Finding the best image per year of Landsat image collection in Google Earth Engine

Question

I am trying to create a series of images with the best image per year using Landsat 7 from 2000-2017 around the legal Amazon (using RBG bands 1-3). The code I have so far pulls up over 70,000 images over the period. It seems like I should be looping through images in each year and finding the least cloudy image, but I know you're not supposed to write loops in GGE.

I'm really new to this platform so I may be missing something obvious!

The end goal is to create a timelapse that I will overlay with my own dataset. The code below is what I have so far, and I have no idea how to proceed from here:

// create polygon around legal amazon
var Amaz = ee.Geometry.Polygon([[-43.0, -18.0], [-69.0, -18.0], [-69.0,5.0], [-43.0, 5.0]]);
Map.addLayer(Amaz)

//dates of interest
var start = ee.Date('2000-01-01');
var end = ee.Date('2018-01-01');

// create image collection

var Amazon_images = ee.ImageCollection('LANDSAT/LE07/C01/T1_SR')
.filterBounds(Amaz)
.filterDate(start, end)
.sort('CLOUD_COVER', false);

// Get number of images
var count = Amazon_images.size();
print('size of collection Amazon_images', count);

Answer 1

Yep, instead of for/while loops, Earth Engine prefers mapping over any type of collection or list (the "Mapping over .." chapters in the documentation are pretty helpful).

We can map over a list of years, filter to an annual Landsat collection, sort by cloud cover and then simply pick the first (least cloudy) image. At each step, the mapping returns a single image (the annual image of least cloud cover), which results in an ImageCollection. I have included the return of a rough image mosaic due to your region of interest being so large.

var roi = ee.Geometry.Polygon([[-43.0, -18.0], [-69.0, -18.0], [-69.0,5.0], 
                               [-43.0, 5.0]]);
Map.addLayer(roi)
Map.centerObject(roi, 4)

var startyear = 2000
var endyear = 2018

var LS7 = ee.ImageCollection('LANDSAT/LE07/C01/T1_SR')
  .filterBounds(roi)
  .filterDate(startyear+'-01-01', endyear+1+'-01-01')

var count = LS7.size();
print('size of collection Amazon_images', count);

// Get least cloudy annual scene / mosaic via the cc metadata property.
// years = [2000, 2001, 2002]
var years = ee.List.sequence(startyear, endyear).getInfo()

var annual_least_cc = years.map(function(year){
  // Get least cloudy annual single scene.
  // var annual = LS7.filterDate(year+'-01-01', (year+1)+'-01-01')
  //                 .sort('CLOUD_COVER')
  // var annual_img = ee.Image(annual.first())
  // Map.addLayer(annual_img.divide(10000), {bands: ['B3', 'B2', 'B1'], min: 0, max: 0.3}, year)

  // Or get mosaic of least cloudy annual scenes.
  var annual = LS7.filterDate(year+'-01-01', (year+1)+'-01-01')
                  .sort('CLOUD_COVER', false) // mosaic puts last element on top. 
  var annual_img = ee.Image(annual.mosaic())
  Map.addLayer(annual_img.divide(10000), {bands: ['B3', 'B2', 'B1'], min: 0, max: 0.3}, year)

  return annual_img
})
annual_least_cc = ee.ImageCollection(annual_least_cc)
print(annual_least_cc)