4

I'm trying to count the number of pixels that are masked by the cloud mask of the image collection given by the Earth Engine dataset. I have clipped my collection with a ROI and now I want to keep the images with 10% or less of clouds (masked pixels). By now, I am only able to count them for a single image but not for the entire collection.

I am now using this code for one image but I am not able to reproduce this for the entire collection.

var Peyto_pixelscount = Peytoclip.first().select('ALBEDO').reduceRegion({
  reducer: ee.Reducer.count(),
  geometry: Peyto_glacier.geometry(),
  scale: 30,
  maxPixels: 1e9
});

print(Peyto_pixelscount)
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  • Thanks for the edit. You could also add a working example of your current code and your (erroneous) trials, then the question is quite complete.
    – Erik
    Jun 2, 2020 at 14:54

2 Answers 2

8

To get to your end goal of filtering your clipped image collection by 10% or less cloud cover, I would write a function that:

  1. counts the number of unmasked pixels in the image
  2. counts the number of pixels in the unmasked image
  3. calculates the percentage of masked pixels and sets this as a metadata property

I would then map this onto your image collection and filter it by the added metadata property.

Note that I'm assuming that your images are only masked by a cloud mask.


function get_cloud_cover_roi(image){ 

  var Peyto_pixelscount = image.select('ALBEDO').reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: image.geometry(),
    scale: 30,
    maxPixels: 1e9
  }).get('ALBEDO')

  var npix = image.select('ALBEDO').unmask().reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: image.geometry(),
    scale: 30,
    maxPixels: 1e9
  }).get('ALBEDO')

  var cloud_cover_roi = ee.Number(1)
      .subtract(ee.Number(Peyto_pixelscount).divide(npix))
      .multiply(100)

  return image.set('cloud_cover_roi', cloud_cover_roi)

}

var Peytoclip_filtered = Peytoclip
    .map(get_cloud_cover_roi)
    .filterMetadata('cloud_cover_roi', 'less_than', 10)

2
  • Thank you ! You are awsome.
    – laurent
    Jun 2, 2020 at 19:30
  • 3
    Hi @laurent, it looks like this answer satisfies your question. If so, would you kindly accept the answer (click check mark below vote tally) so that question browsers know that this is a completed Q&A. If a more suitable answer comes along in the future, you can always change the accepted answer. Jun 4, 2020 at 23:25
0

Similar logic to the previous answer but in a function call. geom could be any region in the image, or image.geometry() for the whole image.

/** 
 * return the percentage of masked pixels in an image region. 
 */
function calculateMaskedPercent(image, band, geom) {
   var maskedPixelCount = image.select(band).reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: geom,
    scale: 30,
    maxPixels: 1e9
  }).get(band);

  var totalPixelCount  = image.unmask().select(band).reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: geom,
    scale: 30,
    maxPixels: 1e9
  }).get(band);
 
  var cloud_cover_roi = ee.Number(1)
      .subtract(ee.Number(maskedPixelCount).divide(totalPixelCount))
      .multiply(100)
  return cloud_cover_roi;
}

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