Using the ee.Reducer.sum() to count the area of different classes of a layer (hansen/global_forest_change_2017_v1_5)

Question

I have used the following function to calculate the area of an area of interest (AOI) of a classification using landsat8 images. This counts the area in square meters of the classification for a given polygon.

var areaChart = ui.Chart.image.byClass({
  image: ee.Image.pixelArea().addBands(lista_imagenes[0]),
  classBand: 'classification', 
  region: geometry,
  scale: 30,
  reducer: ee.Reducer.sum()
});

The problem is that the band of classification of landsat8 is just one (classification band). And the layer I am using has three layers (it is call year of loss).

There is any way to count the classes of the layer given a polygon using a similar question?

Here the code: https://code.earthengine.google.com/495ed315756b7181b4570546be82e2f4

Answer 1

I just found the way to solve the problem, everything is in the following link (google documentation).

https://developers.google.com/earth-engine/tutorial_forest_03a

Here is the relevant part (with .reduceRegion()):

// Load country boundaries from LSIB.
var countries = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017');
// Get a feature collection with just the Congo feature.
var congo = countries.filter(ee.Filter.eq('country_co', 'CF'));

// Get the loss image.
// This dataset is updated yearly, so we get the latest version.
var gfc2017 = ee.Image('UMD/hansen/global_forest_change_2017_v1_5');
var lossImage = gfc2017.select(['loss']);
var lossAreaImage = lossImage.multiply(ee.Image.pixelArea());

var lossYear = gfc2017.select(['lossyear']);
var lossByYear = lossAreaImage.addBands(lossYear).reduceRegion({
  reducer: ee.Reducer.sum().group({
    groupField: 1
    }),
  geometry: congo,
  scale: 30,
  maxPixels: 1e9
});
print(lossByYear);