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I am working with sentinel 2 images in Google earth engine and I have implemented the formula to calculate the EVI and I have added it as a band (in the collection). I know there is a way to implement a mask and show only those pixels with a certain value, but I'm interested to know if by implementing this mask (for example leaving only those pixels with EVI>0) there is a way to extrapolate it to the other bands . Specifically, I would like to calculate total reflectance (DN) in the image of all those pixels with EVI>0 in the NIR band. For this I would have to add the reflectance in the NIR band of all those pixels that have EVI>0 (for each image in the collection). It's possible?

 var addEVI=function(image){
    var EVI = image.expression(
          '2.5 * ((NIR - RED) / (NIR + 6 * RED - 7.5 * BLUE + 1))', {
          'NIR' : image.select('B8').divide(10000),
          'RED' : image.select('B4').divide(10000),
          'BLUE': image.select('B2').divide(10000)}).rename('EVI');
          return image.addBands(EVI);
    }; 

var S2withEVI =S2.map(addEVI);

1 Answer 1

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You can use the following function with 'where' method.

  var maskedNIR = S2withEVI.map(function (img) {

  var img_evi = img.select("EVI");
  var img_nir = img.select("B8");
  
  var new_nir = img_nir.where(img_evi.gte(0), img_nir)
                       .where(img_evi.lt(0), 0);
  
  new_nir = new_nir.updateMask(new_nir.neq(0));
  
  return new_nir.rename("new_NIR");
  
});

Complete code here.

After running link code in GEE code editor, I got result of following picture.

enter image description here

At the above picture, there are three layers: EVI, original B8 and masked B8 (new_NIR) bands. The original B8 band was unmarked for corroborating that new_NIR and EVI bands are both masked at the same areas. All B8 and masked B8 values are coincident in unmasked areas.

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  • Thank you very much for your answer! That's to implement the mask, but I don't know how to work with the other bands yet. I want to calculate the total reflectance (sum of digital numbers) of all those pixels that are not masked (those with EVI>0)
    – Camila
    Commented Jan 30, 2023 at 19:41

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