9.6 圆的数据处理

圆存入数据库的是中心坐标，周长和面积
导入圆也同理，需要反算出半径
比如，导出的geojson数据是：

{
    "type": "Feature",
    "properties":
    {
      "options":
      {
        "color": "#634FF1",
        "fillColor": "#634FF1",
        "fillOpacity": 0.4,
        "radius": "2120.919064",
        "pane": "overlayPane"
      },
      "type": "circle"
    },
    "geometry":
    {
      "type": "Point",
      "coordinates": [113.191452, 23.113523]
    }
  },

半径2120.9，周长是13.33km……