Choose a Sentinel 2 image according to the amount of clouds in a given polygon - Google Earth Engine

Question

I know that it is possible to filter an image according to the amount of clouds in the whole picture.

Is there a way to choose the image according to the amount of cloud for a given polygon instead of the whole image?

// function to create mosaic of AOI
// Mosaic the visualization layers and display (or export).
// This function clips images to the ROI feature collection
var clipToCol = function(image){
  return image.clip(geometry);
};

// Map the function over one year of data
// Load Sentinel-2 
var s2 = ee.ImageCollection('COPERNICUS/S2')
                  .filterDate('2017-06-01', '2017-12-30')
                  .filterBounds(geometry)
                  .map(clipToCol)
                  .sort('CLOUD_COVER')
                  .first();

//Parametros de visualización
  var rgbVis = {
  min: 0.0,
  max: 3000,
  bands: ['B4', 'B3', 'B2'],
  };

  Map.addLayer(s2 , rgbVis, 'RGB')

Answer 1

First of all, the property name that holds the cloud coverage in Sentinel 2 is CLOUD_COVERAGE_ASSESSMENT, not CLOUD_COVER. In you code you are just sorting, which does nothing because that property is not present, but if you try to filter, it will throw an error.

// function to create mosaic of AOI
// Mosaic the visualization layers and display (or export).

// Map the function over one year of data
// Load Sentinel-2 
var s2 = ee.ImageCollection('COPERNICUS/S2')
                  .filterDate('2017-06-01', '2017-12-30')
                  .filterBounds(geometry)

var filtered = s2.filterMetadata('CLOUD_COVERAGE_ASSESSMENT', 'less_than', 50)
// var filtered = s2.filterMetadata('CLOUD_COVER', 'less_than', 50) // doesn't work

print('S2', s2.toList(s2.size()).map(function(img){return [ee.Image(img).date().format('yyyyMMdd'), ee.Image(img).get('CLOUD_COVERAGE_ASSESSMENT')]}))
print('filtered', filtered.toList(filtered.size()).map(function(img){return [ee.Image(img).date().format('yyyyMMdd'), ee.Image(img).get('CLOUD_COVERAGE_ASSESSMENT')]}))

https://code.earthengine.google.com/e2709c19846884520c2369faffc50b64

To compute a "mask percentage" property first you have to mask out clouds:

var cldmask = require('users/fitoprincipe/geetools:cloud_masks')
var masked = filtered.map(cldmask.sentinel2())
// Inspect first masked
var rgbVis = {
  min: 0.0,
  max: 3000,
  bands: ['B4', 'B3', 'B2'],
  };
Map.addLayer(masked.first(), rgbVis, 'Masked')
Map.addLayer(geometry)

An then, I have a function in geetools to compute the percentage of masked pixels.

var tools = require('users/fitoprincipe/geetools:tools')

// Test the first image of the collection
var maskCover = tools.image.maskCover(masked.first(), geometry)
print('first image mask coverage (%):', maskCover)
print('first image cloud coverage (%):', masked.first().get('CLOUD_COVERAGE_ASSESSMENT'))

// Add MASK_COVER property for all images in the collection
var addMaskCover = function(img) {
  var maskCover = tools.image.maskCover(img, geometry)
  return img.set('MASK_COVER', maskCover)
}
var colWithMaskCover = masked.map(addMaskCover)

// filter the collection by MASK_COVER
var newFiltered = colWithMaskCover.filterMetadata('MASK_COVER', 'less_than', 10)

// Inspect results
print('newFiltered', newFiltered)
print(newFiltered.toList(newFiltered.size()).map(function(img){
  return {
  'CLOUD_COVERAGE_ASSESSMENT':ee.Image(img).get('CLOUD_COVERAGE_ASSESSMENT'),
  'MASK_COVER': ee.Image(img).get('MASK_COVER')
  }
}))

https://code.earthengine.google.com/cd36f8a7336e396aab0c1ec99b78043e