A preliminary study of geojson

       

         Recently, when doing data visualization , Mentioned a kind of GeoJSON Formatted data , Here is a comb .

1 brief introduction

        GeoJSON It is a format for encoding various geographic data structures .GeoJSON Objects can represent geometry 、 A feature or set of features .GeoJSON The following geometry types are supported ： spot 、 Line 、 Noodles 、 Multipoint 、 Multi line 、 Polyhedra and geometric sets .GeoJSON The feature in contains a geometric object and other attributes , A feature set represents a series of features .

         A complete GeoJSON The data structure is always a （JSON In terms of ） object . stay GeoJSON in , Object by name / It's worth it -- Also known as the set composition of members .

example ：

{
	"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"
				}
			}
		}
	]
}

 2 GeoJSON object

        GeoJSON Always consists of a single object . This object （ It means the following GeoJSON object ） Represents geometry 、 A feature or set of features .

• GeoJSON Object can have any number of members （ name / It's worth it ）.
• 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 The value of the member must be one of the following ："Point", "MultiPoint", "LineString", "MultiLineString", "Polygon",    "MultiPolygon",   "GeometryCollection", "Feature", perhaps "FeatureCollection".
• GeoJSON Object may have an optional "crs" member , Its value must be an object of a coordinate reference system .
• GeoJSON The object may have a "bbox" member , Its value must be an array of bounding boxes .

Can be in  http://geojson.io  See the effect

2.1 Geometry object

         Geometry is 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 .

2.1.1 spot Point

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

2.1.2 Multipoint MultiPoint

{
	"type": "MultiPoint",
	"coordinates": [
		[100, 0],
		[101, 1]
	]
}

2.1.3 Line LineString

{
	"type": "LineString",
	"coordinates": [
		[100, 0],
		[101, 1]
	]
}

2.1.4 Multi line MultiLineString

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

2.1.5 polygon Polyon

Nonporous

{
    "type": "Polygon",
    "coordinates": [
        [
            [ 100, 0 ],
            [ 101, 0 ],
            [ 101, 1 ],
            [ 100, 1 ],
            [ 100, 0 ]
        ]
    ]
}

Porous

{
    "type": "Polygon",
    "coordinates": [
        [
            [ 100, 0 ],
            [ 101, 0 ],
            [ 101, 1 ],
            [ 100, 1 ],
            [ 100, 0 ]
        ],
        [
            [ 100.2, 0.2 ],
            [ 100.8, 0.2 ],
            [ 100.8, 0.8 ],
            [ 100.2, 0.8 ],
            [ 100.2, 0.2 ]
        ]
    ]
}

2.1.6 Compound polygon MultiPolyon

Disjoint polygons

{
	"type": "MultiPolygon",
	"coordinates": [
		[
			[
				[109.2041015625, 30.088107753367257],
				[115.02685546875, 30.088107753367257],
				[115.02685546875, 32.7872745269555],
				[109.2041015625, 32.7872745269555],
				[109.2041015625, 30.088107753367257]


			]
		],
		[
			[
				[112.9833984375, 26.82407078047018],
				[116.69677734375, 26.82407078047018],
				[116.69677734375, 29.036960648558267],
				[112.9833984375, 29.036960648558267],
				[112.9833984375, 26.82407078047018]
			]
		]
	]
}

Two nested polygons

{
	"type": "MultiPolygon",
	"coordinates": [
		[
			[
				[101.6455078125, 27.68352808378776],
				[114.78515624999999, 27.68352808378776],
				[114.78515624999999, 35.209721645221386],
				[101.6455078125, 35.209721645221386],
				[101.6455078125, 27.68352808378776]
			]
		],
		[
			[
				[104.2822265625, 30.107117887092357],
				[108.896484375, 30.107117887092357],
				[108.896484375, 33.76088200086917],
				[104.2822265625, 33.76088200086917],
				[104.2822265625, 30.107117887092357]
			]
		]
	]
}

Polygons with holes

{
	"type": "MultiPolygon",
	"coordinates": [
		[
			[
				[101.6455078125, 27.68352808378776],
				[114.78515624999999, 27.68352808378776],
				[114.78515624999999, 35.209721645221386],
				[101.6455078125, 35.209721645221386],
				[101.6455078125, 27.68352808378776]


			],
			[
				[104.2822265625, 30.107117887092357],
				[108.896484375, 30.107117887092357],
				[108.896484375, 33.76088200086917],
				[104.2822265625, 33.76088200086917],
				[104.2822265625, 30.107117887092357]

			]
		]
	]
}

2.1.7 Geometric set GeometryCollection

         It is a collection of many basic geographical elements , It can contain points 、 Line 、 Surface elements

{
	"type": "GeometryCollection",
	"geometries": [{
		"type": "Point",
		"coordinates": [108.62, 31.02819]
	}, {
		"type": "LineString",
		"coordinates": [
			[108.896484375, 30.1071178870],
			[108.2184375, 30.91717870],
			[109.5184375, 31.2175780]
		]
	}]
}

2.2 Characteristic object

• The type is Feature Of GeoJSON An object is a feature object
• The feature object must have a name of "geometry" Members of , The value of this geometric member is the geometric object defined above or JSON Of null value .
• Feature to play that must have a name for “properties" Members of , The value of this property member is an object （ whatever JSON Object or JSON Of null value ）

