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draft.txt january 7, 2014
Independent H. Butler
Internet-Draft Hobu Inc.
Intended status: Informational M. Daly
Expires: July 12, 2014 Cadcorp
A. Doyle
MIT
S. Gillies
Mapbox Inc.
T. Schaub
OpenGeo
S. Drees
January 8, 2014
The GeoJSON Format
draft-butler-geojson-00
Abstract
GeoJSON is a geospatial data interchange format based on JavaScript
Object Notation (JSON). It defines several types of JSON objects and
the manner in which they are combined to represent data about
geographic features, their properties, and their spatial extents.
This document recommends a single coordinate reference system based
on WGS 84. Other coordinate reference systems are not recommended.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 12, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Conventions Used in This Document . . . . . . . . . . . . 4
1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. GeoJSON Object . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Geometry Object . . . . . . . . . . . . . . . . . . . . . 6
2.1.1. Position . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2. Point . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3. MultiPoint . . . . . . . . . . . . . . . . . . . . . 7
2.1.4. LineString . . . . . . . . . . . . . . . . . . . . . 7
2.1.5. MultiLineString . . . . . . . . . . . . . . . . . . . 7
2.1.6. Polygon . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.7. MultiPolygon . . . . . . . . . . . . . . . . . . . . 8
2.1.8. Geometry Collection . . . . . . . . . . . . . . . . . 8
2.2. Feature Object . . . . . . . . . . . . . . . . . . . . . 8
2.3. Feature Collection Object . . . . . . . . . . . . . . . . 8
3. Coordinate Reference System Object . . . . . . . . . . . . . 8
3.1. Named CRS . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Linked CRS . . . . . . . . . . . . . . . . . . . . . . . 9
4. Bounding Box . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Geometry Examples . . . . . . . . . . . . . . . . . 12
A.1. Point . . . . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. LineString . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Polygon . . . . . . . . . . . . . . . . . . . . . . . . . 12
A.4. MultiPoint . . . . . . . . . . . . . . . . . . . . . . . 13
A.5. MultiLineString . . . . . . . . . . . . . . . . . . . . . 14
A.6. MultiPolygon . . . . . . . . . . . . . . . . . . . . . . 14
A.7. GeometryCollection . . . . . . . . . . . . . . . . . . . 15
Appendix B. Contributors . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
GeoJSON is a format for encoding data about geographic features using
JavaScript Object Notation (JSON) [RFC4627]. The format is concerned
with features in the broadest sense; any thing with qualities that
are bounded in geographical space may be a feature whether it is a
physical structure or not. The concepts in GeoJSON are not new; they
are derived from pre-existing open geographic information system
standards (for COM, SQL, and XML) and have been streamlined to better
suit web application development using JSON.
GeoJSON comprises the seven concrete geometry types defined in the
OpenGIS Simple Features Implementation Specification for SQL [SFSQL]:
0-dimensional Point and MultiPoint; 1-dimensional curve LineString
and MultiLineString; 2-dimensional surface Polygon and MultiPolygon;
and the heterogeneous GeometryCollection. GeoJSON representations of
instances of these geometry types are analogous to the well-known
binary (WKB) and text (WKT) representations described in that same
specification.
GeoJSON also comprises the types Feature and FeatureCollection.
Feature objects in GeoJSON contain a geometry object with one of the
above geometry types and additional properties. A FeatureCollection
object contains an array of feature objects. This structure is
analogous to that of the Web Feature Service (WFS) response to
GetFeatures requests specified in [WFSv1] or to a KML Folder of
Placemarks [KMLv2.2]. Some implementations of the WFS specification
also provide GeoJSON formatted responses to GetFeature requests, but
there is no particular service model or feature type ontology implied
in the GeoJSON format specification.
Since its initial publication in 2008 [GeoJSON], the GeoJSON format
specification has steadily grown in popularity. It is widely used in
JavaScript web mapping libraries, JSON-based document databases, and
web APIs.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
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1.2. Conventions Used in This Document
The ordering of the members of any JSON object defined in this
document MUST be considered irrelevant, as specified by [RFC4627].
Some examples use the combination of a JavaScript single line comment
(//) followed by an ellipsis (...) as placeholder notation for
content deemed irrelevant by the authors. These placeholders must of
course be deleted or otherwise replaced, before attempting to
validate the corresponding JSON code example.
Whitespace is used in the examples inside this document to help
illustrate the data structures, but is not required. Unquoted
whitespace is not significant in JSON.
