I have two ImagesCollections. I can do mean() reducer before merging them. But after merging them I got error when I was trying to do mean() to the merged imagecollection. See below the code.

//The image collection of Chlorophyll from MODIS Aqua
var chla_Modis = ee.ImageCollection("NASA/OCEANDATA/MODIS-Aqua/L3SMI")
.filterDate('2010-7-04', '2011-3-14')

var seawifsData = ee.ImageCollection("NASA/OCEANDATA/SeaWiFS/L3SMI");
var chla_SeaWiFS = seawifsData
.filterDate('2010-1-04', '2010-12-11')

//merge the two imageCollections
var mergedChla=chla_Modis.merge(chla_SeaWiFS);


var meanChla=mergedChla.mean(); 
  • I copied and pasted your code exactly and there were no errors. You should always report any error messages so we can diagnose the problem. – Jon Apr 13 '18 at 14:56
  • If you change the second line from the bottom to var meanChla=mergedChla.mean(); You may get the error. – Sky Apr 13 '18 at 14:59
  • You should do that in your post. – Jon Apr 13 '18 at 15:02
  • You are right. I've changed the post. – Sky Apr 13 '18 at 15:06

You need to re-cast your merged imagecollection before calling mean(). Try this:

var meanChla=ee.ImageCollection(mergedChla).mean();

I can't explain why. I would guess it has something to do with the server-side not knowing that mergedChla is an ImageCollection.

  • My final goal is to combine the two data sets into one. As you can see there is one overlapped time period between the two data sets. I want get the mean value for each day in the overlapped time period and original value for each day not in the time period. Can you give me any advice? – Sky Apr 13 '18 at 15:48
  • What do you mean by "two data sets"? Are you saying that you only want to use the mean() reducer where they overlap? You should create a new question with as many details as possible. What do you want as an output? An ImageCollection or a single image or...? – Jon Apr 13 '18 at 16:07

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