最近做的项目需要详细了解geojson,因此查了一些资料,现在整理一份标准格式的记录,要理解本文需要首先了解json的基本知识,这里不过多展开,可以去参考w3school上的教程,简言之,json是通过键值对表示数据对象的一种格式,可以很好地表达数据,其全称为JavaScript Object Notation(JavaScript Object Notation),正如这个名称,JavaScript和json联系紧密,但是json可以应用的范围很广,不止于前端,它比XML数据更轻量、更容易解析(某种角度上说xml可以更自由地封装更多的数据)。很多编程语言都有对应的json解析库,例如Python的json库,C#的Newtonsoft.Json,Java的org.json。geojson是用json的语法表达和存储地理数据,可以说是json的子集。
例如下面就是一个点数据:
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| { "type": "FeatureCollection", "features": [ {"type":"Feature", "properties":{}, "geometry":{ "type":"Point", "coordinates":[105.380859375,31.57853542647338] } } ] }
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(注:以下geojson的效果截图都来自geojson.io在线生成)
geojson将所有的地理要素分为Point、MultiPoint、LineString、MultiLineString、Polygon、MultiPolygon、GeometryCollection。首先是将这些要素封装到单个的geometry里,然后作为一个个的Feature(也就是要素);要素放到一个要素集合里,从树状结构来理解FeatureCollection就是根节点,表示为:
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| { "type": "FeatureCollection", "features": [] }
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所有地理要素放在features的列表里。
点要素Point
点要素是最简单的,类型type对应Point,然后坐标是一个1维的数组,里面有两个元素(如果是立体的坐标就是三维x,y,z),分别为经度和纬度。properties里面可以封装各种属性,例如名称、标识颜色等等。
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| {"type":"Feature", "properties":{}, "geometry":{ "type":"Point", "coordinates":[105.380859375,31.57853542647338] } }
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多点要素MultiPoint
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| {"type":"Feature", "properties":{}, "geometry":{ "type":"MultiPoint", "coordinates":[[105.380859375,31.57853542647338], [105.580859375,31.52853542647338] ] } }
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其核心坐标:
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| 105.380859375,31.57853542647338 105.580859375,31.52853542647338
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线要素LineString
线要素就是指线段,记录的是线的端点坐标,可视化时会按照记录顺序联结。对于曲线(如贝塞尔曲线)目前还没有很好的表达,但是在地理数据中,曲线一般会用LineString去拟合,现实地理世界中也没有标准的曲线地理要素。
线要素的坐标coordinates里的二维数组和多点要素基本一样,区别就在type上了。
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| {"type":"Feature", "properties":{}, "geometry":{ "type":"LineString", "coordinates":[[105.6005859375,30.65681556429287], [107.95166015624999,31.98944183792288], [109.3798828125,30.031055426540206], [107.7978515625,29.935895213372444]] } }
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对应的Kml表达:
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| <Placemark> <ExtendedData></ExtendedData> <LineString> <coordinates>108.65753173828125,34.1873818599505 108.72413635253905,34.25154099726973 108.77151489257812,34.16977214177208 108.88481140136719,34.229970811273084 </coordinates> </LineString> </Placemark>
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MultiLineString
也是一个三维数组(和多边形一样);
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| {"type":"Feature", "properties":{}, "geometry":{ "type":"MultiLineString", "coordinates": [ [ [105.6005859375,30.65681556429287], [107.95166015624999,31.98944183792288], [109.3798828125,30.031055426540206], [107.7978515625,29.935895213372444] ], [ [109.3798828125,30.031055426540206], [107.1978515625,31.235895213372444] ] ] } }
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多边形Polygon
注:单个多边形是一个3维数组,可以包含多个二维数组,这种情况和MultiPolygon效果很像。
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| {"type":"Feature", "properties":{}, "geometry":{ "type":"Polygon", "coordinates":[ [ [106.10595703125,33.33970700424026], [106.32568359375,32.41706632846282], [108.03955078125,32.2313896627376], [108.25927734375,33.15594830078649], [106.10595703125,33.33970700424026] ] ] } }
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多多边形MultiPolygon
type 1 两个不会相交的多边形
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| { "type": "Feature", "properties": {}, "geometry": { "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] ] ] ] } }
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type 2 两个镶套的多边形
小的在前面,范围大的在后面,用上4个中括号,但效果不是有洞的
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| { "type": "Feature", "properties": {}, "geometry": { "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] ] ] ] } }
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type 3 有孔洞的多边形
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| { "type": "Feature", "properties": {}, "geometry": { "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] ] ] ] } }
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可以仔细去品味type2和type3的区别。它们对应的kml表达区别是比较大的。
GeometryCollection
GeometryCollection是多种基本地理要素的集合,就是里面可以包含点、线、面要素。
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| { "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]] }] }
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GeometryCollection不需要放在FeatureCollection里:
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| { "type": "FeatureCollection", "features": [] }
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geojson里面还有其他标签表达其他的属性,如外包矩形等,其中特别重要的是坐标系统,一般里面的坐标默认为WGS84,当然也可以是其他坐标系统的坐标,但是要标识。这部分内容之后再补充了。
更具体的内容可以参考rfc7946。