harmonize and smooth nightlights

combine DMSP and VIIRS in one timeseries

Copy the code below or click here

function returnFloat(img){
  return img.float()};

// import virrs data
var DMSP = ee.ImageCollection("NOAA/DMSP-OLS/NIGHTTIME_LIGHTS").select(["avg_vis"],["nightlight"]).map(returnFloat)

var a = 14.758;
var b = 0.448;

// apply power relation to viirs data
// from here: https://www.iirs.gov.in/iirs/sites/default/files/StudentThesis/S_6027253_Reshma_Jeswani_MSc_Thesis.pdf
// maybe use log function instead.
function scaleNL(img){
  var t = img.get("system:time_start");
  img = img.where(img.lt(0.001),0.001);
  return ee.Image(ee.Image(a).multiply(img.pow(b))).set("system:time_start",t).rename("nightlight").float();
}

// import virrs data
var viirs = ee.ImageCollection("NOAA/VIIRS/DNB/MONTHLY_V1/VCMSLCFG").select("avg_rad").map(scaleNL);

// combine dataseries
var nightlights = DMSP.merge(viirs);

// This field contains UNIX time in milliseconds.
var timeField = 'system:time_start';

// select nighlights band
var modeled =  ee.ImageCollection(nightlights.select(['nightlight']));

// Smoothing ---------------------------------------------------------------
var join = ee.Join.saveAll({
  matchesKey: 'images'
});

var diffFilter = ee.Filter.maxDifference({
  difference: 1000 * 60 * 60 * 24 *365,
  leftField: timeField, 
  rightField: timeField
});

var threeNeighborJoin = join.apply({
  primary: modeled, 
  secondary: modeled, 
  condition: diffFilter
});


// smooth image collection
var smoothed = ee.ImageCollection(threeNeighborJoin.map(function(image) {
  var collection = ee.ImageCollection.fromImages(image.get('images'));
  return ee.Image(image).addBands(collection.mean().rename('mean'));
}));


// set display parameters
var display = {
  title: "night lights",
  fontSize: 12,
  hAxis: {title: 'Year'},
  vAxis: {title: "avg_vis"}};


// add chart
print(ui.Chart.image.series(
  smoothed.select(['nightlight', 'mean']), geometry, ee.Reducer.mean(), 30)
    .setSeriesNames(['nightlight', 'mean'])
    .setOptions({
      title: 'smoothed',
      lineWidth: 1,
      pointSize: 3,
}));

// set viz paramater
var viz = {min:0, max:50, palette:['000000','700000','808080','FFFF00','ffffff','ffffff','ffffff']};

// get the 1995 and 2018 image
var img1995 = smoothed.filterDate(ee.Date.fromYMD(1995,1,1),ee.Date.fromYMD(1995,12,31));
var img2018 = smoothed.filterDate(ee.Date.fromYMD(2018,1,1),ee.Date.fromYMD(2018,12,31));

// add the maps
Map.addLayer(ee.Image(img1995.first().select("mean")),viz,"1995");
Map.addLayer(ee.Image(img2018.first().select("mean")),viz,"2018");