前一阵子，徐老师问了我们一个问题，说他的样本数据采样后都变成空了，但是实际是有影像的。后来，他把代码发给了我们。我们对其进行了检测，发现在随时序数据采样时，经常会出现这样的问题。主要原因是去云后时序数据很容易出现空值，而一旦出现空值，sampleRegions函数得到的featureCollection数据集就会有样本丢失，甚至可能为空集的情况。因此，在使用sampleRegions函数的时候，一定要时刻留意采样影像是否有空值。
var img = ee.Image('COPERNICUS/S2_SR/20210109T185751_20210109T185931_T10SEG');Map.setCenter(-122.503881, 37.765588, 18);Map.addLayer(img, {bands: ['B11', 'B8', 'B3'], min: 100, max: 4500}, 'img');
var fcPolygon = ee.FeatureCollection([ ee.Feature(ee.Geometry.Rectangle( -122.50620929, 37.76502806, -122.50552264, 37.76556663), {id: 0}), ee.Feature(ee.Geometry.Rectangle( -122.50530270, 37.76565568, -122.50460533, 37.76619425), {id: 1})]);Map.addLayer(fcPolygon, {color: 'yellow'}, 'fcPolygon');
var fcPolygonSamp = img.sampleRegions({ collection: fcPolygon, scale: 10, geometries: true});print('A feature per pixel (at given scale) in each region', fcPolygonSamp);Map.addLayer(fcPolygonSamp, {color: 'purple'}, 'fcPolygonSamp');
var fcPoint = ee.FeatureCollection([ ee.Feature(ee.Geometry.Point([-122.50309256, 37.76605006]), {id: 0}), ee.Feature(ee.Geometry.Point([-122.50344661, 37.76560903]), {id: 1})]);Map.addLayer(fcPoint, {color: 'cyan'}, 'fcPoint');
var fcPointSamp = img.sampleRegions({ collection: fcPoint, scale: 10});print('A feature per point', fcPointSamp);
