Geojson/ turf ：将多个多边形合并为一个多边形保持孔

如何解决Geojson/ turf ：将多个多边形合并为一个多边形保持孔

所以我想在javascript中合并相邻的多边形，这就是我的代码实际拥有的：

我想删除内部描边但保留边框描边。

所以我想从这个开始：

为此：

我想保留巴黎的洞 - 我可以定义哪些多边形必须分组，但我无法将它们与此代码合并：

var map = L.map('map',{
    center: [46.52863469527167,2.43896484375],zoom: 6,maxZoom: 18,minZoom: 7
});
$http.get("france.json").then(function (response) {
    $scope.communeFr = response.data.features
    $http.get(apiult).then(function (response) {
        $scope.showCommune = response.data.Liste
        $scope.communeFr.map(x => {
            x.show = false
            for (let i = 0; i < $scope.showCommune .length; i++) {
                if (x.properties.insee == $scope.showCommune[i].insee) {
                    x.show = true
                    return
                }
            }                
        })
        L.geoJson($scope.communeFr,{
            filter: function (feature,latlng) {
                return feature.show 
            }
        }).addTo(map);
    });
    
});

更新 - 我尝试使用 turf.union 但输出不正确：

这是我的代码

var Grouppolygons = [];    
$scope.communeFr.map(x => {
    x.show = false
    for (let i = 0; i < $scope.showCommune .length; i++) {
        if (x.properties.insee == $scope.showCommune[i].insee) {
            Grouppolygons.push(x)
        }
    }                
})
var union = turf.union(...Grouppolygons)
L.geoJson(union).addTo(map)

解决方法

草皮文档中关于 union 的关于可以作为参数传递的多边形数量似乎有歧义。请参阅 here 和 here。看起来它必须一次有两个 - 所以这会起作用：

// feature ids to remove e.g. central Paris
var removeIds = [868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887];

// filter features to remove central Paris
var hole = fc.features.filter(f => !removeIds.includes(f.properties.ID_APUR))

// do the union over each feature
var union = hole[0];
for (let i=1; i<hole.length; i++) {
  union = turf.union(union,hole[i]);
}

我选择了一堆巴黎市中心来删除，然后从剩余的特征中我从特征 0 开始，然后 turf.join 所有其他特征（从索引 1 开始）到这个特征。似乎效率低下但有效...

这里有一个小问题：

// new Feature collection with unioned features
var fc2 = {
  "type": "FeatureCollection","features": [union] // note features has to be an array
}

记住将 FeatureCollection 传递给 L.geoJson 并且 features 属性需要是一个数组 - 即使在这种情况下它包含合并区域的单个特征。

工作示例：

// center map on Paris
var map = L.map('mapid',{
    center: [48.856,2.352],zoom: 9,maxZoom: 18,minZoom: 1
});

// add tiles
L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png',{
    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
}).addTo(map);

var url = "https://gist.githubusercontent.com/robinmackenzie/937e5bd42a0412c21281f69b8f0c8614/raw/fbed7c2783366463a250e4bb0ebcf3c5f6d54dfe/greaterParis.geo.json";

// get greater Paris definition
fetch(url)
  .then(response => response.json())
  .then(fc => { 

    // feature ids to remove e.g. central Paris
    var removeIds = [868,887];

    // filter features to remove central Paris
    var hole = fc.features.filter(f => !removeIds.includes(f.properties.ID_APUR))

    // do the union over each feature
    var union = hole[0];
    for (let i=1; i<hole.length; i++) {
      union = turf.union(union,hole[i]);
    }

    // new Feature collection with unioned features
    var fc2 = {
      "type": "FeatureCollection","features": [union] // note features has to be an array
    }

    // add to map
    L.geoJson(fc2).addTo(map);
  
  });

#mapid { height: 200px; }

<link rel="stylesheet" href="https://unpkg.com/leaflet@1.7.1/dist/leaflet.css"/>
<script src="https://unpkg.com/leaflet@1.7.1/dist/leaflet.js"></script>
<script src='https://unpkg.com/@turf/turf@6.3.0/turf.min.js'></script>
<div id="mapid"></div>