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I am trying to make mean Sentinel-2 images of an area for each day from May 1st to June 30th over several years. I've never used google earth engine/javascript before, so it's been a bit challenging.

Here's what I've come up with so far, copy pasted together from various sources:

///cloud mask (included just in case my error comes from here)
function maskS2clouds(image) {
  var qa = image.select('QA60');
  var cloudBitMask = 1 << 10;
  var cirrusBitMask = 1 << 11;
  var mask = qa.bitwiseAnd(cloudBitMask).eq(0)
      .and(qa.bitwiseAnd(cirrusBitMask).eq(0));
  return image.updateMask(mask).divide(10000)
      .copyProperties(image, image.propertyNames());
}

///making an image collection over the relevant area/time period
var tiles = ['32VKM','32VLM']
var hardanger = ee.ImageCollection('COPERNICUS/S2').filter(ee.Filter.dayOfYear(120,180)) 
    .filter(ee.Filter.inList('MGRS_TILE', tiles))
    .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 10))
                  .map(maskS2clouds);
                
//getting the mean image per day:

var days = ee.List.sequence(120, 180);
var byDay = ee.ImageCollection.fromImages(
    days.map(function(y) {
      return hardanger
        .filter(ee.Filter.calendarRange(y, y, 'DAY_OF_YEAR'))
        .reduce(ee.Reducer.mean())
        .set('DAY_OF_YEAR', y);
    })
  );
print(byDay, "byDay");

It seems to work fine and I do get 61 images, which seems about right (some are empty/no bands which I assume is because the satellite does not pass on those days).

However, all the images look exactly the same?

1 Answer 1

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It looks like you did it correctly.

Just sticking the collection on the map and inspecting a point, you can see that the values are different over time. What makes you think they all look the same?

enter image description here

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  • thanks for your quick answer! I just realized that my problem was that I added the same image as a layer 15 times by doing it this way: //Map.addLayer(hardanger, rgbVis, '10'); rather than adding 15 different images. Commented Aug 4, 2021 at 11:51

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