Error generating chart: The image collection is empty in GEE

Question

I am building a Landsat image collection time series and then trying to plot a time series of values for my region of interest by mean reduction. However, upon charting, I get an error message stating the image collection is empty. Why would the collection be empty?

Here is my code:

var Akdarya = ee.FeatureCollection("users/xodjayevshovkat/Akdarya_Crop_Class_Wheat_dissolve");

// Import S2 image collection.
var S2 = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA')
  // Filter collection.
  .filterDate('2018-01-01','2018-12-31')
  .filterBounds(Akdarya)
  .filterMetadata('CLOUDY_PIXEL_PERCENTAGE', 'less_than', 20);

// Define a cloud masking function.
var maskcloud1 = function(image) {
var QA60 = image.select(['QA60']);
  return image.updateMask(QA60.lt(1))//.clip(cotton);
};

// Define an index function (return only NDVI).
var NDVI = function(image) {
  return image.expression(
    '(NIR - RED) / (NIR + RED)', 
    {
      'NIR': image.select('B8'), 
      'RED': image.select('B4'), 
    }).rename('NDVI').copyProperties(image, image.propertyNames());
};

// Apply the cloud mask function.
var NDVIcol = S2.map(maskcloud1)
 // Apply the NDVI transformation.
 .map(NDVI);

// Calculate the per-pixel median NDRE for all images in collection.
var NDVImed = NDVIcol.median();

// Define vis params.
var NDVIvis = {
  palette: ['#d73027', '#f46d43', '#fdae61', '#fee08b', '#d9ef8b', '#a6d96a', 
  '#d73027', '#f46d43', '#fdae61', '#fee08b', '#d9ef8b', '#a6d96a',
  '#d73027', '#f46d43', '#fdae61', '#fee08b', '#d9ef8b', '#a6d96a', 
  '#d73027', '#f46d43', '#fdae61', '#fee08b', '#d9ef8b', '#a6d96a', 
  '#d73027', '#f46d43', '#fdae61', '#fee08b', '#d9ef8b', '#a6d96a', 
  '#d73027', '#f46d43', '#fdae61', '#fee08b', '#d9ef8b', '#a6d96a'],
  min: -0.5,
  max: 0.9};

// Plot a time series of 
var regions = Akdarya.toList(Akdarya.size())
var regionCount = Akdarya.size().evaluate(function (count) {
  for (var i = 0; i < count; i++) {
    var plotNDVI = ui.Chart.image.seriesByRegion({
      imageCollection:NDVIcol,
      regions: ee.FeatureCollection([ee.Feature(regions.get(i))]),
      reducer: ee.Reducer.mean(),  //median //mean
      xProperty: 'system:time_start',
      seriesProperty: 'Farmers'})
      .setChartType('LineChart').setOptions({
        title: 'NDVI Akdarya 2018',
        hAxis: {title: 'Date'},
        vAxis: {title: 'NDVI'},
    });
 print(plotNDVI);    
  }
})
// Display the results.
//print(plotNDVI);
Map.addLayer(NDVImed.clipToCollection(Akdarya), NDVIvis, 'NDVI');

Answer 1

Generally, if a collection is empty it is because applied filters are too restrictive and no collection elements pass. In this case it is because you are filtering by the "CLOUDY_PIXEL_PERCENTAGE" property, which does not exist for the "LANDSAT/LC08/C01/T1_TOA" collection (it is for Sentinel-2) so no images pass the conditional test.

I can't replicate your example because your imported asset is not publicly shared. I've mocked up an example that illustrates the issue with filtering by the incorrect cloud cover property.

// Filter by 'CLOUDY_PIXEL_PERCENTAGE' property.
var col = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA')
  .filterBounds(ee.Geometry.Point(-120.6, 37.6))
  .filterMetadata('CLOUDY_PIXEL_PERCENTAGE', 'less_than', 20);

print("Filter by 'CLOUDY_PIXEL_PERCENTAGE' property", "Collection size", col.size()); 

// Filter by 'CLOUD_COVER' property.
var col = ee.ImageCollection('LANDSAT/LC08/C01/T1_TOA')
  .filterBounds(ee.Geometry.Point(-120.6, 37.6))
  .filterMetadata('CLOUD_COVER', 'less_than', 20);

print("Filter by 'CLOUD_COVER' property", "Collection size", col.size());