以下測試均基於mongo v4.0 win10

一、GeoJSON

　　GeoJSON是一種基於json的經緯度描述資料格式。在這裡主要服務於2dsphere索引查詢。

　　基本格式  <type:"",coordinates:[]> ，type 對應的型別有Point、LineString(老版本有寫Line的)、Polygon、MultiPoint、MultiLineString、MultiPolygon。coordinates對應格式可參照【表1】。

更詳細的請自行百度。

二、2dsphere索引。

　　基於地球的經緯度。舉例如下

　　存在點 【28，2】，則以下區域是否與該點存在交集（$geoIntersects）的情況為

　　從LineString的四個例子中我們可以得出：getIntersects對LineString的支援性並不高，不過考慮到現實場景中的查詢，都是以一個面積為參考的，所以LineString的測驗只瞭解一個結果，不再深入研究。

　　從Polygon的四個例子中我們可以暫時得出：GeoJSON的陣列是不在乎順序的，為了驗證這個觀點，我們再補做方向性的測試資料。

三、geoIntersects的方向性研究

　　根據【表2編號1-5】到驗證了以上的猜想，GeoJSON陣列代表的區域是 ： 陣列中的點連線成線，將經緯度網劃分成兩塊，取面積小的那一塊作為選定區域，進行篩選判定。

　　但是如果所選範圍擴大到整個半球甚至更大，GeoJSON的陣列該如何選擇判定區域？從【表2編號7-16】中，我們什麼也不能得出來，整個查詢呈現出一種混亂的狀態。又進行大量的對比實驗，做了各種統計分析圖後，結論依舊混亂，我甚至還去文具店買了一個地球儀。。。

　　網上進行了大量查詢，最後還是在官網找到了一些資訊。3.0版本以後$within(表示包含)、$geoIntersects的Polygon查詢支援新的引數：crs。可以指定特殊的索引範圍劃分方式。如EPSG:4326，判定區域時具有嚴格的逆時針順序。更多更全的知識請移步 https://docs.mongodb.com/manual/reference/operator/query/geoIntersects/#geointersects-big-poly

　　但是經過測驗發現，4.0版本默認了EPSG:4326，加與不加，影響不大，測驗如下（$within與$geoIntersects結果一致）：

1 > db.people.find({"add":{"$within":{"$geometry":{"type":"Polygon","coordinates":[[[27,1],[29,1],[29,3],[27,3],[27,1]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
2 { "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
3 > db.people.find({"add":{"$within":{"$geometry":{"type":"Polygon","coordinates":[[[29,1],[29,3],[27,3],[27,1],[29,1]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
4 { "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
5 > db.people.find({"add":{"$within":{"$geometry":{"type":"Polygon","coordinates":[[[29,3],[27,3],[27,1],[29,1],[29,3]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
6 { "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
7 > db.people.find({"add":{"$within":{"$geometry":{"type":"Polygon","coordinates":[[[27,3],[27,1],[29,1],[29,3],[27,3]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
8 { "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
9 > db.people.find({"add":{"$within":{"$geometry":{"type":"Polygon","coordinates":[[[27,1],[27,3],[29,3],[29,1],[27,1]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});

　　甚至會出現這種莫名其妙的結果：

> db.people.find({"add":{"$geoIntersects":{"$geometry":{"type":"Polygon","coordinates":[[[0,1],[79,1],[79,3],[0,3],[0,1]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
{ "_id" : ObjectId("5bcafe07377b37ef8e160bc7"), "add" : { "type" : "Point", "coordinates" : [ 2, 2 ] } }
{ "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
{ "_id" : ObjectId("5bb4290d0c2287bbf737427d"), "name" : "ww wu", "age" : 40, "add" : { "type" : "Point", "coordinates" : [ 50, 2 ] } }
> db.people.find({"add":{"$geoIntersects":{"$geometry":{"type":"Polygon","coordinates":[[[0,1],[124,1],[124,3],[0,3],[0,1]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
{ "_id" : ObjectId("5bcafe07377b37ef8e160bc7"), "add" : { "type" : "Point", "coordinates" : [ 2, 2 ] } }
{ "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
> db.people.find({"add":{"$geoIntersects":{"$geometry":{"type":"Polygon","coordinates":[[[0,1],[120,1],[120,3],[0,3],[0,1]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
{ "_id" : ObjectId("5bcafe07377b37ef8e160bc7"), "add" : { "type" : "Point", "coordinates" : [ 2, 2 ] } }
{ "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
{ "_id" : ObjectId("5bb4290d0c2287bbf737427d"), "name" : "ww wu", "age" : 40, "add" : { "type" : "Point", "coordinates" : [ 50, 2 ] } }

　　可以看出，mongo對於2ds索引的支援度並不高。那麼我們完全按照官方文件的格式進行測試，看能不能得出一個有規律的結果。

　　官方文件的格式：

db.places.find(
   {
     loc: {
       $geoIntersects: {
          $geometry: {
             type : "Polygon",
             coordinates: [
               [
                 [ -100, 60 ], [ -100, 0 ], [ -100, -60 ], [ 100, -60 ], [ 100, 60 ], [ -100, 60 ]
               ]
             ],
             crs: {
                type: "name",
                properties: { name: "urn:x-mongodb:crs:strictwinding:EPSG:4326" }
             }
          }
       }
     }
   }
)

 　　格式定義如下：

　　1、首尾相接（以前測試的陣列都是滿足的）

　　2、逆時針方向書寫

> db.people.find({"add":{"$geoIntersects":{"$geometry":{"type":"Polygon","coordinates":[[[0,3],[0,-1],[179,-1],[179,3],[0,3]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});
{ "_id" : ObjectId("5bcafe07377b37ef8e160bc7"), "add" : { "type" : "Point", "coordinates" : [ 2, 2 ] } }
{ "_id" : ObjectId("5bcb0145377b37ef8e160bcb"), "add" : { "type" : "Point", "coordinates" : [ 2, 4 ] } }
{ "_id" : ObjectId("5bb4c46d0c2287bbf737427e"), "name" : "王五", "age" : 26, "sex" : "男", "add" : { "type" : "Point", "coordinates" : [ 28, 2 ] } }
{ "_id" : ObjectId("5bcb0158377b37ef8e160bcc"), "add" : { "type" : "Point", "coordinates" : [ 2, -2 ] } }
{ "_id" : ObjectId("5bb4290d0c2287bbf737427d"), "name" : "ww wu", "age" : 40, "add" : { "type" : "Point", "coordinates" : [ 50, 2 ] } }
> db.people.find({"add":{"$geoIntersects":{"$geometry":{"type":"Polygon","coordinates":[[[0,3],[0,1],[179,1],[179,3],[0,3]]],crs:{type:"name",properties:{name:"urn:x-mongodb:crs:strictwinding:EPSG:4326"}}}}}});

　　講道理，完全沒有道理。

　　總結，2ds對於小範圍的within(包含)，geoIntersects（相交）的支援是正常的，超過半球或者接近半球時，判定方式讓人摸不著頭腦。如果有懂的大神看見了。麻煩指教以下。