【GEE】优雅地实现年度、季度、月度甚至旬度影像合成（附完整代码）

var Beijing = ee.Geometry.Polygon(
          [[[116.29730339279362, 39.99599891062294],
            [116.29730339279362, 39.84222494267003],
            [116.48681755294987, 39.84222494267003],
            [116.48681755294987, 39.99599891062294]]], null, false);
var roi = Beijing;
Map.addLayer(roi, {'color':'grey'}, 'studyArea');
Map.centerObject(roi);

var year = 2020;

// reomove cloud for Landsat-8
function rmL8Cloud(image) { 
  var cloudShadowBitMask = (1 << 3); 
  var cloudsBitMask = (1 << 5); 
  var qa = image.select('pixel_qa'); 
  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0) 
                 .and(qa.bitwiseAnd(cloudsBitMask).eq(0));
  return image.updateMask(mask).toFloat().divide(10000)
              .copyProperties(image)
              .copyProperties(image, ['system:time_start','system:time_end','system:footprint']);
}

var l8 = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
           .filterBounds(roi)
           .filterDate(ee.Date.fromYMD(year,1,1),ee.Date.fromYMD(year+1,1,1))
           .map(rmL8Cloud)
           .select(['B2','B3','B4','B5','B6']);

// year composite
var l8_year =  l8.median().clip(roi);
print("l8_year",l8_year);

// half-year composite
var l8_halfyear = ee.List.sequence(0, 1*11,6).map(function(n) {
  var start = ee.Date.fromYMD(year,1,1).advance(n, 'month');
  var end = start.advance(6, 'month');
  var tmpMedian = l8.filterDate(start, end).median().clip(roi).set("system:time_start", start.millis());
  return tmpMedian;
}).flatten();
var l8_halfyear = ee.ImageCollection.fromImages(l8_halfyear);
print("l8_halfyear",l8_halfyear);

// season composite
var l8_season = ee.List.sequence(0, 1*11,3).map(function(n) {
  var start = ee.Date.fromYMD(year,1,1).advance(n, 'month');
  var end = start.advance(3, 'month');
  var tmpMedian = l8.filterDate(start, end).median().clip(roi).set("system:time_start", start.millis());
  return tmpMedian;
}).flatten();
var l8_season = ee.ImageCollection.fromImages(l8_season);
print("l8_season",l8_season);

// month composite
var l8_month = ee.List.sequence(0, 1*11,1).map(function(n) {
  var start = ee.Date.fromYMD(year,1,1).advance(n, 'month');
  var end = start.advance(1, 'month');
  var tmpMedian = l8.filterDate(start, end).median().clip(roi).set("system:time_start", start.millis());
  return tmpMedian;
}).flatten();
var l8_month = ee.ImageCollection.fromImages(l8_month);
print("l8_month",l8_month);

// l8_halfmonth
var l8_halfmonth = ee.List.sequence(1,12).map(function(item){
  var start_date = ee.Date.fromYMD(year, item, 1);
  var mid_date = start_date.advance(15,'day');
  var end_date = start_date.advance(1,'month');
  var pre_month_img = l8.filterDate(start_date,mid_date).mean()
                            .set('system:time_start',start_date.millis())
                            .set('time',start_date.format('YYYY-MM-dd'));
  var pro_month_img = l8.filterDate(mid_date,end_date).mean()
                            .set('system:time_start',start_date.millis())
                            .set('time',mid_date.format('YYYY-MM-dd'));  
  return ee.ImageCollection([pre_month_img,pro_month_img]);
});

// define iterate function
function accumulate(imageCol, list) {
  return ee.ImageCollection(list).merge(imageCol);
}
// flatten each element in the l8_halfmonth collection
var l8_halfmonth = l8_halfmonth.iterate(accumulate,ee.ImageCollection([]));
var l8_halfmonth= ee.ImageCollection(l8_halfmonth);
print("l8_halfmonth",l8_halfmonth);

// show the image
var visParam = {bands:['B6','B5','B4'],min:0,max:0.3};
Map.addLayer(l8_year, visParam, 'l8_year');