Google Earth Engine（GEE）——下载影像有很多分影像，没有用mosaic或者qualityMosaic()函数导致的影像没有镶嵌在一起

var table = ee.FeatureCollection("users/bqt2000204051/beijing");
var roi = table;
//landsat 8 
function NDVI_landsat_8(image) {
              var ndvi = image.normalizedDifference(['B5', 'B4']);
  return image.addBands(ndvi.rename('NDVI'));
}
var imageCollection = ee.ImageCollection("LANDSAT/LC08/C01/T1_SR").select(['B5', 'B4'])
for(var i=2019;i<=2020;i++){
  for(var j=1;j<=12;j++){
    var data_collection = imageCollection.filter(ee.Filter.calendarRange(j,  j,'month'))
  .filter(ee.Filter.calendarRange(i, i, 'year'))
  .map(function(img) {
    return img.set('year', img.date().get('year'));
  }).map(NDVI_landsat_8).filterBounds(roi);
    var HY_collection = data_collection.qualityMosaic('NDVI').clip(roi);
    print(data_collection,i+"_year_"+j+"_month");
    Map.addLayer(HY_collection,{},i+'-'+j);
    Export.image.toDrive({
      image: HY_collection.int8(),
      description: i+'-'+j,
      crs: "EPSG:32649",
      scale: 30,
      region: roi,
      maxPixels: 1e13,
      folder: 'ShanXi'
    })
  }
}
Map.addLayer(roi,{},'hengyang Boundary');