GeoJSON介绍

hh3755 2013-09-17

展开全文

GeoJSON是一种地理数据的描述格式。GeoJSON可以描述的对象包括：几何体，要素和要素集。
这里几何体(Geometry)的类型有我们熟悉的点(Point),线(LineString),面(Polygon), 多点(MultiPoint),多线(MultiLineString),多面( MultiPolygon)和几何体集合(GeometryCollection)。
要素(Feature)包含了几何体信息以及附加的一些属性信息。
要素集(a collection of features)即为要素的集合。

GeoJSON的结构同JSON并增加了一些约束条件：
GeoJSON对象必须包含一个type属性，type的值为其描述的对象(如前所述为下列选项之一：”Point”, “MultiPoint”, “LineString”, “MultiLineString”, “Polygon”, “MultiPolygon”, “GeometryCollection”, “Feature”, “FeatureCollection”)；
几何体(Geometry)必须包含坐标信息，要素(Feature)必须包含一个几何体信息，同时至少有一项属性信息。

GeoJSON格式示例：
点

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

线

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

面
没有洞的面：

{ "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，数组第一项是外环，其他是内环:

{ "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] ]
    ]
 }

更多介绍和示例可参考这里

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).

Contents

1. Introduction

2. GeoJSON Objects

3. Coordinate Reference System Objects

3.2.1. Link Objects

Appendix B. Contributors

1. Introduction

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.

1.1. Examples

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"}
         }
       }
     ]
   }

1.2. Definitions

JavaScript Object Notation (JSON), and the terms object, name, value, array, and number, are defined in IETF RTC 4627, athttp://www./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, athttp://www./rfc/rfc2119.txt.

2. GeoJSON Objects

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).

2.1 Geometry Objects

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.

2.1.1. Positions

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.

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.

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.

2.1.5. MultiLineString

For type "MultiLineString", the "coordinates" member must be an array of LineString coordinate arrays.

2.1.6. Polygon

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.

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 Objects

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".

2.3. Feature Collection Objects

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 Objects

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).

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: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"
    }
  }

3.2. Linked CRS

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).

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:///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 Boxes

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": [
    ...
    ]
  }

Appendix A. Geometry Examples

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

Point coordinates are in x, y order (easting, northing for projected coordinates, longitude, latitude for geographic coordinates):

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

LineString

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

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

Polygon

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] ]
    ]
 }

MultiPoint

Coordinates of a MultiPoint are an array of positions:

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

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] ]
    ]
  }

MultiPolygon

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]]]
    ]
  }

GeometryCollection

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] ]
      }
  ]
}