看到身边不少同学，都喜欢用GEE来下载遥感影像数据。一是因为非常便捷，二是因为GEE平台上的遥感影像多数已经过地形校正或大气校正，使用起来非常喜人。Hence，本期介绍如何在GEE上下载哨兵2号影像数据。

//////define your study area here!
var studyarea = area1;
Map.addLayer(studyarea)

////////////////////////////prepare sentinel imagecollection/////////////////////////////////////
/**
 * Function to mask clouds using the Sentinel-2 QA band
 * @param {ee.Image} image Sentinel-2 image
 * @return {ee.Image} cloud masked Sentinel-2 image
 */
function maskS2clouds(image) {
  var qa = image.select('QA60');
  // Bits 10 and 11 are clouds and cirrus, respectively.
  var cloudBitMask = 1 << 10;
  var cirrusBitMask = 1 << 11;
  // Both flags should be set to zero, indicating clear conditions.
  var mask = qa.bitwiseAnd(cloudBitMask).eq(0)
      .and(qa.bitwiseAnd(cirrusBitMask).eq(0));
  return image.updateMask(mask).divide(10000);
}  //去云处理


var dataset = ee.ImageCollection('COPERNICUS/S2_SR')
                  .filterDate('2020-01-01', '2020-12-31')   //影像时间
                  // Pre-filter to get less cloudy granules.
                  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE',20))  //云量
                  .map(maskS2clouds)  //去云
                  .map(function(image) {
                   var ndvi = image.normalizedDifference(['B8', 'B4']).rename('B5');
                   return image.addBands(ndvi);
                   })   //添加指数
                   .map(function(image){
                   var B1 = image.select("B2").rename("B1");//blue
                   var B2 = image.select("B3").rename("B2");//green
                   var B3 = image.select("B4").rename("B3");//red
                   var B4 = image.select("B8").rename("B4");//nir
                   return image.addBands(B1)
                               .addBands(B2)
                               .addBands(B3)
                               .addBands(B4)
                   })  //重命名
                   .select(["B1","B2","B3","B4","B5"/*,"B6","B7","B8","B9"*/]);  //影像集
                   
var study_img = dataset.median().clip(studyarea);  //中值影像，裁剪出研究区影像
Map.addLayer(study_img, {bands:["B4", "B3", "B2"], min:0, max:0.3}, 'RGB');

//导出哨兵2号影像数据
Export.image.toDrive({
  image:study_img.select(["B1","B2","B3","B4","B5"]),
  description:"sentinel2_AREA",
  region:studyarea,
  scale:10,
  maxPixels:1e13
})