Extract and plot from an Array in GEE

Question

I want to use the LandTrendr (LT) functions in GEE and adjust them to my needs. Basically, I want to the a similar thing that this earthengine.app is doing (only NBR for a start).

I can run the analysis, compute the values. But I cannot figure out how to produce the figure.

// load the LandTrendr.js module
var ltgee = require('users/emaprlab/public:Modules/LandTrendr.js');

// define parameters
var startYear = 1985;
var endYear = 2017;
var startDay = '06-20';
var endDay = '09-20';
var aoi = ee.Geometry.Point(-122.8848, 43.7929);
var index = 'NBR';
var ftvList = ["NBR"];
var runParams = { 
  maxSegments:            6,
  spikeThreshold:         0.9,
  vertexCountOvershoot:   3,
  preventOneYearRecovery: true,
  recoveryThreshold:      0.25,
  pvalThreshold:          0.05,
  bestModelProportion:    0.75,
  minObservationsNeeded:  6
};


// apply LandTrendr.js functions
var lt = ltgee.runLT(startYear, endYear, startDay, endDay, aoi, index, ftvList, runParams);
print(lt);

This should result in an array 4 x nYears dimension array. With years in (0,0,1) and calculated values in rows 2 to 4.

Now, how to plot row 2 over years. And if I create a buffer around the aoi, do I get the values for the other pixels too?

Answer 1

If you want the chart the original data, you cannot use the ltgee.runLT() convenience method. Though you can still use the library to build your time-series, and extract the fitted values. Then it's a matter of massaging the data so you can chart it.

// load the LandTrendr.js module
var ltgee = require('users/emaprlab/public:Modules/LandTrendr.js');

// define parameters
var startYear = 1985;
var endYear = 2017;
var startDay = '06-20';
var endDay = '09-20';
var aoi = ee.Geometry.Point(-122.8848, 43.7929);
var index = 'NBR';
var ftvList = ["NBR"];
var maskThese = ['cloud', 'shadow', 'snow']

var collection = ltgee.buildSRcollection(startYear, endYear, startDay, endDay, aoi, maskThese)
var timeSeries = ltgee.buildLTcollection(collection, index, ftvList)
var lt = ee.Algorithms.TemporalSegmentation.LandTrendr({ 
  timeSeries:             timeSeries,
  maxSegments:            6,
  spikeThreshold:         0.9,
  vertexCountOvershoot:   3,
  preventOneYearRecovery: true,
  recoveryThreshold:      0.25,
  pvalThreshold:          0.05,
  bestModelProportion:    0.75,
  minObservationsNeeded:  6
})
var fittedImage = ltgee.getFittedData(lt, startYear, endYear, index)
var fittedCollection = ee.ImageCollection(fittedImage.bandNames()
  .map(function (yearString) {
    yearString = ee.String(yearString)
    return fittedImage.select(yearString)
      .set('system:time_start', ee.Date.parse('yyyy', yearString).millis())
      .rename('Fitted')
  })
)

var chart = ui.Chart.image.series({
    imageCollection: fittedCollection.merge(
      timeSeries.map(function (image) { return image.select([1], ['Original']) })
    ), 
    region: aoi, 
    reducer: ee.Reducer.median(), 
    scale: 30
  })

print(chart)

https://code.earthengine.google.com/27a73e0df57e28d3dc1ae4190f5eb52b

Now, how to plot row 2 over years. And if I create a buffer around the aoi, do I get the values for the other pixels too?

You control which value you get with region and reducer in the chart.