1.3. Definitions
o JavaScript Object Notation (JSON), and the terms object, name,
value, array, number, true, false, and null are to be interpreted
as defined in [RFC4627].
o Inside this document the term "geometry type" refers to the seven
case-sensitive strings: "Point", "MultiPoint", "LineString",
"MultiLineString", "Polygon", "MultiPolygon", and
"GeometryCollection".
o As another shorthand notation the term GeoJSON types refers to the
nine case-sensitve strings "Feature", "FeatureCollection" and the
geometry types listed above.
1.4. Example
A GeoJSON feature collection:
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{
"type": "FeatureCollection",
"features": [{
"type": "Feature",
"geometry": {
"type": "Point",
"coordinates": [102.0, 0.5]
},
"properties": {
"prop0": "value0"
}
}, {
"type": "Feature",
"geometry": {
"type": "LineString",
"coordinates": [
[102.0, 0.0],
[103.0, 1.0],
[104.0, 0.0],
[105.0, 1.0]
]
},
"properties": {
"prop0": "value0",
"prop1": 0.0
}
}, {
"type": "Feature",
"geometry": {
"type": "Polygon",
"coordinates": [
[
[100.0, 0.0],
[101.0, 0.0],
[101.0, 1.0],
[100.0, 1.0],
[100.0, 0.0]
]
]
},
"properties": {
"prop0": "value0",
"prop1": {
"this": "that"
}
}
}]
}
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2. GeoJSON Object
GeoJSON always consists of a single object. This object (referred to
as the GeoJSON object below) represents a geometry, feature, or
collection of features.
o The GeoJSON object MUST have a member with the name "type". The
value of the member MUST be one of the GeoJSON types.
o The GeoJSON object MAY have any number of other members.
o A GeoJSON object MAY have a "bbox" member, the value of which MUST
be a bounding box array (see 4. Bounding Boxes).
2.1. Geometry Object
A geometry object is a GeoJSON object where the "type" value is one
of the geometry types. A GeoJSON geometry object of any type other
than "GeometryCollection" MUST have a member with the name
"coordinates". The value of the coordinates member is always an
array. The structure of the elements in this array is determined by
the type of geometry.
2.1.1. Position
A position is the fundamental geometry construct. The "coordinates"
member of a geometry object is composed of either:
o one position (in the case of a Point geometry),
o an array of positions (LineString or MultiPoint geometries),
o an array of arrays of positions (Polygons, MultiLineStrings),
o or a multidimensional array of positions (MultiPolygon).
A position is represented by an array of numbers. There MUST be two
or more elements. The first two elements will be longitude and
latitude, or easting and northing, precisely in that order and using
decimal numbers. Altitude or elevation MAY be included as an
optional third element.
Additional position elements MAY be included but MUST follow the
three specified above and MAY be ignored by software. Interpretation
and meaning of additional elements is beyond the scope of this
specification.
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Examples of positions and geometries are provided in "Appendix A.
Geometry Examples".
2.1.2. Point
For type "Point", the "coordinates" member MUST be a single position.
2.1.3. MultiPoint
For type "MultiPoint", the "coordinates" member MUST be an array of
positions.
2.1.4. LineString
For type "LineString", the "coordinates" member MUST be an array of
two or more positions.
2.1.5. MultiLineString
For type "MultiLineString", the "coordinates" member MUST be an array
of LineString coordinate arrays.
2.1.6. Polygon
To specify a constraint specific to polygons, it is useful to
introduce the concept of a linear ring:
o A linear ring is a closed LineString with 4 or more positions.
o The first and last positions are equivalent (they represent
equivalent points).
o A linear ring is the boundary of a surface or the boundary of a
hole in a surface.
Though a linear ring is not explicitly represented as a GeoJSON
geometry type, it leads to a canonical formulation of the Polygon
geometry type definition as follows:
o For type "Polygon", the "coordinates" member MUST be an array of
linear ring coordinate arrays.
o For Polygons with more than one of these rings, the first MUST be
the exterior ring and any others MUST be interior rings. The
exterior ring bounds the surface and the interiors rings (if
present) bound holes within the surface.
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2.1.7. MultiPolygon
For type "MultiPolygon", the "coordinates" member MUST be an array of
Polygon coordinate arrays.
2.1.8. Geometry Collection
A GeoJSON object with type "GeometryCollection" is a geometry object
which represents a collection of geometry objects. A geometry
collection MUST have a member with the name "geometries". The value
corresponding to "geometries" is an array. Each element in this
array is a GeoJSON geometry object.
