GeoJSON介紹

GeoJSON是一種地理數據的描述格式。GeoJSON可以描述的對象包括：幾何體，要素和要素集。

這里幾何體(Geometry)的類型有我們熟悉的點(Point),線(LineString),面(Polygon), 多點(MultiPoint),多線(MultiLineString),多面( MultiPolygon)和幾何體集合(GeometryCollection)。

要素(Feature)包含了幾何體信息以及附加的一些屬性信息。

要素集(a collection of features)即為要素的集合。

GeoJSON的結構同JSON并增加了一些約束條件：

GeoJSON對象必須包含一個type屬性，type的值為其描述的對象(如前所述為下列選項之一：”Point”, “MultiPoint”, “LineString”, “MultiLineString”, “Polygon”, “MultiPolygon”, “GeometryCollection”, “Feature”, “FeatureCollection”)；

幾何體(Geometry)必須包含坐標信息，要素(Feature)必須包含一個幾何體信息，同時至少有一項屬性信息。

GeoJSON格式示例：

點{"type":"Point","coordinates":[100.0,0.0]}

線{"type":"LineString","coordinates":[[100.0,0.0],[101.0,1.0]]}

面

沒有洞的面：{"type":"Polygon","coordinates":[[[100.0,0.0],[101.0,0.0],[101.0,1.0],[100.0,1.0],[100.0,0.0]]]}

有洞的面，數組第一項是外環，其他是內環:{"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]]]}

更多介紹和示例可參考這里

The GeoJSON Format Specification

Authors:

Howard Butler (Hobu Inc.)

Martin Daly (Cadcorp)

Allan Doyle (MIT)

Sean Gillies (UNC-Chapel Hill)

Tim Schaub (OpenGeo)

Christopher Schmidt (MetaCarta)

Revision:

1.0

Date:

16 June 2008

Copyright:

Copyright ? 2008 by the Authors. This work is licensed under a Creative Commons Attribution 3.0

United States License.

Abstract

GeoJSON is a geospatial data interchange format based on JavaScript Object

Notation (JSON).

GeoJSON is a format for encoding a variety of geographic data structures. A

GeoJSON object may represent a geometry, a feature, or a collection of

features. GeoJSON supports the following geometry types: Point, LineString,

Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection.

Features in GeoJSON contain a geometry object and additional properties, and a

feature collection represents a list of features.

A complete GeoJSON data structure is always an object (in JSON terms). In

GeoJSON, an object consists of a collection of name/value pairs -- also called

members. For each member, the name is always a string. Member values are either

a string, number, object, array or one of the literals: "true", "false", and

"null". An array consists of elements where each element is a value as

described above.

A GeoJSON feature collection:

{ "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"}

}

}

]

}

JavaScript Object Notation (JSON), and the terms object, name, value, array,

and number, are defined in IETF RTC 4627, at

http://www.ietf.org/rfc/rfc4627.txt.

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 IETF RFC 2119, at

http://www.ietf.org/rfc/rfc2119.txt.

GeoJSON always consists of a single object. This object (referred to as the

GeoJSON object below) represents a geometry, feature, or collection of

features.

The GeoJSON object may have any number of members (name/value pairs).

The GeoJSON object must have a member with the name "type". This member's

value is a string that determines the type of the GeoJSON object.

The value of the type member must be one of: "Point", "MultiPoint",

"LineString", "MultiLineString", "Polygon", "MultiPolygon",

"GeometryCollection", "Feature", or "FeatureCollection". The case of the type

member values must be as shown here.

A GeoJSON object may have an optional "crs" member, the value of which must

be a coordinate reference system object (see 3. Coordinate Reference System

Objects).

A GeoJSON object may have a "bbox" member, the value of which must be a

bounding box array (see 4. Bounding Boxes).

A geometry is a GeoJSON object where the type member's value is one of the

following strings: "Point", "MultiPoint", "LineString", "MultiLineString",

"Polygon", "MultiPolygon", or "GeometryCollection".

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 for the elements in this array is determined by

the type of geometry.

A position is the fundamental geometry construct. The "coordinates" member of a

geometry object is composed of one position (in the case of a Point geometry),

an array of positions (LineString or MultiPoint geometries), an array of arrays

of positions (Polygons, MultiLineStrings), or a multidimensional array of

positions (MultiPolygon).

