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Following this example I have merged 2 image collections to create a synthetic twice daily Aqua and Terra composite. Now I want to generate the median image of this combined collection. For a single collection this already works with:

var collection = ee.ImageCollection('MODIS/006/MOD09GQ');
var filtered_date = collection.filterDate('2010-10-01', '2010-12-10');
var medianImage = filtered_date.median();
Map.addLayer(medianImage, {bands:['sur_refl_b02'], min:100, max:4500});

However, the merge seems to interfere with the following script.

var collection_1 = ee.ImageCollection('MODIS/006/MOD09GQ');          
var collection_2 = ee.ImageCollection('MODIS/006/MYD09GQ');
var collection_date_1 = collection_1.filterDate('2010-10-01', '2010-12-10');
var collection_date_2 = collection_2.filterDate('2010-10-01', '2010-12-10');
print('Terra collection', collection_date_1);
print('Aqua collection', collection_date_2);

// merge collections
var collection_merge = collection_date_1.merge(collection_date_2);
print('Merged collection',collection_merge);

// sort merged collection
var collection_merge_sorted = collection_merge.sort("system:time_start");
print('Sorted merged collection', collection_merge_sorted)

var medianImage_10 = collection_merge_sorted.median();
Map.addLayer(medianImage_10, {bands:['sur_refl_b02'], min:100, max:4000});

The script returns that 'collection_merge_sorted.median is not a function', which seems to be caused by the merge itself. How to avoid this?

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Call ee.ImageCollection:

var medianImage_10 = ee.ImageCollection(collection_merge_sorted).median();

Link: https://code.earthengine.google.com/273462eda06defe2b6c2958cc41fabca

  • If you are only calculating the median there is also no need to sort - you can directly call ee.ImageCollection(collection_date_1.merge(collection_date_2)).median() – Kersten May 31 '18 at 20:23

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