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Quantifying Forest Change

Let's start with the calculation needed to create a band that shows pixels where the Hansen et al. data show both loss and gain.

The Hansen et al. dataset has a band whose pixels are 1 where loss occurred and 0 otherwise (loss) and a band that is 1 where gain has occurred and a 0 otherwise (gain). To create a band where pixels in both the loss and the gain bands have a 1, you can use the and() logical method on images. The and() method is called like image1.and(image2) and returns an image in which pixels are 1 where both image1 and image2 are 1, and 0 elsewhere:

Code Editor (JavaScript)

// Load the data and select the bands of interest.
var gfc2014 = ee.Image('UMD/hansen/global_forest_change_2015');
var lossImage = gfc2014.select(['loss']);
var gainImage = gfc2014.select(['gain']);

// Use the and() method to create the lossAndGain image.
var gainAndLoss = gainImage.and(lossImage);

// Show the loss and gain image.
Map.addLayer(gainAndLoss.updateMask(gainAndLoss),
    {palette: 'FF00FF'}, 'Gain and Loss');

The result, zoomed into Arkansas with satellite view, should look something like Figure 1.

Loss Arkansas — Figure 1. Pixels with forest loss and gain in Arkansas.

Combining this example with the result from the previous section, it's now possible to recreate the figure from the beginning of the tutorial:

Code Editor (JavaScript)

// Displaying forest, loss, gain, and pixels where both loss and gain occur.
var gfc2014 = ee.Image('UMD/hansen/global_forest_change_2015');
var lossImage = gfc2014.select(['loss']);
var gainImage = gfc2014.select(['gain']);
var treeCover = gfc2014.select(['treecover2000']);

// Use the and() method to create the lossAndGain image.
var gainAndLoss = gainImage.and(lossImage);

// Add the tree cover layer in green.
Map.addLayer(treeCover.updateMask(treeCover),
    {palette: ['000000', '00FF00'], max: 100}, 'Forest Cover');

// Add the loss layer in red.
Map.addLayer(lossImage.updateMask(lossImage),
    {palette: ['FF0000']}, 'Loss');

// Add the gain layer in blue.
Map.addLayer(gainImage.updateMask(gainImage),
    {palette: ['0000FF']}, 'Gain');

// Show the loss and gain image.
Map.addLayer(gainAndLoss.updateMask(gainAndLoss),
    {palette: 'FF00FF'}, 'Gain and Loss');

Quantifying Forest Change in a Region of Interest

Now that you're more familiar with the bands in the Hansen et al. dataset, we can use concepts learned so far to compute statistics about forest gain and loss in a region of interest. For this we'll need to use vector data (points, lines, and polygons). A vector dataset is represented as a FeatureCollection in Earth Engine. (Learn more about feature collections and how to import vector data.)

In this section, we'll compare the total amount of forest loss that happened within the Congo Republic in the year 2012 to the amount of forest loss that happened within the country's protected areas at the same time.

As you learned in the Earth Engine API tutorial, the key method for calculating statistics in an image region is reduceRegion(). (Learn more about reducing image regions.) For example, suppose we want to calculate the number of pixels estimated to represent forest loss during the study period. For that purpose, consider the following code:

Code Editor (JavaScript)

// Load country features from Large Scale International Boundary (LSIB) dataset.
var countries = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017');

// Subset the Congo Republic feature from countries.
var congo = countries.filter(ee.Filter.eq('country_na', 'Rep of the Congo'));

// Get the forest loss image.
var gfc2014 = ee.Image('UMD/hansen/global_forest_change_2015');
var lossImage = gfc2014.select(['loss']);

// Sum the values of forest loss pixels in the Congo Republic.
var stats = lossImage.reduceRegion({
  reducer: ee.Reducer.sum(),
  geometry: congo,
  scale: 30
});
print(stats);

This example uses the ee.Reducer.sum() reducer to sum the values of the pixels in lossImage within the congo feature. Because lossImage consists of pixels that have a value of 1 or 0 (for loss or not loss, respectively), the sum of these values is equivalent to the number of pixels of loss in the region.