2.2. Feature Object
A GeoJSON object with the type "Feature" is a feature object.
o A feature object MUST have a member with the name "geometry". The
value of the geometry member is a geometry object as defined above
or a JSON null value.
o A feature object MUST have a member with the name "properties".
The value of the properties member is an object (any JSON object
or a JSON null value).
o If a feature has a commonly used identifier, that identifier
SHOULD be included as a member of the feature object with the name
"id".
2.3. Feature Collection Object
A GeoJSON object with the type "FeatureCollection" is a feature
collection object. An object of type "FeatureCollection" MUST have a
member with the name "features". The value corresponding to
"features" is an array. Each element in the array is a feature
object as defined above.
3. Coordinate Reference System Object
The coordinate reference system (CRS) of a GeoJSON object is
determined by its OPTIONAL "crs" member (referred to as the CRS
object below). If an object has no crs member, then its parent or
grandparent object's crs member SHALL be acquired. If no crs member
can be so acquired, the default CRS SHALL apply to the GeoJSON
object.
The default coordinate reference system for all GeoJSON objects SHALL
be a geographic coordinate reference system, using the WGS84 datum,
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and with longitude and latitude units of decimal degrees. This
coordinate reference system is equivalent to the OGC's
urn:ogc:def:crs:OGC::CRS84 [OGCURN]. An OPTIONAL third position
element SHALL be the height in meters above the WGS 84 reference
ellipsoid. For widest interoperability, GeoJSON data SHOULD use this
default coordinate reference system and omit CRS objects.
If an application demands a different coordinate reference system,
the following specifications shall be followed:
o CRS SHOULD be defined as highly in GeoJSON object hierarchy as
possible, ideally in the FeatureCollection, and SHOULD NOT be
repeated or overridden in child objects.
o The value of a member named "crs" is a JSON object (referred to as
the CRS object below) or JSON null. If the value of CRS is null,
no CRS can be assumed.
o A non-null CRS object has two mandatory members: "type" and
"properties".
o The value of the type member indicates the type of CRS object.
o The value of the properties member must be an object.
o The presence of a CRS object SHALL NOT change the ordering of
coordinates specified in section 2.1.1.
3.1. Named CRS
A CRS object may indicate a coordinate reference system by name. In
this case, the value of its "type" member must be the string "name".
The value of its "properties" member must be an object containing a
"name" member. The value of that "name" member must be a string
identifying a coordinate reference system. OGC CRS URNs such as
"urn:ogc:def:crs:OGC::CRS84" are RECOMMENDED over legacy identifiers
such as "EPSG:4326".
"crs": {
"type": "name",
"properties": {
"name": "urn:ogc:def:crs:OGC::CRS84"
}
}
3.2. Linked CRS
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A CRS object may link to CRS parameters on the Web. In this case, the
value of its "type" member must be the string "link", and the value
of its "properties" member must be a Link object.
A link object has one required member: "href", and one optional
member: "type". The value of the required "href" member must be a
dereferenceable URI. The value of the optional "type" member must be
a string that hints at the format used to represent CRS parameters at
the provided URI. Suggested values are: "proj4", "ogcwkt",
"esriwkt", but others can be used::
"crs": {
"type": "link",
"properties": {
"href": "http://example.com/crs/42",
"type": "proj4"
}
}
Relative links may be used to direct processors to CRS parameters in
an auxiliary file:
"crs": {
"type": "link",
"properties": {
"href": "data.crs",
"type": "ogcwkt"
}
}
4. Bounding Box
A GeoJSON object MAY have a member named "bbox" to include
information on the coordinate range for its geometries, features, or
feature collections. The value of the bbox member MUST be an array
of length 2*n where n is the number of dimensions represented in the
contained geometries, with the lowest values for all axes followed by
the highest values. The axes order of a bbox follows the axes order
of geometries.
Example of a bbox member on a feature:
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{
"type": "Feature",
"bbox": [-180.0, -90.0, 180.0, 90.0],
"geometry": {
"type": "Polygon",
"coordinates": [
[
[-180.0, 10.0],
[20.0, 90.0],
[180.0, -5.0],
[-30.0, -90.0]
]
]
}
//...
}
Example of a bbox member on a feature collection:
{
"type": "FeatureCollection",
"bbox": [100.0, 0.0, 105.0, 1.0],
"features": [
//...
]
}
5. Security Considerations
This memo raises no security issues.
6. IANA Considerations
There are no IANA considerations beyond those in [RFC4267].
