Overlapping multiple images with cloud masked?

Question

I am now using fmask algorithm to eliminate the cloud covered areas on Landsat images.

I choose a period of time (Jan 1st 2017 to Dec 31st 2017), and selected the study area (Florida). I want to get all of the Landsat 8 images under the above mentioned conditions, then apply the algorithm to mask the cloud.

I hoped the images taken for the same area but on different observation days could all get cloud masked, overlapped together and then I can do some statistical calculation.

However, I can only get the cloud masked images for Dec 31st 2017. I have tried a lot but still cannot overlap the images for the whole year.

My code is attached below

//Choose country using GEE Feature Collection

var region = ee.FeatureCollection('TIGER/2016/States')
    .filter(ee.Filter.or(
        ee.Filter.eq('NAME', 'Florida')));

//Add region outline to layer ‐ for selected states

//Map.addLayer(region,{}, 'Florida');

var landsat8 = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR');

// Fmask classification values var FMASK_CLEAR_GROUND = 0; var FMASK_WATER = 2; var FMASK_CLOUD_SHADOW = 3; var FMASK_SNOW = 4; var FMASK_CLOUD = 5;

var mosaic = landsat8 .filterBounds(region) .filterDate('2017-01-01', '2017-12-31') .mosaic();

var getQABits = function(image, start, end, newName) {
    // Compute the bits we need to extract.
    var pattern = 0;
    for (var i = start; i <= end; i++) {
       pattern += Math.pow(2, i);
    }
    // Return a single band image of the extracted QA bits, giving the band
    // a new name.
    return image.select([0], [newName])
                  .bitwiseAnd(pattern)
                  .rightShift(start);
};

// A function to mask out cloudy pixels.
var cloud_shadows = function(image) {
  // Select the QA band.
  var QA = image.select(['pixel_qa']);
  // Get the internal_cloud_algorithm_flag bit.
  return getQABits(QA, 3,3, 'Cloud_shadows').eq(0);
  // Return an image masking out cloudy areas.
};

// A function to mask out cloudy pixels.
var clouds = function(image) {
  // Select the QA band.
  var QA = image.select(['pixel_qa']);
  // Get the internal_cloud_algorithm_flag bit.
  return getQABits(QA, 5,5, 'Cloud').eq(0);
  // Return an image masking out cloudy areas.
};

var maskClouds = function(image) {
  var cs = cloud_shadows(image);
  var c = clouds(image);
  image = image.updateMask(cs);
  return image.updateMask(c);
};

var mosaic_free = maskClouds(mosaic);

var visParams = {bands: ['B4', 'B3', 'B2'],min: [0,0,0],max: [2000, 2000, 2000]};

Map.addLayer(mosaic, visParams, 'With clouds'); 
Map.addLayer(mosaic_free, visParams, 'Cloud free'); 

Could anybody help me to figure out what's wrong with that?

Answer 1

First, what does mosaic?

Composites all the images in a collection, using the mask.

Returns: Image

Since mosaic returns an image object, use this function at the end of the process, because if you removes clouds for a single image, you can't fill gaps. Apply mosaic.map(maskClouds) (as ImageCollection) instead maskClouds(mosaic) (as Image).

But, is mosaic what you need for a nice composite? With mosaic:

With median reducer:

CODE: https://code.earthengine.google.com/432c55d13f4651bc4e46d9cab4ab2f9c