Unfortunately, running the script as it is results in an error like:

The default maximum number of pixels in reduceRegion() is 10 million. This error message indicates that the Congo Republic covers about 383 million Landsat pixels. Luckily, reduceRegion() takes many parameters, one of which (maxPixels) lets you control how many pixels are used in the computation. Specifying this parameter allows the computation to succeed:

Code Editor (JavaScript)

// Load country features from Large Scale International Boundary (LSIB) dataset.
var countries = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017');

// Subset the Congo Republic feature from countries.
var congo = countries.filter(ee.Filter.eq('country_na', 'Rep of the Congo'));

// Get the forest loss image.
var gfc2014 = ee.Image('UMD/hansen/global_forest_change_2015');
var lossImage = gfc2014.select(['loss']);

// Sum the values of forest loss pixels in the Congo Republic.
var stats = lossImage.reduceRegion({
  reducer: ee.Reducer.sum(),
  geometry: congo,
  scale: 30,
  maxPixels: 1e9
});
print(stats);

By expanding the object printed to the console, observe that the result is 4897933 pixels of forest lost. You can clean up the printout in the console a bit by labeling the output and getting the result of interest from the dictionary returned by reduceRegion():

Code Editor (JavaScript)

print('pixels representing loss: ', stats.get('loss'));

Calculating Pixel Areas

You're almost ready to answer the question of how much area was lost in the Congo Republic, and how much of that was in protected areas. The remaining part is to convert pixels into actual area. This conversion is important because we don't necessarily know the size of the pixels input to reduceRegion(). To help compute areas, Earth Engine has the ee.Image.pixelArea() method which generates an image in which the value of each pixel is the pixel's area in square meters. Multiplying the loss image with this area image and then summing over the result gives us a measure of area:

Code Editor (JavaScript)

// Load country features from Large Scale International Boundary (LSIB) dataset.
var countries = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017');

// Subset the Congo Republic feature from countries.
var congo = countries.filter(ee.Filter.eq('country_na', 'Rep of the Congo'));

// Get the forest loss image.
var gfc2014 = ee.Image('UMD/hansen/global_forest_change_2015');
var lossImage = gfc2014.select(['loss']);
var areaImage = lossImage.multiply(ee.Image.pixelArea());

// Sum the values of forest loss pixels in the Congo Republic.
var stats = areaImage.reduceRegion({
  reducer: ee.Reducer.sum(),
  geometry: congo,
  scale: 30,
  maxPixels: 1e9
});
print('pixels representing loss: ', stats.get('loss'), 'square meters');

Now the result is 4,372,575,052 square meters lost over the study period.

You are now ready to answer the question at the start of this section - how much forest area was lost in the Congo Republic in 2012, and how much of that was in protected areas?

Code Editor (JavaScript)

// Load country features from Large Scale International Boundary (LSIB) dataset.
var countries = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017');

// Subset the Congo Republic feature from countries.
var congo = ee.Feature(
  countries
    .filter(ee.Filter.eq('country_na', 'Rep of the Congo'))
    .first()
);

// Subset protected areas to the bounds of the congo feature
// and other criteria. Clip to the intersection with congo.
var protectedAreas = ee.FeatureCollection('WCMC/WDPA/current/polygons')
  .filter(ee.Filter.and(
    ee.Filter.bounds(congo.geometry()),
    ee.Filter.neq('IUCN_CAT', 'VI'),
    ee.Filter.neq('STATUS', 'proposed'),
    ee.Filter.lt('STATUS_YR', 2010)
  ))
  .map(function(feat){
    return congo.intersection(feat);
  });

// Get the loss image.
var gfc2014 = ee.Image('UMD/hansen/global_forest_change_2015');
var lossIn2012 = gfc2014.select(['lossyear']).eq(12);
var areaImage = lossIn2012.multiply(ee.Image.pixelArea());

// Calculate the area of loss pixels in the Congo Republic.
var stats = areaImage.reduceRegion({
  reducer: ee.Reducer.sum(),
  geometry: congo.geometry(),
  scale: 30,
  maxPixels: 1e9
});
print(
  'Area lost in the Congo Republic:',
  stats.get('lossyear'),
  'square meters'
);

// Calculate the area of loss pixels in the protected areas.
var stats = areaImage.reduceRegion({
  reducer: ee.Reducer.sum(),
  geometry: protectedAreas.geometry(),
  scale: 30,
  maxPixels: 1e9
});
print(
  'Area lost in protected areas:',
  stats.get('lossyear'),
  'square meters'
);

The output indicates that of the 348,036,295 square meters of forest lost in the Congo Republic in 2012, 11,880,976 of those were in protected areas, as represented in the World Database on Protected Areas table.

The only changes between this script and the one just prior are the addition of the protected area information and changing the script from looking at overall loss to looking at loss in 2012. This required two changes. First, there's a new lossIn2012 image which has a 1 where loss was recorded in 2012, 0 otherwise. Second, because the name of the band is different (lossyear instead of loss) the property name had to change in the print statement.

In the next section, we will explore some advanced methods for computing and charting forest loss for every year, instead of just one year as we did in this section.