7. References
7.1. Normative References
[GeoJSON] Butler, H., Daly, M., Doyle, A., Gillies, S., Schaub, T.,
and C. Schmidt, "The GeoJSON Format Specification", June
2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
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7.2. Informative References
[KMLv2.2] Wilson, T., "OGC KML", OGC 07-147r2, April 2008.
[OGCURN] Whiteside, A., "Definition identifier URNs in OGC
namespace", OGC 07-092r3, January 2009.
[SFSQL] OpenGIS Consortium, Inc., "OpenGIS Simple Features
Specification For SQL Revision 1.1", OGC 99-049, May 1999.
[WFSv1] Vretanos, P., "Web Feature Service Implementation
Specification", OGC 02-058, May 2002.
[WGS84] National Imagery and Mapping Agency, "Department of
Defense World Geodetic System 1984, Third Edition", 1984.
Appendix A. Geometry Examples
Each of the examples below represents a valid and complete GeoJSON
object.
A.1. Point
Point coordinates are in x, y order (easting, northing for projected
coordinates, longitude, latitude for geographic coordinates):
{
"type": "Point",
"coordinates": [100.0, 0.0]
}
A.2. LineString
Coordinates of LineString are an array of positions (see "2.1.1.
Position"):
{
"type": "LineString",
"coordinates": [
[100.0, 0.0],
[101.0, 1.0]
]
}
A.3. Polygon
Coordinates of a Polygon are an array of LinearRing (cf. "2.1.6
Polygon") coordinate arrays. The first element in the array
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represents the exterior ring. Any subsequent elements represent
interior rings (or holes).
No holes:
{
"type": "Polygon",
"coordinates": [
[
[100.0, 0.0],
[101.0, 0.0],
[101.0, 1.0],
[100.0, 1.0],
[100.0, 0.0]
]
]
}
With holes:
{
"type": "Polygon",
"coordinates": [
[
[100.0, 0.0],
[101.0, 0.0],
[101.0, 1.0],
[100.0, 1.0],
[100.0, 0.0]
],
[
[100.2, 0.2],
[100.8, 0.2],
[100.8, 0.8],
[100.2, 0.8],
[100.2, 0.2]
]
]
}
A.4. MultiPoint
Coordinates of a MultiPoint are an array of positions::
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{
"type": "MultiPoint",
"coordinates": [
[100.0, 0.0],
[101.0, 1.0]
]
}
A.5. MultiLineString
Coordinates of a MultiLineString are an array of LineString
coordinate arrays:
{
"type": "MultiLineString",
"coordinates": [
[
[100.0, 0.0],
[101.0, 1.0]
],
[
[102.0, 2.0],
[103.0, 3.0]
]
]
}
A.6. MultiPolygon
Coordinates of a MultiPolygon are an array of Polygon coordinate
arrays:
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{
"type": "MultiPolygon",
"coordinates": [
[
[
[102.0, 2.0],
[103.0, 2.0],
[103.0, 3.0],
[102.0, 3.0],
[102.0, 2.0]
]
],
[
[
[100.0, 0.0],
[101.0, 0.0],
[101.0, 1.0],
[100.0, 1.0],
[100.0, 0.0]
],
[
[100.2, 0.2],
[100.8, 0.2],
[100.8, 0.8],
[100.2, 0.8],
[100.2, 0.2]
]
]
]
}
A.7. GeometryCollection
Each element in the geometries array of a GeometryCollection is one
of the geometry objects described above:
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{
"type": "GeometryCollection",
"geometries": [{
"type": "Point",
"coordinates": [100.0, 0.0]
}, {
"type": "LineString",
"coordinates": [
[101.0, 0.0],
[102.0, 1.0]
]
}]
}
Appendix B. Contributors
The GeoJSON format is the product of discussion on the GeoJSON
mailing list: http://lists.geojson.org/listinfo.cgi/geojson-
geojson.org.
Comments are solicited and should be addressed to the GeoJSON mailing
list at geojson@lists.geojson.org or to the GeoJSON issue tracker at
https://github.com/geojson/draft-geojson/issues.
Authors' Addresses
H. Butler
Hobu Inc.
M. Daly
Cadcorp
A. Doyle
MIT
S. Gillies
Mapbox Inc.
Email: sean.gillies@gmail.com
URI: http://sgillies.net
T. Schaub
OpenGeo
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S. Drees
Rheinaustr. 62
Bonn 53225
DE
Email: stefan@drees.name
URI: http://sdre.es/
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@tschaub
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tschaub commented Jan 8, 2014

Duplicated "and and" removed in geojson/draft-geojson@a7a5140.

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