Google Earth Engine(GEE) 學習筆記二十九：下載文件一

06-26

來自專欄 GEE 開發

失蹤人口重新上線，接下來一段時間我可能會斷斷續續更新一個關於GEE的一個在實際工作中使用遇到的問題以及解決方法。

今天首先探討一下在實際工作中最為常用的一個功能，下載GEE中的數據或者結果，關於GEE中下載數據方式我了解到的目前主要有以下兩種方式，如果有新的方式請大家留言探討一下。

第一種方式：getDownloadURL方式，這是這篇文章主要介紹的內容。
第二種方式：Export方式，這種方式在之前的文章中也提起過，只不過沒有詳細介紹。篇幅所限，下一篇文章中會詳細介紹各種Export方式。

首先，我們查看GEE的API，getDownloadURL方法總共有兩個：一個是FeatureCollection，一個是Image的。這個也非常好理解，我們在GEE需要下載的也就是矢量數據和柵格數據，那麼也就是這兩種數據集合。需要明確的一點是GEE不支持直接下載ImageCollection，非常重要。下面我會依次介紹這兩個方法：

1. ee.FeatureCollection.getDownloadURL()參數

Get a download URL.
Returns a download URL or undefined if a callback was specified.
Arguments:
this:featurecollection (FeatureCollection):
The FeatureCollection instance.
format (String, optional):
The format of download, one of:
"csv", "json", "kml", "kmz".
selectors (List<String>|String, optional):
Selectors that should be used to determine which attributes will be downloaded.

filename (String, optional):
Name of the file to be downloaded.
callback (Function, optional):
An optional callback. If not supplied, the call is made synchronously.
Returns: Object|String

參數說明：

format：生成的下載格式，支持四種各種csv、json、kml、kmz。如果需要ESRI shapefile格式，那麼需要生成比如kml格式，然後使用QGIS或者Arcgis本地轉換一下；
selectors：篩選下載的矢量數據中包含哪些屬性，默認是包含全部屬性；
filename：下載的文件名稱；
callback：這個參數幾乎不會用到，回調方法。

上傳的Assert文件下載示例：

//使用的是GEE中世界地區邊界這個數據集，展示的是中國台灣省var fc = ee.FeatureCollection(USDOS/LSIB/2013);//篩選台灣省var sfc = fc.filter(ee.Filter.eq("cc", "TW"));//這一步非常重要，直接使用sfc下載數據那麼只能獲取相關數據結構，數據內容會全部丟失var taiwan = ee.FeatureCollection(sfc.toList(sfc.size()));//地圖載入台灣省Map.addLayer(taiwan, {color:"00ff00"}, taiwan);Map.centerObject(taiwan, 7);print(taiwan);//輸出的kml包含全部屬性var url1 = taiwan.getDownloadURL({ format: "kml", filename: "taiwan1"});//在輸出面板中展示URLprint("url1 is: ", url1);//輸出的kml只包含cc屬性var url2 = taiwan.getDownloadURL({ format: "kml", selectors: ["cc"], filename: "taiwan2"});print("url2 is: ", url1);

輸出結果：

使用QGIS查看兩個文件中的屬性：

包含全部屬性的文件

只包含cc屬性的文件

對於直接使用代碼生成的FeatureCollection下載示例：

var roi = /* color: #d63000 */ee.Geometry.Polygon( [[[115.46424865722656, 39.47489550075251], [115.54664611816406, 39.47171528483254], [115.55419921875, 39.524699902077586], [115.47111511230469, 39.52734807268146]]]);//featureCollection生成下載的URLvar roi_col = ee.FeatureCollection(roi);var url3 = roi_col.getDownloadURL({ format: "json", filename: "bounds"});print("url3 is: ", url3);

2. ee.Image.getDownloadURL() 參數說明：

Get a Download URL
Returns returns a download URL, or undefined if a callback was specified.
Arguments:
this:image (Image):
The Image instance.

params (Object):
An object containing download options with the following possible values:
- name: a base name to use when constructing filenames.
- bands: a description of the bands to download. Must be a list of dictionaries, each with the following keys:
+ id: the name of the band, a string, required.
+ crs: an optional CRS string defining the band projection.
+ crs_transform: an optional list of 6 numbers specifying an affine transform from the specified CRS, in row-major order:
[xScale, xShearing, xTranslation, yShearing, yScale, yTranslation]
+ dimensions: an optional list of two integers defining the width and height to which the band is cropped.
+ scale: an optional number, specifying the scale in meters of the band; ignored if crs and crs_transform is specified.

- crs: a default CRS string to use for any bands that do not explicitly specify one.
- crs_transform: a default affine transform to use for any bands that do not specify one, of the same format as the crs_transform of bands.
- dimensions: default image cropping dimensions to use for any bands that do not specify them.
- scale: a default scale to use for any bands that do not specify one; ignored if crs and crs_transform is specified.
- region: a polygon specifying a region to download; ignored if crs and crs_transform is specified.
callback (Function, optional):
An optional callback. If not supplied, the call is made synchronously.
Returns: Object|String

參數說明：

params各種參數，這裡說一下常用的，其他的參數用的機會不多（bands波段參數列表，這個參數不會用，??）

name 下載文件的名稱
crs 投影信息，一般來講都是使用默認的投影，然後本地使用其他軟體轉換投影信息
scale 解析度
region 下載的區域

callback回調方法，幾乎不用

下載示例：

//download image url demovar roi = /* color: #d63000 */ee.Geometry.Polygon( [[[115.46424865722656, 39.47489550075251], [115.54664611816406, 39.47171528483254], [115.55419921875, 39.524699902077586], [115.47111511230469, 39.52734807268146]]]); var l8 = ee.ImageCollection("LANDSAT/LC08/C01/T1_RT_TOA") .filterBounds(roi) .filterDate("2017-01-01", "2017-12-31") .map(ee.Algorithms.Landsat.simpleCloudScore) .map(function(img) { img = img.updateMask(img.select("cloud").lt(1)); return img; }) .sort("system:time_start");var landsat = l8.mosaic().clip(roi);Map.addLayer(landsat, {bands: ["B3", "B2", "B1"], min:0, max:0.3}, "landsat");Map.centerObject(roi, 12);Map.addLayer(roi, {color: red}, roi);//第一種獲取geometry的json字元串方式var roiInfo = roi.getInfo();var region1 = JSON.stringify(roiInfo);var url1 = landsat.select(B1).getDownloadURL({scale: 30, region: region1, name:"landsat8-B1"});print("url1 is: ", url1);var url2 = landsat.select(B2).getDownloadURL({scale: 100, region: region1, name:"landsat8-B2"});print("url2 is: ", url2);//第二種獲取geometry的json字元串方式（推薦）var region2 = ee.Geometry(roi).toGeoJSONString();var url3 = landsat.select(B3).getDownloadURL({scale: 30, region: region2, name:"landsat8-B3"});print("url3 is: ", url3);//篩選多個波段同時下載var url4 = landsat.select(["B3", "B2", "B1"]).getDownloadURL({scale: 30, region:region2, name:"landsat8-RGB"});print("url4 is: ", url4);

運行結果：