Google My Maps中的KML格式
這篇要來紀錄一下，如何產生一個 Google My Maps 能夠閱讀、解析、顯示的 KML 內容格式。
KML範例檔案的取得
直接從Google My Maps建立地圖並匯出KML
最簡單的實驗就是先到 Google My Maps 上面建立一個地圖，然後點擊地圖名稱右側的「
」，然後選擇「匯出成 KML/KMZ」。這樣就能觀察由 Google My Maps 產生的KML檔案格式，方便自己生成KML檔時進行驗證或比對。
參考Google KML說明中的範例檔案
KML的格式內容
這邊我們就以 Google My Maps 可以讀懂的 KML 單元來做實驗與說明。為什麼說「Google My Maps可以讀懂」，是因為坊間一些App所匯出的KML檔案，在你從App匯出並匯入Google My Maps後，會發現一些格式或單元不相容，而導致匯入失敗的情況。
新增一個地標
最簡單 Google My Maps 可以匯入的 KML 檔案，大概就類似下方這樣包含一個地標（Placemark）的內容。Placemark中以<name>來描述這個地標的名稱，以<description>紀錄地點的說明，以及透過<Point>來描述這個點的座標位置。
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
<Placemark>
<name>地標1</name>
<description></description>
<Point>
<coordinates>121.41738,25.11393,0</coordinates>
</Point>
</Placemark>
</kml>在<Placemark>中使用<Point>來紀錄一個點，透過<coordinates>來描述座標的位置。裡面的內容為：
<coordinates>經度,緯度,高度</coordinates>
高度指的是海拔高度，單位為公尺。
地圖的座標支援種類有哪些？依照Google Maps Help上的說明，目前有支援以下幾種：
- Decimal degrees (DD)： 41.40338, 2.17403
- Degrees, minutes, and seconds (DMS)： 41°24’12.2″N 2°10’26.5″E
- Degrees and decimal minutes (DMM)： 41 24.2028, 2 10.4418
那如何知道一個地點的座標數值呢？最簡單的方式就是開啟Google Maps，在你所要查詢座標的位置點點擊滑鼠右鍵，這樣就會跳出一個功能方塊，最上面的數值就是這個位置的座標。
在想要進一步新增地標時，會發現到兩個情況。
- 匯入 Google My Maps 時，必須要在有空圖層的情況下，才能匯入KML資料。
- 無法新增超過一個Placemark的話，在上面範例中，如果依樣畫葫蘆，只會顯示最後一筆的地標資料。
加入圖層
上面遇到這兩個問題可以合起來說明，就是在問題1的情況下可以知道，基本上在Google My Maps的設計上，地標基本上都要包含於一個圖層之內的。而圖層在這裡對應的XML標籤名稱為「Folder」，在圖層「Folder」的兩個基本屬性為名稱與說明，名稱的XML標籤為「name」、說明的XML標籤為「description」，這兩個屬性對應到Google My Maps中，就是點擊圖層時，上面出現的一列名稱，以及下方可以容納多列資料的說明空間。然後再將多個地標（Placemarks）放到圖層（Folder），這樣就能在一個圖層中正確顯示多個地標，同時解決了上面段落的兩個問題。
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
<Folder>
<name>Placemarks</name>
<description></description>
<Placemark>
<name>地標1</name>
<description></description>
<Point>
<coordinates>121.41738,25.11393,0</coordinates>
</Point>
</Placemark>
<Placemark>
<name>地標2</name>
<description></description>
<Point>
<coordinates>121.41838,25.11293,0</coordinates>
</Point>
</Placemark>
</Folder>
</kml>路徑繪製
路徑的部份同樣是用<Placemark>，只不過之前單一地點是以<point>來標明，路徑的話是以<LineString>來描述。<LineString>主要是透過<coordinates>標籤來描述一連串的路徑資料，關於<coordinates>內的資料格式，可以參考上面「新增一個地標」章節中的說明，另外，<tessellate>屬性就字面上的翻譯是「鑲嵌」，屬性是個boolean值，當為1的時候為True，代表這條LineString要鑲嵌在地形上，也就是路徑隨著這地形高低而起伏，因此在絕大部分一般的使用上，應該都是設為1。
<Folder>
<name>二子坪步道步行路線</name>
<Placemark>
<name>二子坪步道步行路線</name>
<LineString>
<tessellate>1</tessellate>
<coordinates>
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</coordinates>
</LineString>
</Placemark>參考資料：
- Google的KML檔案格式介紹
- Google KML Reference
- Google 的 KML範例檔案
- Google的KML描述說明：KML 教程 | “Keyhole 标记语言” | Google Developers