【高精地图】GEOJSON标准格式学习

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原文链接：https://www.jianshu.com/p/852d7ad081b3

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本文深入解析GeoJSON格式，涵盖点、线、面等地理要素的表示方法，及其在FeatureCollection中的组织方式，适合地理信息系统和地图开发者参考。

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：

{
  "type": "FeatureCollection",
  "features": [
        {"type":"Feature",
        "properties":{},
        "geometry":{
            "type":"Point",
            "coordinates":[105.380859375,31.57853542647338]
            }
        }
    ]
}

例子图片

（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：

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

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。

{"type":"Feature",
    "properties":{},
    "geometry":{
        "type":"Point",
        "coordinates":[105.380859375,31.57853542647338]
        }
    }

多点要素MultiPoint

{"type":"Feature",
    "properties":{},
    "geometry":{
        "type":"MultiPoint",
        "coordinates":[[105.380859375,31.57853542647338],
                [105.580859375,31.52853542647338]
            ]
        }
        }

其核心坐标：

105.380859375,31.57853542647338
105.580859375,31.52853542647338

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。

{"type":"Feature",
    "properties":{},
    "geometry":{
        "type":"LineString",
        "coordinates":[[105.6005859375,30.65681556429287],
        [107.95166015624999,31.98944183792288],
        [109.3798828125,30.031055426540206],
        [107.7978515625,29.935895213372444]]
        }
    }

对应的Kml表达：

<Placemark>
    <ExtendedData></ExtendedData>
    <LineString>
        <coordinates>108.65753173828125,34.1873818599505 108.72413635253905,34.25154099726973 108.77151489257812,34.16977214177208 108.88481140136719,34.229970811273084
        </coordinates>
    </LineString>
</Placemark>

MultiLineString

也是一个三维数组（和多边形一样）；

{"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]
            ]
        ]
                }
    }

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。

{"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]
                        ]
                      ]
        }
    }

多多边形MultiPolygon

type 1 两个不会相交的多边形

{
  "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]
        <span class="token punctuation">]</span>
    <span class="token punctuation">]</span><span class="token punctuation">,</span>
    <span class="token punctuation">[</span>
        <span class="token punctuation">[</span>
            <span class="token punctuation">[</span><span class="token number">112.9833984375</span><span class="token punctuation">,</span><span class="token number">26.82407078047018</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">116.69677734375</span><span class="token punctuation">,</span><span class="token number">26.82407078047018</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">116.69677734375</span><span class="token punctuation">,</span><span class="token number">29.036960648558267</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">112.9833984375</span><span class="token punctuation">,</span><span class="token number">29.036960648558267</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">112.9833984375</span><span class="token punctuation">,</span><span class="token number">26.82407078047018</span><span class="token punctuation">]</span>
        <span class="token punctuation">]</span>
    <span class="token punctuation">]</span>
<span class="token punctuation">]</span>
         <span class="token punctuation">}</span>

}

两个不相交的多边形

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

    {
      "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]
                ]
            ]
        ]
      }
    }

两个镶套的多边形

type 3 有孔洞的多边形

{
      "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]
        <span class="token punctuation">]</span><span class="token punctuation">,</span>
        <span class="token punctuation">[</span>
            <span class="token punctuation">[</span><span class="token number">104.2822265625</span><span class="token punctuation">,</span><span class="token number">30.107117887092357</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">108.896484375</span><span class="token punctuation">,</span><span class="token number">30.107117887092357</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">108.896484375</span><span class="token punctuation">,</span><span class="token number">33.76088200086917</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">104.2822265625</span><span class="token punctuation">,</span><span class="token number">33.76088200086917</span><span class="token punctuation">]</span><span class="token punctuation">,</span>
            <span class="token punctuation">[</span><span class="token number">104.2822265625</span><span class="token punctuation">,</span><span class="token number">30.107117887092357</span><span class="token punctuation">]</span>
        
        <span class="token punctuation">]</span>
    <span class="token punctuation">]</span>
<span class="token punctuation">]</span>
            <span class="token punctuation">}</span>

}

有孔洞的多边形

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。

{
    "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]]
         }]
}

GeometryCollection不需要放在FeatureCollection里：

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

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。