this-that> are identical, because the value of
the unknown parameter 'foo' is bitwise identical after percent-
decoding; parameter names are case insensitive, and coordinates
and uncertainty are mathematically identical.
o The comparison operation on <geo:70,20;foo=1.00;bar=white> and
<geo:70,20;foo=1;bar=white> in a legacy implementation is
undefined, because the normalization rules for 'foo' are not
known, and hence the implementation cannot identify whether or not
'1.00' is identical to '1' for the 'foo' parameter.
o Comparing <geo:47,11;foo=blue;bar=white> and <geo:
47,11;bar=white;foo=blue> returns true, because parameter order is
insignificant in comparison operations.
o The comparison operation on <geo:22,0;bar=Blue> and <geo:
22,0;BAR=blue> is undefined, because even though parameter names
are case insensitive, this is not necessarily the case for the
values of the unknown 'bar' parameter.
7. GML Mappings
The Geographic Markup Language (GML) by the Open Geospatial
Consortium (OGC) is a set of XML schemas that represent geographical
features. Since GML is widely accepted, this document includes
instructions on how to transform 'geo' URIs from and to GML
fragments. The instructions in this section are not normative.
For the following sections, "%lat%", "%lon%", "%alt%", and "%unc%"
are placeholders for latitude, longitude, altitude, and uncertainty
values, respectively. The mappings use WGS-84 and are defined in the
following sections.
Note: GML fragments in other reference systems could be used as well
if a transformation into "urn:ogc:def:crs:EPSG::4979" or
"urn:ogc:def:crs:EPSG::4326" is defined and applied before the
mapping step. Such transformations are typically not lossless.
GML uses the 'double' type from XML schema, and the mapping examples
assume that numbers in the form of "3.32435e2" in GML are properly
converted to fixed point when placed into the 'geo' URI.
7.1. 2D GML 'Point'
A 2D GML 'Point' [RFC5491] is constructed from a 'geo' URI that has
two coordinates and an uncertainty ('u') parameter that is absent or
zero. A GML point is always converted to a 'geo' URI that has no
uncertainty parameter.
'geo' URI:
geo:%lat%,%lon%
GML fragment:
<Point srsName="urn:ogc:def:crs:EPSG::4326"
xmlns="http://www.opengis.net/gml">
<pos>%lat% %lon%</pos>
</Point>
Note that a 'geo' URI with an uncertainty value of zero is converted
to a GML 'Point', but a GML 'Point' cannot be translated to a 'geo'
URI with zero uncertainty.
7.2. 3D GML 'Point'
A 3D GML 'Point' [RFC5491] is constructed from a 'geo' URI that has
three coordinates and an uncertainty parameter that is absent or
zero. A GML point is always converted to a 'geo' URI that has no
uncertainty parameter.
'geo' URI:
geo:%lat%,%lon%,%alt%
GML fragment:
<Point srsName="urn:ogc:def:crs:EPSG::4979"
xmlns="http://www.opengis.net/gml">
<pos>%lat% %lon% %alt%</pos>
</Point>
7.3. GML 'Circle'
A GML 'Circle' [RFC5491] is constructed from a 'geo' URI that has two
coordinates and an uncertainty parameter that is present and non-
zero.
'geo' URI:
geo:%lat%,%lon%;u=%unc%
GML fragment:
<gs:Circle srsName="urn:ogc:def:crs:EPSG::4326"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0">
<gml:pos>%lat% %lon%</gml:pos>
<gs:radius uom="urn:ogc:def:uom:EPSG::9001">
%unc%
</gs:radius>
</gs:Circle>
7.4. GML 'Sphere'
A GML 'sphere' [RFC5491] is constructed from a 'geo' URI that has
three coordinates and an uncertainty parameter that is present and
non-zero.
'geo' URI:
geo:%lat%,%lon%,%alt%;u=%unc%
GML fragment:
<gs:Sphere srsName="urn:ogc:def:crs:EPSG::4979"
xmlns:gml="http://www.opengis.net/gml"
xmlns:gs="http://www.opengis.net/pidflo/1.0">
<gml:pos>%lat% %lon% %alt%</gml:pos>
<gs:radius uom="urn:ogc:def:uom:EPSG::9001">
%unc%
</gs:radius>
</gs:Sphere>
8. IANA Considerations
8.1. 'geo' URI Scheme
This document creates the 'geo' URI scheme in the IETF part of the
URI scheme tree, according to the guidelines in BCP 115 (RFC 4395)
[RFC4395]. The definitions required for the assignment are contained
in Section 3.
8.2. URI Parameter Registry
This document creates a new IANA Registry named "'geo' URI
Parameters", according to the information in Section 4 and the
definition in this section.
8.2.1. Registry Contents
When registering a new 'geo' URI Parameter, the following information
MUST be provided:
o Name of the Parameter.
o Whether the Parameter accepts no value ("No value"), values from a
predefined set ("Predefined"), or values constrained by a syntax
only ("Constrained").
o Reference to the RFC or other permanent and readily available
public specification defining the parameters and the new values.
