change detection 1: year of detection

using landtrendr to determine year of change

LandTrendr (Landsat-based detection of Trends in Disturbance and Recovery) is an approach to extract spectral trajectories of land surface change from yearly Landsat time-series stacks (LTS) . The method brings together two themes in time-series analysis of LTS: capture of short-duration events and smoothing of long-term trends.

reference: Kennedy, R. E., Yang, Z., & Cohen, W. B. (2010). Detecting trends in forest disturbance and recovery using yearly Landsat time series: 1. LandTrendr—Temporal segmentation algorithms. Remote Sensing of Environment, 114(12), 2897-2910.

Step 1: get the collection and area of interest.

// get annual collection
var annualSRcollection = ee.ImageCollection("projects/servir-mekong/yearlyComposites")

// set the area of interest
var chl = ee.FeatureCollection("users/servirmekong/Vietnam/CHL_Boundary").geometry();

Step 2: add the function to calculate the nbr

// define function to calculate a spectral index to segment with LT
var nbr = function(img) {

    var index = img.normalizedDifference(['nir', 'swir1'])
                   .select([0], ['NBR'])
                   .set('system:time_start', img.get('system:time_start'));
    return img.addBands(index) ;

Step 3: apply the function to the image collection

var ltCollection =

Step 4: Define the segmentation parameters

// define the segmentation parameters:
var run_params = {
  maxSegments:            6,
  spikeThreshold:         0.9,
  vertexCountOvershoot:   3,
  preventOneYearRecovery: true,
  recoveryThreshold:      0.25,
  pvalThreshold:          0.05,
  bestModelProportion:    0.75,
  minObservationsNeeded:  6

Step 5: Multiply the segmentation index by the distDir to ensure that vegetation loss is associated with a positive spectral delta

ltCollection = {
                 .set('system:time_start', img.get('system:time_start'))

Step 6: Run the landtrendr algorithm

//----- RUN LANDTRENDR -----
run_params.timeSeries = ltCollection;
var lt = ee.Algorithms.TemporalSegmentation.LandTrendr(run_params);

Step 7: define disturbance mapping filter parameters

// define disturbance mapping filter parameters
 // delta filter for 1 year duration disturbance, <= will not be included as disturbance - units are in units of segIndex defined in the following function definition
var treeLoss1  = 175;
// delta filter for 20 year duration disturbance, <= will not be included as disturbance - units are in units of segIndex defined in the following function definition
var treeLoss20 = 200;
// pre-disturbance value threshold - values below the provided threshold will exclude disturbance for those pixels - units are in units of segIndex defined in the following function definition
var preVal     = 400;
 // minimum mapping unit for disturbance patches - units of pixels
var mmu        = 15;      

// assemble the disturbance extraction parameters
var distParams = {
  tree_loss1: treeLoss1,
  tree_loss20: treeLoss20,
  pre_val: preVal

Step 7: add function to extract greatest disturbance based on spectral delta between vertices.

// ----- function to extract greatest disturbance based on spectral delta between vertices
var extractDisturbance = function(lt, distDir, params, mmu) {
// select only the vertices that represents a change
var vertexMask = lt.arraySlice(0, 3, 4);
var vertices = lt.arrayMask(vertexMask);

// construct segment start and end point years and index values
var left = vertices.arraySlice(1, 0, -1);
var right = vertices.arraySlice(1, 1, null);
var startYear = left.arraySlice(0, 0, 1);
var startVal = left.arraySlice(0, 2, 3);
var endYear = right.arraySlice(0, 0, 1);
var endVal = right.arraySlice(0, 2, 3);

var dur = endYear.subtract(startYear);
var mag = endVal.subtract(startVal);

// concatenate segment start year, delta, duration, and starting spectral index value to an array
var distImg =[startYear.add(1), mag, dur, startVal.multiply(distDir)]).toArray(0);

// sort the segments in the disturbance attribute image delta by spectral index change delta
var distImgSorted = distImg.arraySort(mag.multiply(-1));

// slice out the first (greatest) delta
var tempDistImg = distImgSorted.arraySlice(1, 0, 1).unmask(ee.Image(ee.Array([[0],[0],[0],[0]])));

// make an image from the array of attributes for the greatest disturbance
var finalDistImg =,0,1).arrayProject([1]).arrayFlatten([['yod']]),

// filter out disturbances based on user settings
var threshold = ee.Image(['dur']))
.multiply((params.tree_loss20 - params.tree_loss1) / 19.0)

// apply the filter mask
finalDistImg = finalDistImg.mask(threshold).int16();

// patchify the remaining disturbance pixels using a minimum mapping unit
if(mmu > 1){
var mmuPatches =['yod'])
.connectedPixelCount(mmu, true)
finalDistImg = finalDistImg.updateMask(mmuPatches);

return finalDistImg;

Step 8: Run the extraction function

// run the dist extract function
var distImg = extractDisturbance('LandTrendr'), distDir, distParams);

Step 8: add the map and legend

Map.addLayer(['yod']).clip(chl), viz, 'Year of Detection');    // add disturbance year of detection to map

// set position of panel
var legend = ui.Panel({
style: {
position: 'bottom-left',
padding: '8px 15px'

// Create legend title
var legendTitle = ui.Label({
value: 'Year',
style: {
fontWeight: 'bold',
fontSize: '18px',
margin: '0 0 4px 0',
padding: '0'

// Add the title to the panel

// create the legend image
var lon = ee.Image.pixelLonLat().select('latitude');
var gradient = lon.multiply((viz.max-viz.min)/100.0).add(viz.min);
var legendImage = gradient.visualize(viz);

// create text on top of legend
var panel = ui.Panel({
widgets: [


// create thumbnail from the image
var thumbnail = ui.Thumbnail({
image: legendImage,
params: {bbox:'0,0,10,100', dimensions:'10x200'},
style: {padding: '1px', position: 'bottom-center'}

// add the thumbnail to the legend

// create text on top of legend
var panel = ui.Panel({
widgets: [





  1. Would you please explain how you made the yearly composite

    this annual collection
    var annualSRcollection = ee.ImageCollection(“projects/servir-mekong/yearlyComposites”)


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