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I create a Sentinel-2 cloud free mosaic image with Google Earth Engine and exported the image to an Earth Engine asset.

I know the image that I exported was composed by several images that taken at the same place in different time.

I want to obtain the pixel value change of my region of interest. However, I have problem with exporting every pixel value of those images (take at same place but different time).

Could anyone tell me how to do? Please help me!!!

Here's the code with text.

// Filter an image collection by date and region to make a
// min pixel composite.


// Create a S2 composite for 2015-06-01 to 2017-10-24, and filter by
// the bounds of the FeatureCollection.
var COPERNICUS = ee.ImageCollection('COPERNICUS/S2')
    .filterDate('2015-06-01', '2017-10-24')
    .filterBounds(geometry);

// Select the min pixel.
var min = COPERNICUS.min();

// Select the red, green and blue bands.
var result = min.select('B4', 'B3', 'B2');
Map.addLayer(result, {gain: '0.1, 0.1, 0.1',scale:10});
Map.setCenter(135.3735, 34.4794, 5);

var band4 = result.select('B4').rename('b4_min')
.addBands(result.select('B3').rename('b3_min'))
.addBands(result.select('B2').rename('b2_min'));

// Export the image to an Earth Engine asset.
Export.image.toAsset({
  image: result,
  description: 'S2min10m',
  assetId: 'S2min10m',
  scale: 10,
  maxPixels: 1e10,
  region: geometry,
});
  • Hi @BEAR Thank you for your advice. I edited the question and added the code as text. – dor.F Dec 7 '17 at 4:55
  • What do you mean by "pixel value change"? The difference between minimum and maximum or some other multi-temporal statistic? – Kersten Dec 7 '17 at 11:30
  • Hi @Kersten. Thanks for your reply. I mean the "multi-temporal statistic". (pixel values of the same pixel but different time.) – dor.F Dec 8 '17 at 7:04
  • So you want to export the whole image stack - why? You can calculate these statistics in GEE just like you used .min() for the minimum. You can get pretty much any statistic you want (min, max, stdDev, percentiles, ...) - see the reference: developers.google.com/earth-engine/api_docs#eereducerpercentile – Kersten Dec 8 '17 at 8:08
  • @Kersten Thank you for the information. Because I want to know the pixel number of changes,the time of change and the magnitude of change. In order to analysis land use changes or some other phenomenon. – dor.F Dec 9 '17 at 12:29
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I am also a beginner in GEE, but I think that you can achieve your demand through calculating local derivatives (subsequent value minus previous one and dividing by difference of time), and co-erecting them with the timestamps. This may give you something like, peak values and troughs that could be easily retrieved. I also feel that the satellite data could be noisy so some sort of curve smoothing may be necessary.

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