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

2.3 Feature set object

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

{
  "type": "FeatureCollection",
  "features": []
}

3 Application in various databases

3.1 mysql

1 geometry type

        MySQL Provides data types geometry Used to store coordinate information ,geometry Type supports the following three kinds of data storage

data structure	Example	explain
POINT( spot )	POINT(113.3 40.08)	Used to store point information , Contains longitude and latitude information
LINESTRING( Line )	LineString(84.070 33.801,99.52 30.292)	Used to store route information
POLYGON( Noodles )	POLYGON((84.070 33.801, 84.100 33.801,84.070 33.801))	Used to store face data

2 Format spatial data type

         The plaintext structure of the spatial data stored in the database displayed by the visualization tool is as seen in the above example , The structure is not easy to be parsed by the client , therefore MySQL Several spatial functions are provided to parse and format spatial data ,geojson yes gis Standard format for spatial data presentation , The front-end map framework and the related framework of back-end spatial analysis will support geojson Format

operation	function
geojson -> geometry	ST_GeomFromGeoJSON
geometry -> geojson	ST_ASGEOJSON
geometry character string -> geometry	ST_GEOMFROMTEXT

 3 Example

new table

CREATE TABLE `geojson`  (
  `id` int(0) NOT NULL AUTO_INCREMENT,
  `geojson` geometry NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

  insert data

insert into geojson(geojson) VALUES (ST_GeomFromGeoJSON('{"type": "Point", "coordinates": [121.0, 31.0]}'));

Inquire about

select id,ST_AsGeoJSON(geojson) as geojson from geojson;

Common space functions

name	describe
ST_INTERSECTS()	Judge whether two geometries intersect
ST_DISTANCE()	The distance between two geometries
ST_CONTAIONS()	Whether the geometry contains
ST_ISVALID()	Is geometry valid
ST_WITHIN()	Whether geometry is inside , Results and ST_CONTAIONS() contrary

SELECT
	id,
	ST_AsGeoJSON ( geojson ) AS geojson 
FROM
	geojson 
WHERE
	ST_CONTAINS (
		ST_GeomFromText ( 'POLYGON((100 30,120 30,120 36,100 36,100 30))' ),
	geojson 
	)

3.2 mongodb

1 Support geospatial index

• 2dsphere Indexes
  • Used on maps of the earth's surface type
  • It can be used in Legacy Coordinate Paris Save the latitude and longitude field and use GeoJSON Format saved points 、 Line 、 Polygon field

db.world.ensureIndex({"geometry" : "2dsphere"})

• 2d Indexes
  • For aspheric surfaces （ Game map , Time continuous data, etc ）, have access to 2d Index substitution 2dsphere
  • Support flat queries and some spherical queries , But spherical support is not very good
  • Only points can be indexed

2 Support geospatial query

         Main support intersection （intersection）, contain （within）, And close to （nearness）

• $geoIntersects
  • Point out the document intersecting with the query location
  • Support operators
    • $geometry Appoint GeoJSON Format geometry
• $geoWithin
  • Point out the documents completely contained in a certain area
  • Support operators
    • $box
      • Find all documents within the rectangle
    • $center
      • Find out all documents within the circle
    • $polygon
      • Find out all documents within the polygon
    • $centerSphere
      • Query all documents within the sphere circle
    • $geometry
      • Appoint GeoJSON Format geometry
• $near
  • Point out the document from the nearest to the farthest from the query location
  • Support operators
    • $maxDistance
      • Specify the maximum distance of query results
    • $minDistance
      • Specify the minimum distance of query results
    • $geometry
      • Appoint GeoJSON Format points
• $nearSphere
  • Use spherical geometry to calculate the distance near the spherical surface , Point out the document from the nearest to the farthest from the query location
  • Support operators
    • $maxDistance
      • Specify the maximum distance of query results
    • $minDistance
      • Specify the minimum distance of query results
    • $geometry
      • Appoint GeoJSON Format points

         Query documents with intersecting positions

db.getCollection('GeoEntity').find({
     location: {
       $geoIntersects: {
          $geometry: {
             type: "Polygon" ,
             coordinates: [
               [ [ 30, 20 ], [ 30, 40 ], [ 40, 40 ],[ 40, 20 ],[ 30, 20 ]]
             ]
          }
       }
     }
   })

         Query cross location documents

db.getCollection('GeoEntity').find({ location: { $geoWithin: { $box: [ [ 30, 30 ], [ 32, 33 ]] } } })

3.3 redis

1 Geospatial commands

• geoadd
  • geoadd key longitude latitude member [ longitude latitude member ... ]
  • Add the specified space element to the specified key in
• geodist
  • geodist key member1 member2 [ unit ]
  • unit Representative unit
  • Returns the distance between specified positions
• geohash
  • geohash key member [ member ... ]
  • Return to a standard geospatial geohash character string
• geopos
  • geopos key member [ member ... ]
  • Back to the latitude and longitude of geospatial
• georadius
  • georadius key longitude latitude radius m|km|ft|mi [ WITHCOORD ] [ WITHDIST ] [ WITHHASH ] [ COUNT count ]
  • Query the collection of all geospatial elements within the specified radius
  • longitude longitude
  • latitude latitude
  • radius Radius distance number
  • WITHDIST The distance between the centers of the position elements is also returned
  • WITHCOORD Return the longitude and latitude of the location element as well
  • WITHHASH With 52 Bit signed integer returns
  • COUNT Before returning all matching positions count Elements
• georadiusbymember
  • georadiusbymember key member radius m|km|ft|mi [ WITHCOORD ] [ WITHDIST ] [ WITHHASH ] [ COUNT count ]
  • Query a geospatial element that matches the maximum distance within the specified radius