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I was downloading surface reflectance data from GEE from March 1st 2017 to October 31st 2017. I should be getting data every 16 days because of the nature of Landsat 8. However, there is no data in August 2017 and when I hit October, it started showing data from May 4th.

What's the reason for this?

// 定义经纬度坐标
var lon = 100.37183333333333;
var lat = 38.854350000000004;

// 定义日期范围
var startDate = '2017-03-01';
var endDate = '2017-10-31';

// 定义 Landsat 8 的影像集合
var collection = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
  .filterBounds(ee.Geometry.Point(lon, lat))
  .filterDate(startDate, endDate);

// 选择波段 1-7
var bands = ['B1', 'B2', 'B3', 'B4', 'B5', 'B7'];

// 转换为 0-1 之间的地表反射率
var scaledCollection = collection.map(function(image) {
  return image.select(bands).divide(10000);
});

// 提取中心像元的地表反射率
var centerPixel = scaledCollection.map(function(image) {
  var value = image.reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: ee.Geometry.Point(lon, lat),
    scale: 30
  });
  return ee.Feature(null, value);
});

// 获取影像的获取时间
var time = scaledCollection.map(function(image) {
  var date = image.date().format('YYYY-MM-dd');
  return ee.Feature(null, {'Date': date});
});

// 将结果导出为 CSV 文件
Export.table.toDrive({
  collection: centerPixel,
  description: '2017',
  fileFormat: 'CSV'
});

With the result:

enter image description here

2
  • 2
    Please include a minimal reproducible example Commented Oct 24, 2023 at 6:21
  • 1
    Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
    – Community Bot
    Commented Oct 24, 2023 at 6:27

1 Answer 1

1
var collection = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
  .filter(ee.Filter.eq("WRS_PATH",133))
  .filter(ee.Filter.eq("WRS_ROW", 33))
  .filterDate("2017-03-01", "2017-10-31")
  .sort("system:time_start")
print(collection.aggregate_array("system:index")

The code above will show you two things

  1. That you can use .sort("system:time_start") to have your collection sorted according by date. You can use this later also for your centerPixel feature collection so that the exported table is sorted. This solves your issue that "when I hit October, it started showing data from May 4th."

  2. Indeed, there are no images for this collection in path/row 133/33 in August 2017:

[ "LC08_133033_20170310", "LC08_133033_20170326", "LC08_133033_20170411", "LC08_133033_20170427", "LC08_133033_20170513", "LC08_133033_20170529", "LC08_133033_20170614", "LC08_133033_20170630", "LC08_133033_20170716", "LC08_133033_20170902", "LC08_133033_20170918", "LC08_133033_20171004", "LC08_133033_20171020" ]

However.. if you use the LANDSAT/LC08/C02/T1_L2 collection instead, you get one image in August:

[ "LC08_133033_20170310", "LC08_133033_20170326", "LC08_133033_20170411", "LC08_133033_20170427", "LC08_133033_20170529", "LC08_133033_20170614", "LC08_133033_20170630", "LC08_133033_20170716", "LC08_133033_20170817", "LC08_133033_20170902", "LC08_133033_20170918", "LC08_133033_20171004", "LC08_133033_20171020" ]

There is actually another image in August 1st that is available in EarthExplorer*:

enter image description here

but note that it has the L1GT processing flag. According to the Landsat documentation,

All Landsat scenes are attempted to process to L1TP, using Ground Control Points (GCPs) and a digital elevation model (DEM). In some cases (and more likely in older Landsat data), scene and/or sensor issues, or insufficient reference data can cause L1TP processing to fail. Scene issues include snow, ice, and clouds, which prevent an accurate registration of GCPs within a scene. Sensor issues include measurement/outliers in the spacecraft or instrument telemetry of an interval which also affect the usability of the GCPs. In these scenarios, a L1GT or a L1GS product will be created instead.

According to Google Earth Engine (see here),

Note that surface reflectance products are only generated for Landsat images with processing level L1TP.

Note that the image is available in the LANDSAT/LC08/C02/T2 level 1 tier 2 product, but not T1.

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