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I'm following this example to create zonal statistics for EVI images in Google Earth Engine. In my code (GEE code here) I have a FeatureCollection of points, that creates a 3km buffer for each point, and with it, finally, calculate the mean value for each polygon.

It looks like a very straightforward code but I'm having this headache to get it returning the mean value

When I print(reducer_mean) it should return the mean values for each region. Instead it's retuning null:

properties: Object (5 properties)
0_0: null
1_1: null
2_2: null

I'm wondering if someone could help me to figure out where the inconsistency of the code are. Because I can manage to find it.

Snapshot of my code:

function create_Landsat_img( year, name ){
  // settings for landsat 8
  var imgCol = "LANDSAT/LC08/C01/T1_TOA";
  var band_nir= "B5";
  var band_red= "B4";
  var band_blue= "B2";
  
  var img = ee.ImageCollection(imgCol)
  .filterBounds(ROI)
  .filterDate(year+"-01-01", year+"-12-31")
  .filterMetadata('CLOUD_COVER', 'less_than', 95)
  .mean()
  .clip(ROI);
  
  // calculate the evi
  var evi = img.expression("2.5 * ((nir - red) / (nir + 6 * red - 7.5 * blue + 1))",
    {
      nir : img.select(band_nir),
      red : img.select(band_red),
      blue: img.select(band_blue)
      });
  
  // add the evi to the map
  Map.addLayer(evi,{min:0,max:0.85,palette:"blue,red,yellow,green,darkgreen"}, name);
  return evi;
}

//--- START SCRIPT ---
//------------------------------------------------------------
// Create a function for Buffering a feature, using the buffer_size
// property of the feature.
var BufferFeature = function(f) {
  f = ee.Feature(f);
  var buffer_size = f.get('buffer_size');
  return f.buffer(buffer_size);   
};

var BufferFeaturesByDistance = function (fc, buffer_size) {
  // Set a buffer_size property on a feature. Note that this function needs
  // to be defined within the BufferFeaturesByDistance function, so it can
  // make use of the buffer_size parameter.
  var SetBufferSize = function(f) {
    return f.set({'buffer_size': buffer_size});
  };

  return POI.map(SetBufferSize).map(BufferFeature);
};

var buffered = BufferFeaturesByDistance(POI, 3000); // 3km buffer
//------------------------------------------------------------
// Create EVI layers
var i;
var num_years_range = 3; 
var images_list = [];
for(i = num_years_range; i >= 1; i--){
  var year = 2022;
  var yearImg = year-i;
  
  year = yearImg.toString();
  
  var nameLayer= "EVI_"+year;
  var EVI_Layer = create_Landsat_img(year, nameLayer);
  images_list.push( EVI_Layer );
  
}
//------------------------------------------------------------

var imgCol = ee.ImageCollection.fromImages(images_list);
print(imgCol);

var rename_band = function(img){
  return img.select([0], [img.id()]);
};

// stack all the images into a single image.
var stacked_image = imgCol.map(rename_band).toBands(); //iterate(combine, stacked_image);
print(stacked_image);

// determine scale to perform reduceRegions.
var scale = imgCol.first().projection().nominalScale();

// calculate the timeseries for each feature.
var reducer_mean = ee.Image(stacked_image).reduceRegions({collection: buffered, reducer: ee.Reducer.mean(), scale: scale});

print(reducer_mean);
Export.table.toDrive({
  collection: reducer_mean,
  description: 'tableOutput',
  fileFormat: 'KML',
  folder: 'GoogleEarthEngine'
});

// Add to layer
Map.addLayer(buffered, {color:'green'}, 'buffered');
// Zooming to layer
Map.centerObject(POI);
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  • I think the scale you are setting is much too large. I tried setting the scale to 500 and it worked great.
    – M. Nicolas
    Jan 11, 2022 at 22:58

1 Answer 1

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Try reducing the scale; you're currently using the reducer at 1 degree (~110km), which is much larger than the size of your geometries:

print(imgCol.first().projection().nominalScale())
     111319.49079327357

Setting the scale to 30 meters (the scale of bands B2, B4 and B5), fixes the issue:

var reducer_mean = ee.Image(stacked_image).reduceRegions({collection: buffered, reducer: ee.Reducer.mean(), scale: 30});

print(reducer_mean)
    properties: Object (5 properties)
    0_0: 0.4079238581747526
    1_1: 0.39326260869897006
    2_2: 0.35812035383587515
    buffer_size: 3000
    class: 0

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