Unless specific instructions exist for a Parameter (like the
definition of a Sub-registry), the following information MUST be
provided when registering new values for existing "Predefined" 'geo'
URI Parameters:
o Name of the Parameter.
o Reference to the RFC or other permanent and readily available
public specification defining the new values.
The following table provides the initial values for this registry:
Parameter Name Value Restriction Reference(s)
----------------------------------------------------------
crs Predefined [RFC5870]
u Constrained [RFC5870]
8.2.2. Registration Policy
The Registration Policy for 'geo' URI Parameters and their value
definitions is "Specification Required" (which implies "Designated
Expert"), as defined in [RFC5226].
8.3. Sub-Registry for 'crs' Parameter
This document creates a new IANA Sub-registry named "'geo' URI 'crs'
Parameter Values", based on the Registry specified in Section 8.2 and
the information in this section and Section 4. The syntax of the
'crs' parameter is constrained by the ABNF given in Section 3.3.
8.3.1. Registry Contents
When registering a new value for the 'crs' parameter, the following
information MUST be provided:
o Value of the parameter.
o Reference to the RFC or other permanent and readily available
public specification defining the use of the CRS in the scope of
the 'geo' URI. The specification should contain information that
is similar to the WGS-84-specific text given in this document.
o Reference to the definition document of the CRS. If a URN is
assigned to the CRS, the use of such URN as reference is
preferred. Note that different URNs may exist for the
2-dimensional and 3-dimensional case.
The following table provides the initial values for this registry:
crs Value CRS definition(s) Reference(s)
-----------------------------------------------------------
wgs84 urn:ogc:def:crs:EPSG::4326 [RFC5870]
urn:ogc:def:crs:EPSG::4979 [RFC5870]
8.3.2. Registration Policy
The registration policy for the "'geo' URI 'crs' Parameter Values"
Registry shall require both "Specification Required" and "IESG
Approval", as defined in [RFC5226].
Section 1 contains some text about the motivation for when to
introduce new 'crs' values.
9. Security Considerations
Because the 'geo' URI is not tied to any specific protocol and
identifies a physical location rather than a network resource, most
of the general security considerations on URIs (Section 7 of RFC
3986) do not apply. However, the following (additional) issues
apply:
9.1. Invalid Locations
The URI syntax (Section 3.3) makes it possible to construct 'geo'
URIs that don't identify a valid location. Applications MUST NOT use
URIs with such values and SHOULD warn the user when such URIs are
encountered.
An example of such a URI referring to an invalid location would be
<geo:94,0> (latitude "beyond" north pole).
9.2. Location Privacy
A 'geo' URI by itself is just an opaque reference to a physical
location, expressed by a set of spatial coordinates. This does not
fit the "Location Information" definition according to Section 5.2 of
GEOPRIV Requirements [RFC3693], because there is not necessarily a
"Device" involved.
Because there is also no way to specify the identity of a "Target"
within the confines of a 'geo' URI, it also does not fit the
specification of a "Location Object" (Section 5.2 of RFC 3693).
However, if a 'geo' URI is used in a context where it identifies the
location of a Target, it becomes part of a Location Object and is
therefore subject to GEOPRIV rules.
Therefore, when 'geo' URIs are put into such contexts, the privacy
requirements of RFC 3693 MUST be met.
10. Acknowledgements
Martin Thomson has provided significant text around the definition of
the "uncertainty" parameter and the GML mappings.
The authors further wish to acknowledge the helpful contributions
from Carl Reed, Bill McQuillan, Martin Kofal, Andrew Turner, Kim
Sanders, Ted Hardie, Cullen Jennings, Klaus Darilion, Bjoern
Hoehrmann, Alissa Cooper, and Ivan Shmakov.
Alfred Hoenes has provided an extremely helpful in-depth review of
the document.
11. References
11.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5491] Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV
Presence Information Data Format Location Object (PIDF-LO)
Usage Clarification, Considerations, and Recommendations",
RFC 5491, March 2009.
11.2. Informative References
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.
[WGS84] National Imagery and Mapping Agency, "Department of
Defense World Geodetic System 1984, Third Edition",
NIMA TR8350.2, January 2000.
[ISO.6709.2008]
International Organization for Standardization, "Standard
representation of geographic point location by
coordinates", ISO Standard 6709, 2008.
Authors' Addresses
Alexander Mayrhofer
IPCom GmbH
Karlsplatz 1/2/9
Wien A-1010
Austria
Phone: +43 1 5056416 34
Email: alexander.mayrhofer@ipcom.at
URI: http://www.ipcom.at/
Christian Spanring
73 Josephine Ave
Somerville 02144
Email: christian@spanring.eu
URI: http://www.spanring.eu/