本次用到的函数：统计影像数量的总的函数
size()
Returns the number of elements in the collection.
Arguments:
this:collection (FeatureCollection):
The collection to count.
Returns: Integer
count()
Reduces an image collection by calculating the number of images with a valid mask at each pixel across the stack of all matching bands. Bands are matched by name.
通过计算所有匹配波段的堆栈中每个像素上有有效掩码的图像数量来减少图像集合。波段是按名称匹配的。
Arguments:
this:collection (ImageCollection):
The image collection to reduce.
Returns: Image
代码：大家自己更换自己的研究区即可
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//加载我们的影像
var CHINA = ee.FeatureCollection('users/bqt2000204051/CHINA').geometry();
//加载指定的研究区到map上
Map.addLayer(CHINA, {'color':'grey'