I am trying to create a time series of the total area of water pixels in a particular region using the ee.Image.pixelArea() method. I understand that this method creates a raster whereby the value of each pixel is the corresponding area of a specific pixel. This means that we would need a raster of strictly value 1 to multiply with that raster, to get the area, as is shown in Quantifying Forest Change and Calculating Area in Google Earth Engine.

I previously saw an answer to Counting number of pixel identified as water from a collection of landsat image using Google Earth Engine, which is similar to the output I want, except the answer does not convert the NDWI band to 1 (water pixels = 1, non-water = 0) but immediately multiplies the NDWI raster with the ee.Image.pixelArea(), which contradicts the two other articles i mentioned previously.

Despite me dividing the water pixels extracted by NDWI by itself (so everything = 1), my resultant values would be exactly the same if I had not done that (i.e. change water1 to water01, and erase the subsequent line where water01 = water1.divide(water1), which baffles me.

Can anyone provide some insight as to why this is so, or point out exactly what is wrong with my script?

// last modified on 9/7/2020
// script to extract total area of water pixels from given area (fp), using landsat 8
// two outputs: one where clouds are masked, one where clouds are not masked

var roi = ee.Geometry.Polygon([

// loading image collection 
var landsat8= ee.ImageCollection('LANDSAT/LC08/C01/T1_SR').filterBounds(roi)
.filterDate('2013-01-01', '2015-07-31')
var visParams = {
  bands: ['B4', 'B3', 'B2'],
  min: 0,
  max: 3000,
  gamma: 1.4,
Map.addLayer(landsat8, visParams,'original images');

// Function to extract water, then calculate area of water pixel 
var waterfunction = function(image){
  //add the NDWI band to the image
  var ndwi = image.normalizedDifference(['B3', 'B5']).rename('NDWI');
  //get pixels above the threshold
  var water1 = ndwi.gt(-0.2);
  //convert all pixels to value 1
  var water01 = water1.divide(water1);
  //extract only water pixels 
  image = image.addBands(water01).updateMask(water01);

// now to calculate water area, multiply water01 (all values are 1) by ee.Image.pixelArea; since that image gives us the area of each pixel
// then rename the band 
  var waterArea = water01

// adding area of water as a band
  image = image.addBands(waterArea);

// calculate area 
  var stats = waterArea.reduceRegion({
    reducer: ee.Reducer.sum(), 
    geometry: roi, 
    scale: 30,

  return image.set(stats);

// function to mask clouds out
function maskL8sr(image) {
  // Bits 3 and 5 are cloud shadow and cloud, respectively.
  var cloudShadowBitMask = (1 << 3);
  var cloudsBitMask = (1 << 5);
  // Get the pixel QA band.
  var qa = image.select('pixel_qa');
  // Both flags should be set to zero, indicating clear conditions.
  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0)
  return image.updateMask(mask);

// mapping water function over my collection 
var collection = landsat8.map(waterfunction);
var visParams2 = {min: -0.2, 
  max: 1, 
    bands: 'NDWI', 
    palette: ['00FFFF', '0000FF']};

var NDWICollection = collection.select('NDWI')
Map.addLayer(NDWICollection.first(), visParams2,'water');
//creating a chart 
var title = {
  title: 'Total area of water pixels (No clouds masked out)',
  hAxis: {title: 'Time'},
  vAxis: {title: 'Area (sq km)'},

// var cloudChart = ui.Chart.image.series({
//   imageCollection: collection.select('waterArea'), 
//   region: fp, 
//   reducer: ee.Reducer.sum(), 
//   scale: 30,
// })
// .setOptions(title);
// print(cloudChart);

// masking clouds, then mapping water function over the collection
var cloudlessCollection = landsat8.map(maskL8sr).map(waterfunction);
var title2 = {
  title: 'Total area of water pixels (Clouds masked out)',
  hAxis: {title: 'Time'},
  vAxis: {title: 'Area (sq km)'},
var cloudlessChart = ui.Chart.image.series({
  imageCollection: cloudlessCollection.select('waterArea'), 
  region: roi, 
  reducer: ee.Reducer.sum(), 
  scale: 30,

1 Answer 1


This line already makes a binary map of values 1 (values greater than -0.2) and 0 (values lower than -0.2). No need to divide.

//get pixels above the threshold
var water1 = ndwi.gt(-0.2);

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