Statistics of each image in an ImageCollection in Google Earth Engine

Question

I have an ImageCollection that I made calculation with so that it now contains images that show a ratio of one band within a given Polygon. I would like to get the mean and standard deviation for each image of the collection within the polygon, so I have a value for each image afterwards. I keep on receiving the error of ReduceRegion not working for all the codes I am trying. Maybe you can help me to understand this better and solve the problem.

I tried this code for example:

I edited code but unfortunately still the same error. I read it could be because of null values but I do have a function sorting out null values so that shouldn't be the issue

var reducers = ee.Reducer.mean().combine({
  reducer2: ee.Reducer.stdDev(),
  sharedInputs: true
});

function stats (image) {
  var stats1 = image.reduceRegion({
    reducer: reducers,
    geometry: MODISofstation,
    scale: 30,
    bestEffort: true
  });
  return image.set(stats1);

var statistics = ImageCollection.map(stats);
print (statistics);

Answer 1

I completed with your snippet code my script used in your previous question as follows:

var maskL8 = function(image) {
  var qa = image.select('BQA');

  var mask = qa.bitwiseAnd(1 << 4).eq(0);
  return image.updateMask(mask);
};

var pt = ee.Geometry.Point([-1.869490554037725, 41.191353356841205]);

var polygon = ee.Geometry.Polygon(
        [[[-1.8966130515963187, 41.18786546760974],
          [-1.8736104270846, 41.173136775751885],
          [-1.8413380882174124, 41.176754656054],
          [-1.8389348289400687, 41.19974936510326],
          [-1.861250807943975, 41.207498570723196],
          [-1.8732671043306937, 41.19690775979286],
          [-1.8883733055025687, 41.2000076867413]]]);

Map.addLayer(pt);
Map.centerObject(pt);

var start = '2017-01-01';
var finish = '2017-06-01';

var l8 = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA')
  .filterDate (start,finish)
  .filterBounds(pt)
  .filter("WRS_ROW <= 122")
  .map(maskL8);

var myB8 = l8.select("B8");

//receiving the values at pt for each image

var getB8 = function(image) {

  // Reducing region and getting value
  var value_B8 = ee.Image(image)
    .reduceRegion(ee.Reducer.first(), pt)
    .get('B8');

  return value_B8;
};

var count = myB8.size();

var listOfImages = l8.toList(count);

var B8_list = listOfImages.map(getB8);

print("B8 list", B8_list);

var nonNulls = B8_list.filter(ee.Filter.neq('item', null));

print("Non Null Values", nonNulls);

var idxList = nonNulls.map(function extract (ele) {
  
  var idx1 = nonNulls.indexOf(ele);
  
  var idx2 = B8_list.indexOf(nonNulls.get(idx1)); 
  
  return idx2;
  
});

print("Indices of Non Null Values", idxList);

//Ratio function

var RatioALL = idxList.map(function  calculateRatio (ele) {
  
  var idx = ele;

  var value = ee.Number(B8_list.get(idx));
  
  var image = listOfImages.get(idx);
  
  var ratio1 = ee.Image(image).select('B8')
                              .clip(polygon)
                              .divide(value);

  var time = ee.Image(image).get('system:time_start');

  return ratio1.set('system:time_start', time);
  
});

print("Original Image Collection", myB8);

print ("Ratio All for Non Null Values", RatioALL);

var getStats = function(image) {
  
  var reducers = ee.Reducer.mean().combine({
  reducer2: ee.Reducer.stdDev(),
  sharedInputs: true
  });

  var stats1 = ee.Image(image).reduceRegion({
    reducer: reducers,
    geometry: polygon,
    scale: 30,
    bestEffort: true
  });
  
  return ee.Image(image).set(stats1);

};

var statistics = RatioALL.map(getStats);

print("statistics", statistics);

After running it in GEE code editor, I got expected result; as it can be observed in following image:

Answer 2

When you map over collections, you have to return something that can go back into a collection. Dictionaries can't do that. Instead, put the stats on the image and return that:

function stats (image) {
  var stats1 = image.reduceRegion({
    reducer: reducers,
    bestEffort: true,
  });
  return image.set(stats1);
}

Also, you need to specify your polygon in the reduceRegion and probably a scale.