在 Google Earth Engine 中汇总图像集合中每张图像的像素值

Question

I am a newbie in GEE. I want to calculate a sum / mean of a pixel value (rainfall data from CHIRPS satellite) in one region of every image data in an ImageCollection. Basically i want to calculate the mean daily rainfall in my study area.

Here is my code so far:

//define study area
var aoi = ee.FeatureCollection(juana)

//load & filtering ImageCollection
var precipitation = ee.ImageCollection('UCSB-CHG/CHIRPS/DAILY')
.filterDate('2021-01-01', '2021-12-31')
.select('precipitation');

//clipped ImageCollection to Study Area
var clipped = precipitation.map(function(precipitation){
return precipitation.clip(aoi);
});

//plot in map
Map.addLayer(clipped)

Here is the result in map:

Every pixel has a daily rainfall data like this:

But what i need is i want to sum/averaging those pixel in the study area and make a daily graphic. What should i do?

Answer 1

You use ee.Image().reduceRegion() to calculate aggregate statistics for an area of an image. In your case, you want a time-series of the stats, so you want to do this for every image in you collection. To do that, you use ee.ImageCollection.map() . You have relevant sections in the docs here and here . Something like this:

var dailyStats = precipitation.map(function (image) {
  var reduced = image.reduceRegion({
    reducer: ee.Reducer.sum()
      .combine(ee.Reducer.mean(), null, true), 
    geometry: aoi.geometry(), 
    scale: 5000, 
    maxPixels: 1e13
  })
  return ee.Feature(null, reduced)
    .set('system:time_start', image.get('system:time_start'))
})

var meanChart = ui.Chart.feature.byFeature({
  features: dailyStats,
  xProperty: 'system:time_start', 
  yProperties: 'precipitation_mean'
})

var sumChart = ui.Chart.feature.byFeature({
  features: dailyStats,
  xProperty: 'system:time_start', 
  yProperties: 'precipitation_sum'
})
print('mean', meanChart)
print('sum', sumChart)

https://code.earthengine.google.com/e6e4af4147004c6cbd9fe0f20dd7c9fe