Geojson format description (detailed format)

1. brief introduction

GeoJSON Is a format that encodes geographic data structures .GeoJSON Objects can represent geometric information 、 An element or collection of elements .GeoJSON The following geometry types are supported ：Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon and GeometryCollection.GeoJSON The element in contains a geometric object and an attribute object , A feature set represents a collection of multiple features . A complete GeoJSON The data structure is always a JSON object . stay GeoJSON in , Objects consist of a collection of key value pairs . For every member , Names are always strings . The value of the member is either character string 、 Numbers 、 object 、 Array , Or "true","false" and "null", The array and object are composed of the values of the above types .

geojson Data acquisition address ：http://datav.aliyun.com/tools/atlas/#&lat=31.769817845138945&lng=104.29901249999999&zoom=4

2. GeoJSON object

GeoJSON Always consists of a single object . This object represents geometry 、 An element or collection of elements , An object can have any number of members （ Key value pair ）.

1. GeoJSON The object must have a name of "type" Members of . The value of this member is determined by GeoJSON The string determined by the type of the object .

{
    
	"type": "FeatureCollection",
	"features": [
		//  Other content 
	]
}

2. type The value of the member must be one of the following ：“Point”, “MultiPoint”, “LineString”, “MultiLineString”, “Polygon”, “MultiPolygon”, “GeometryCollection”, “Feature”, perhaps “FeatureCollection”.（ Similar to the code above ）

3. GeoJSON Object may have an optional "crs" member , Its value must be an object of a coordinate reference system （ see 3. Coordinate reference system object ）.

{
    
	"type": "FeatureCollection",
	"crs": {
    
		"type": "name",
		"properties": {
    
			"name": "urn:ogc:def:crs:OGC:1.3:CRS84"
		}
	},
	"features": [
		//  Other content 
	]
}

GeoJSON The object may have a "bbox" member , Its value must be an array of bounding boxes **（ see 4. Bounding box ）**.

{
    
	"type": "FeatureCollection",
	"bbox": [-180.0, -90.0, 180.0, 90.0],
	"features": [
		//  Other content 
	]
}

2.1 Geometry object

The geometric (Geometry) It's a kind of GeoJSON object , At this time type The value of the member is one of the following strings ："Point", "MultiPoint", "LineString", "MultiLineString", "Polygon", "MultiPolygon", perhaps "GeometryCollection". except “GeometryCollection” Any other type of GeoJSON Geometric objects must have a name of "coordinates" Members of .coordinates The value of a member is always an array . The structure of the elements in this array is determined by the geometric type .

{
    
	"type": "FeatureCollection",
	"features": [
		{
    
			"type": "GeoJSON",
			"geometry": {
    
				"type": "MultiPolygon",
				"coordinates": [
					[
						[
							[
								108.953361,
								34.292663
							],
							[
								108.953404,
								34.289539
							],
							[
								108.950857,
								34.289529
							]
							//  Multiple latitude and longitude points 
						]
					]
				]
			}
		}
	]
}

2.1.1.Position（ Base point ）

position Is the basic geometry . Of geometric objects "coordinates" The members consist of a position（ Here is the geometric point ）、position Array （ Line or geometric multipoint ）,position Array of arrays （ Noodles 、 Multi line ） perhaps position Multidimensional array of （ In many ways ） form .

position Represented by an array of numbers （ It can be understood as latitude and longitude ）. Must have at least two elements , There can be more elements . The order of the elements must follow x,y,z The order （ The east direction of coordinates in the projection coordinate reference system 、 To the north 、 Elevation or coordinate longitude in geographic coordinate reference system 、 latitude 、 Altitude ）. Any number of other elements is allowed, and this article does not discuss the project .

2.1.2. type type ：Point（ spot ）

Right type "Point" Come on ,“coordinates" The member must be a separate position.

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

2.1.3. type type ：MultiPoint（ Multipoint ）

Right type "MultiPoint" Come on ,"coordinates" Members must be position Array .

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

2.1.4. type type ：LineString（ Line ）

Right type "LineString" Come on ,“coordinates" Members must be two or more position Array of .

A linear ring has 4 One or more position A closed line of . The first and last positions are the same （ They represent the same point ）. Although linear rings do not explicitly act as GeoJSON Geometric type , However, it is mentioned in the definition of surface geometry type .

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

2.1.5. type type ：MultiLineString（ Multi line ）

Right type “MultiLineString" Come on ,"coordinates" The member must be an array of linear coordinates .

{
    
    "type": "MultiLineString",
    "coordinates": [
        [ [100.0, 0.0], [101.0, 1.0] ],
        [ [102.0, 2.0], [103.0, 3.0] ]
    ]
}

2.1.6. type type ：Polygon（ Noodles ）

Right type "Polygon" Come on ,"coordinates" The member must be an array of linear circular coordinates . For faces with multiple rings , The first ring must be an outer ring , Others must be internal rings or holes , And the direction of the inner ring and the outer ring is opposite .

Without holes ：

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

Porous （ The direction of the outer ring and the inner ring is always opposite ）

{
    
    "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] ]
    ]
}

2.1.7. type type ：MultiPlygon（ In many ways ）

Right type "MultiPlygon" Come on ,"coordinates" The member must be an array of face coordinates .

{
    
    "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]]
        ]
    ]
}

2.1.8. type type ：Geometry Collection（ Geometric set ）

The type is "GeometryCollection" Of GeoJSON An object is a collection object , It represents a collection of geometric objects . A geometric set must have a name "geometries" Members of . And "geometries" The corresponding value is an array . Each element in this array is a GeoJSON Geometry object .

