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I just got acquainted with the ‘Joins’ command. I wrote the following code to practice. But instead of 18 elements, it displays 324 elements. How to make this code with the same join structure to give me 18 two-band elements from two sentinel Image Collections and why does this code show 324 elements?

code link: https://code.earthengine.google.com/f0aacb653bfaff59a5f31cd963557750

code :

Map.centerObject(geometry);

// Function - Calculate S2_1 NDVI
var ndvi1 = function(img){
var bands = img.select(['B2','B3','B4','B8']).multiply(0.0001)
.clip(geometry);
var index = bands.normalizedDifference(['B8','B4']).rename('NDVI_S2');
return index
.copyProperties(img,['system:time_start','system:time_end','system:index']);
 };

 // Get S2_1 NDVI images
 var S2_1 = ee.ImageCollection('COPERNICUS/S2_SR')
 .filterBounds(geometry)
 .filterDate('2018-10-24','2019-06-30')
 .filter(ee.Filter.lte('CLOUDY_PIXEL_PERCENTAGE',20))
 .map(ndvi1);

 print('S2 NDVI ImageCollection',S2_1);

 // Function - Calculate S2_2 NDVI
 var ndvi2 = function(img){
 var bands = img.select(['B2','B3','B4','B8']).multiply(0.0001)
 .clip(geometry);
 var index = bands.normalizedDifference(['B8','B4']).rename('NDVI_S2');
 return index
 .copyProperties(img,['system:time_start','system:time_end','system:index']);
 };

// Get S2_2 NDVI images
var S2_2 = ee.ImageCollection('COPERNICUS/S2_SR')
.filterBounds(geometry)
.filterDate('2018-10-24','2019-06-30')
.filter(ee.Filter.lte('CLOUDY_PIXEL_PERCENTAGE',20))
.map(ndvi2);

print('S2 NDVI ImageCollection',S2_2);


// Combine S2_1 and S2_2 Image Collections using Date
// Specify the join type
var join_type = ee.Join.inner();

// Set the join parameter
var filter = ee.Filter.equals({
leftField: 'Date',
rightField: 'Date'
});

// Execute the join
var inner_join = ee.ImageCollection(join_type.apply(S2_1,S2_2,filter));

// Flatten joined images into a single image with 2 bands
 var S2_Combined = inner_join.map(function(feature) {
  return ee.Image.cat(feature.get('primary'), feature.get('secondary'));
  });

print('Combined S2 Collection:',S2_Combined);
Amir Mohamad Abasi
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1 Answers1

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Your datasets do not have the uniform 'Date' property that you generated in your previous question so there is nothing to join. You will need to create that again if you want to use 'Date' as the join parameter. That is why it is giving you every possible combination of joined fields which is 18 images * 18 images = 324.

Andrew Notohamiprodjo
  • 41
  • 4