remote sensing - Two Different Areas for the same Image in Google Earth Engine

IMPORTS:

var polygon = /* color: #d63000 */ee.Geometry.Polygon(
        [[[-119.33364769821117, 46.05123532178373],
          [-119.3233620672313, 45.869732769408905],
          [-119.04111088542663, 45.873079023065166],
          [-119.0396574679861, 46.045448840018565]]]),
    landsat8 = ee.ImageCollection("LANDSAT/LC8_L1T_TOA_FMASK");

FIRST CODE: In the First Code, Collection is Filtered and Mosaic is applied even though it is only one image.

var PIXEL_SCALE = 30; // Meters. Resolution of most Landsat 8 bands
var PIXEL_AREA = PIXEL_SCALE * PIXEL_SCALE; // Square meters.


// Fmask classification values
var FMASK_CLEAR_GROUND = 0;
var FMASK_WATER = 1;
var FMASK_CLOUD_SHADOW = 2;
var FMASK_SNOW = 3;
var FMASK_CLOUD = 4;

var mosaic = landsat8
  .filterBounds(polygon)

  .filterDate('2016-08-01', '2016-08-30')
  .mosaic();

// Update the mask on our mosaic to mask cloud and cloud shadow pixels
var fmask = mosaic.select('fmask');
var cloudMask = fmask.neq(FMASK_CLOUD).and(fmask.neq(FMASK_CLOUD_SHADOW));
var maskedMosaic = mosaic.updateMask(cloudMask);


Map.addLayer(fmask, {min:0, max:4, palette:'green, blue, black, cyan, 

white'}, 'Fmask');
Map.addLayer(maskedMosaic.select('B4'), {min:0, max:0.5, palette:'yellow, 
green'}, 'Masked NIR');
Map.setCenter(-119.34, 45.97, 8);

// Calculate the number of pixels of each classification in our polygon
var regionCoverHistogram = mosaic.select('fmask')
 .reduceRegion(ee.Reducer.frequencyHistogram(), polygon, PIXEL_SCALE);
print('Fmask class pixel count within region', regionCoverHistogram);



var waterPixelCount = ee.Dictionary(regionCoverHistogram.get('fmask'))
  .get(FMASK_WATER.toString());

var waterArea = ee.Number(waterPixelCount).multiply(PIXEL_AREA);
print('Water Area (sq meters) in region', waterArea);

SECOND CODE:

Whereas in the Second Code, I am directly taking the Image ID of the same image and running the same algorithm.

var PIXEL_SCALE = 30; // Meters. Resolution of most Landsat 8 bands

var PIXEL_AREA = PIXEL_SCALE * PIXEL_SCALE; // Square meters.

// Fmask classification values
var FMASK_CLEAR_GROUND = 0;
var FMASK_WATER = 1;
var FMASK_CLOUD_SHADOW = 2;
var FMASK_SNOW = 3;
var FMASK_CLOUD = 4;



var mosaic  = ee.Image('LANDSAT/LC8_L1T_TOA_FMASK/LC80440282016227LGN00');

// Update the mask on our mosaic to mask cloud and cloud shadow pixels
var fmask = mosaic.select('fmask');
var cloudMask = fmask.neq(FMASK_CLOUD).and(fmask.neq(FMASK_CLOUD_SHADOW));
var maskedMosaic = mosaic.updateMask(cloudMask);


Map.addLayer(fmask, {min:0, max:4, palette:'green, blue, black, cyan, 
white'}, 'Fmask');

Map.addLayer(maskedMosaic.select('B4'), {min:0, max:0.5, palette:'yellow, 
green'}, 'Masked NIR');
Map.setCenter(-119.34, 45.97, 8);

//
// Calculating Region Cover Statistics
//

// Calculate the number of pixels of each classification in our polygon
var regionCoverHistogram = mosaic.select('fmask')

 .reduceRegion(ee.Reducer.frequencyHistogram(), polygon, PIXEL_SCALE);
print('Fmask class pixel count within region', regionCoverHistogram);


var waterPixelCount = ee.Dictionary(regionCoverHistogram.get('fmask'))
.get(FMASK_WATER.toString());

var waterArea = ee.Number(waterPixelCount).multiply(PIXEL_AREA);
print('Water Area (sq meters) in region', waterArea);

I am getting two Different Water Areas for the same image. Why?

Answer

You are getting different answers because the first code is using the default projection of the mosaic (WGS84) and the second one is using the default projection of the image (EPSG:32628). The reprojection is significantly altering the QA band.

You should always specify a crs to reduceRegion so you know in what projection the calculation is being done.