[GIS] Retrieve image statistics as numbers in Google Earth Engine

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I am working with a simple piece of code for obtaining EVI index from two Landsat8 images and then calculating the difference between both. 

var img_t0 = ee.Image('LANDSAT/LC8_L1T_TOA_FMASK/LC82330892017001LGN00');
var img_t1 = ee.Image('LANDSAT/LC8_L1T_TOA_FMASK/LC82330892017081LGN00');

function EVI(image) {
  var step1 = image.expression(
    '2.5 * ((NIR - RED) / (NIR + 6 * RED - 7.5 * BLUE + 1))', {
      'NIR': image.select('B5'),
      'RED': image.select('B4'),
      'BLUE': image.select('B2')
      });
  var step2 = step1.updateMask(image.select(['fmask']).eq(0));  // mask everything different than 'clear'
  return step2
}

var n0 = EVI(img_t0);
var n1 = EVI(img_t1);

var diff = n0.subtract(n1);

// Mean and standard deviation reducers.
var meanReducer = ee.Reducer.mean();
var sigmaReducer = ee.Reducer.stdDev();

// Use the reducer to get the mean and SD of the image.
var mean = ee.Number(diff.reduceRegion({
  reducer: meanReducer,
  bestEffort: true,
}).get('constant'));

var sigma = ee.Number(diff.reduceRegion({
  reducer: sigmaReducer,
  bestEffort: true,
}).get('constant'));

print(mean, sigma, mean+2*sigma)

From the resulting image, I need to get the mean and standard deviation for further calculations. Despite the fact that I can print the parameters (as numbers), I can not retrieve their values as such type. When I use them in an arithmetic operation, I am getting NaN as result.

I understand that the reducer property gives you a dictionary as a result, but I have tried several methods (link casting the variables with ee.Number or using brackets [ ] for getting the key value without success).

So the question is ¿How can I retrieve those values like numbers or additional variables for using them later?

Best Answer

The cause of your issue is that you are mixing server-side Earth Engine objects (mean, sigma) with client-side operators (+ 2 *) in the following line:

print(mean, sigma, mean+2*sigma)

If you want to do the arithmetic operations on the client, you can call .getInfo() on the objects to convert them from Earth Engine objects to client-side Javascript objects. This is generally not recommended for a variety of reasons: it is less efficient given that it makes multiple network requests to the server; the approach doesn't scale to large data instructions; it will block execution of the Code Editor script until it completes.

// Example of forcing client-side evaluation (discouraged).
print(mean, sigma, mean.getInfo()+2*sigma.getInfo());

Instead, use server-side methods to describe the arithmetic:

// Server-side evaluation.
print(mean, sigma, mean.add(sigma.multiply(2)));

For a detailed discussion of this topic, see the Client vs. Server section of the Earth Engine documentation.

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