{
    
    "type": "GeometryCollection",
    "geometries": [
        {
     "type": "Point",
            "coordinates": [100.0, 0.0]
        },
        {
     "type": "LineString",
            "coordinates": [ [101.0, 0.0], [102.0, 1.0] ]
        }
    ]
}

2.2 Feature object

The type is "Feature" Of GeoJSON The object is Feature object .

1. The element object must have a name of "geometry" Members of , The value of this geometric member is defined above geometry Object or JSON Of null value .
2. The element object must have a name of “properties" Members of , The value of this property member is an object （ whatever JSON Object or JSON Of null value ）.
3. If the element is a common identifier , Then this identifier should contain the name “id” Members of the object .

{
    
	"type": "FeatureCollection",
	"features": [
		{
    
			"type": "Feature",
			"properties": {
    
				"adcode": 610102,
				"name": " New town ",
				"center": [
					108.959903,
					34.26927
				],
				"centroid": [
					108.991696,
					34.271932
				],
				"childrenNum": 0,
				"level": "district",
				"parent": {
    
					"adcode": 610100
				},
				"subFeatureIndex": 0,
				"acroutes": [
					100000,
					610000,
					610100
				]
			},
			"geometry": {
    
				"type": "MultiPolygon",
				"coordinates": [
					[
						[
							[
								108.953361,
								34.292663
							],
							[
								108.953404,
								34.289539
							],
							[
								108.950857,
								34.289529
							]
							//  Multiple latitude and longitude points 
						]
					]
				]
			}
		}
	]
}

2.2.1 “properties" member

“properties" The content of the member is to save Feature object Other attribute information , For example Feature object Of name Information, etc .

2.3. FeatureCollection object

The type is "FeatureCollection" Of GeoJSON An object is a feature set object .

The type is "FeatureCollection" The object of must have a name "features" Members of . And “features" The corresponding value is an array . Each element in this array is defined above feature object .

You can refer to 2.2 Example

3. Coordinate reference system object

GeoJSON Object's coordinate reference system （CRS） By it "crs" Members to determine . If the object does not crs member , So it's a parent or a grandparent crs Members may be acquired as its crs. If this has not yet been achieved crs member , So the default CRS Will be applied to GeoJSON object .

1. default CRS Is a geographic coordinate reference system , It uses WGS84 data , The units of length and height are decimal .
2. The name is "crs" The value of a member must be JSON object （ It means the following CRS object ） perhaps JSON Of null. If CRS The value of is null, Then assume that there is no CRS 了 .
3. crs Members should be located at （ Feature set 、 features 、 Of geometric order ） In the hierarchy GeoJSON Top level of object , And it should not be repeated or overwritten in self objects or grandchildren .
4. Non empty CRS Object has two mandatory objects :“type" and "properties”.
5. type The value of a member must be a string , This string indicates CRS Type of object .
6. The value of a property member must be an object .
7. CRS The coordinate order cannot be changed **（ see 2.1.1. Location ）**

{
    
	"type": "FeatureCollection",
	"crs": {
    
		"type": "name",
		"properties": {
    
			"name": "urn:ogc:def:crs:OGC:1.3:CRS84"
		}
	},
	"features": [
		//  Other content 
	]
}

3.1. CRS

CRS Objects can pass through name To indicate the coordinate reference system . under these circumstances , its "type" The value of a member must be a string "name". its "properties" The value of a member must contain "name" Member's object . This "name" The value of the member must be a string that identifies the coordinate reference system . such as “urn:ogc:def:crs:OGC:1.3:CRS84" Of OGC CRS Of URN Should take precedence over old identifiers such as "EPSG:4326" Selected .

3.2. On-line CRS

CRS Objects can also be connected to the Internet CRS Parameters . under these circumstances , its "type" The value of a member must be a string "link", its "properties" The value of a member must be a linked object .

1. The connection object consists of a required member ：“href”, And an optional member :“type”.
2. Essential "href" The value of a member must be dereferenced URI（ Unified resource identification ）.
3. Optional "type" The value of a member must be a string , And this string implies the provided URI Used in to indicate CRS Parameter format . The recommended value is :“proj4”,“ogcwkt”,esriwkt", However, other values can be used .

{
    
	"type": "FeatureCollection",
	"crs": {
    
	    "type": "link",
	    "properties": {
    
	        "href": "http://example.com/crs/42",
	        "type": "proj4"
	    }
	},
	"features": [
		//  Other content 
	]
}

The auxiliary file of the relative path can also be used as CRS Deal directly with ：

{
    
	"type": "FeatureCollection",
	"crs": {
    
	    "type": "link",
	    "properties": {
    
	        "href": "data.crs",
	        "type": "ogcwkt"
	    }
	},
	"features": [
		//  Other content 
	]
}

4.Bounding Boxes

To include geometry、feature perhaps feature collection Coordinate range information ,GeoJSON The object may have a name of "bbox Members of .bbox The value of a member must be 2*n Array , here n Is the number of dimensions of the contained geometric objects , And the maximum and minimum values of all coordinate axes .bbox The order of the coordinate axes of follows the order of the geometric coordinate axes . besides ,bbox The coordinate reference system of is assumed to match where it is located GeoJSON Object's coordinate reference system .

feature On the object bbox Examples of members ：

{
    
    "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]
        ]]
    }
}

feature collection In the object bbox Examples of members ：

{
    
    "type": "FeatureCollection",
    "bbox": [100.0, 0.0, 105.0, 1.0],
    "features": [
      //  Other content 
    ]
}

Reference resources ：
https://zhuanlan.zhihu.com/p/510882183
https://blog.csdn.net/qq_36213352/article/details/117534921