Set band value as property in an image collection Google Earth Engine

Question

I have function which calculates monthly means from an image collection

var UK_SST = ee.ImageCollection('NOAA/CDR/SST_WHOI/V2').select('sea_surface_temperature')
    .filterDate(ee.Date('1990-01-01'), ee.Date('2019-01-01'))
    .filterBounds(ROI)


// month data for dataset

var startyear = 1990; 
var endyear = 2019; 

var startmonth = 1;
var endmonth = 12;

var startdate = ee.Date.fromYMD(startyear, startmonth, 1);
var enddate = ee.Date.fromYMD(endyear , endmonth, 1);

var years = ee.List.sequence(startyear, endyear);

var months = ee.List.sequence(1, 12);
var monthlySST =  ee.ImageCollection.fromImages(
  years.map(function (y) {
    return months.map(function(m) {
      var w = UK_SST.filter(ee.Filter.calendarRange(y, y, 'year'))
                .filter(ee.Filter.calendarRange(m, m, 'month'))
                .mean();
      return w.set('year', y)
          .set('month', m)
          .set('system:time_start', ee.Date.fromYMD(y, m, 1));

    });
  }).flatten()
);

I'm looking to set the band value as a property so I can export the image collection as a csv with the band values contained. However, when I try to set this

  .set("sst_mean", w.get("sea_surface_temperature"))

In this part of the function.

  return w.set('year', y)
          .set('month', m)
          .set("sst_mean", res.get("sea_surface_temperature"))
          .set('system:time_start', ee.Date.fromYMD(y, m, 1));

Nothing gets set. Any ideas?

Answer 1

Your w is an image where each pixel contains monthly means. Maybe you're looking the mean across your entire ROI? If so, you have to call w.reduceRegion().

Try something like this:

// Picking the scale from the first image in the collection
var scale = UK_SST.first().projection().nominalScale()

var months = ee.List.sequence(1, 12);
var monthlySST =  ee.FeatureCollection(
  years.map(function (y) {
    return ee.FeatureCollection(months.map(function(m) {
      var w = UK_SST.filter(ee.Filter.calendarRange(y, y, 'year'))
                .filter(ee.Filter.calendarRange(m, m, 'month'))
                .mean();
      var dict = w.reduceRegion({
        reducer: ee.Reducer.mean(),
        geometry: ROI,
        scale: scale,
        maxPixels: 1e13
      })
      var sst_mean = ee.Algorithms.If(
        dict.contains('sea_surface_temperature'),
        dict.get('sea_surface_temperature'),
        null
      )
      return ee.Feature(null, {
        'sst_mean': sst_mean,
          'year': y,
          'month': m,
          'system:time_start': ee.Date.fromYMD(y, m, 1)
      });
    }));
  })
).flatten().filter(ee.Filter.notNull(['sst_mean']));

https://code.earthengine.google.com/963e8883883e2b2b0444eaeea5c23a33