A position is represented by an array of numbers. There must be at least two

elements, and may be more. The order of elements must follow x, y, z order

(easting, northing, altitude for coordinates in a projected coordinate

reference system, or longitude, latitude, altitude for coordinates in a

geographic coordinate reference system). Any number of additional elements are

allowed -- interpretation and meaning of additional elements is beyond the

scope of this specification.

Examples of positions and geometries are provided in Appendix A. Geometry

Examples.

For type "Point", the "coordinates" member must be a single position.

For type "MultiPoint", the "coordinates" member must be an array of positions.

For type "LineString", the "coordinates" member must be an array of two or more

positions.

A LinearRing is closed LineString with 4 or more positions. The first and last

positions are equivalent (they represent equivalent points). Though a

LinearRing is not explicitly represented as a GeoJSON geometry type, it is

referred to in the Polygon geometry type definition.

For type "MultiLineString", the "coordinates" member must be an array of

LineString coordinate arrays.

For type "Polygon", the "coordinates" member must be an array of LinearRing

coordinate arrays. For Polygons with multiple rings, the first must be the

exterior ring and any others must be interior rings or holes.

For type "MultiPolygon", the "coordinates" member must be an array of Polygon

coordinate arrays.

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.

A GeoJSON object with the type "Feature" is a feature object.

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.

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).

If a feature has a commonly used identifier, that identifier should be

included as a member of the feature object with the name "id".

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.

The coordinate reference system (CRS) of a GeoJSON object is determined by its

"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 may be acquired. If

no crs member can be so acquired, the default CRS shall apply to the GeoJSON

object.

The default CRS is a geographic coordinate reference system, using the WGS84

datum, and with longitude and latitude units of decimal degrees.

The value of a member named "crs" must be 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.

The crs member should be on the top-level GeoJSON object in a hierarchy (in

feature collection, feature, geometry order) and should not be repeated or

overridden on children or grandchildren of the object.

A non-null CRS object has two mandatory members: "type" and "properties".

The value of the type member must be a string, indicating the type of CRS

object.

The value of the properties member must be an object.

CRS shall not change coordinate ordering (see 2.1.1. Positions).

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:1.3:CRS84" shall be preferred over

legacy identifiers such as "EPSG:4326":

"crs": {

"type": "name",

"properties": {

"name": "urn:ogc:def:crs:OGC:1.3:CRS84"

}

}

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 (see 3.2.1. Link Objects).

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"

}

}

To include information on the coordinate range for geometries, features, or

feature collections, a GeoJSON object may have a member named "bbox". The value

of the bbox member must be a 2*n array 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. In addition, the coordinate reference system for the bbox is

assumed to match the coordinate reference system of the GeoJSON object of which

it is a member.

Example of a bbox member on a feature:

{ "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": [

...

]

}

Each of the examples below represents a complete GeoJSON object. Note that

unquoted whitespace is not significant in JSON. Whitespace is used in the

examples to help illustrate the data structures, but is not required.

Point coordinates are in x, y order (easting, northing for projected

coordinates, longitude, latitude for geographic coordinates):

{ "type": "Point", "coordinates": [100.0, 0.0] }

Coordinates of LineString are an array of positions (see 2.1.1. Positions):

{ "type": "LineString",

"coordinates": [ [100.0, 0.0], [101.0, 1.0] ]

}

Coordinates of a Polygon are an array of LinearRing coordinate arrays. The

first element in the array 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] ]

]

}

Coordinates of a MultiPoint are an array of positions:

{ "type": "MultiPoint",

"coordinates": [ [100.0, 0.0], [101.0, 1.0] ]

}

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] ]

]

}

Coordinates of a MultiPolygon are an array of Polygon coordinate arrays:

{ "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]]]

]

}

Each element in the geometries array of a GeometryCollection is one of the

geometry objects described above:

{ "type": "GeometryCollection",

"geometries": [

{ "type": "Point",

"coordinates": [100.0, 0.0]

},

{ "type": "LineString",

"coordinates": [ [101.0, 0.0], [102.0, 1.0] ]

}

]

}

The GeoJSON format specification is the product of discussion on the GeoJSON

list:

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