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I have extracted sea surface temperature(SST) images from Google Earth Engine for a specific time period and region

var start = ee.Date(2012-01-01);
  var end = ee.Date(start).advance(1,'month');
  var date_range = ee.DateRange(start,end);

var sstImg = ee.ImageCollection('NASA/OCEANDATA/MODIS-Aqua/L3SMI') 
.filterBounds(reefs_500) //Filter by Study Area
.filterDate(date_range) // startdate, enddate
.select('sst')
.sort('system:time_start', false);

I want to calculate the long term mean SST per month for the region and then subtract that value from the image collection above.

I have calculated the mean for each month from 2002-2012 and have this as an image collection for 12 different images one per month.

var sst2002_12 = ee.ImageCollection('NASA/OCEANDATA/MODIS-Aqua/L3SMI')
    .select('sst')
    .filterDate(ee.Date('2002-01-01'), ee.Date('2012-12-31'))

var months = ee.List.sequence(1, 12);

var byMonth = ee.ImageCollection.fromImages(
      months.map(function (m) {
        return sst2002_12.filter(ee.Filter.calendarRange(m, m, 'month'))
                    .select(0).mean()
                    .set('month', m);
}));

How can I subtract only the mean from the image for the same month e.g. should the image come from January, it takes the Jan (or 01) value from the image collection?

I have this code to subtract a specific value from each image.

var series_daily = sstImg.map(function(image) {
    return image.subtract(mean)
    .set('system:time_start', image.get('system:time_start'));
});

But not sure how to adapt it so it only picks the correct value to do the subtracting from the list.

1 Answer 1

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I would use ee.Date.get to get the month from each image in sstImg. Then I would use that information to filter byMonth. Example:

var series_daily = sstImg.map(function(image) {
    
    var m = image.date().get('month') // get month 
    var mean = byMonth.filter(ee.Filter.eq('month',m)).first() // filter by month 

    return image.subtract(mean)
    .set('system:time_start', image.get('system:time_start'));
})
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  • Thanks, that makes sense. However I'm getting an error. Error in map(ID=A2016305): Image.subtract, argument 'image2': Invalid type. Expected type: Image<unknown bands>. Actual type: ImageCollection. My code is here if want to check it out code.earthengine.google.com/… Feb 2, 2021 at 9:53
  • Hi @mikejwilliamson. Thanks for the heads-up. I have editted my code snippet. Hope it is de-bugged (still have not ran it; thank you for your link, but I was unable to access the script).
    – korndog
    Feb 2, 2021 at 16:57
  • yeah I kind of figured out the issue and did the same thing, so that's great. Works fine now. Feb 2, 2021 at 17:12

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