Google Earth Engine – Adding Each Image to Previous Image Error

google-earth-enginejavascriptmodis

The below code has written to add each image to the previous image using iteration function but I don't know why returns negative values while positive expected!

Code link: https://code.earthengine.google.com/0df6e1a39abc8849660447b23abc8cbe

Map.centerObject(table);

var lst = function(img){
  var bands = img.select('LST_Day_1km').multiply(0.02).clip(table);
  return bands.subtract(273.15)
  .copyProperties(img,['system:time_start','system:time_end']);
};


var startYear = '2001';
var endYear = '2021';

var modisLST = ee.ImageCollection("MODIS/006/MOD11A2")
.filterDate(startYear, endYear)
.filter(ee.Filter.calendarRange(5,9))
.filterBounds(table)
.map(lst);


var time0 = modisLST.first().get('system:time_start');

var first = ee.List([
  ee.Image(0).set('system:time_start', time0).select([0],['LST_Day_1km'])
  ]);
  

var accumulate = function(img, list){
  
  var previous = ee.Image(ee.List(list).get(-1))
  var added = img.add(previous)
  .set('system:time_start', img.get('system:time_start'));
  
  return ee.List(list).add(added);
};


var cummulative = ee.ImageCollection(ee.List(modisLST.iterate(accumulate, first)));

print(
  ui.Chart.image.series(cummulative, table, ee.Reducer.mean(), 1000, 'system:time_start')
  )

Best Answer

The values seem to be correct, maybe you intended to filter to the months 5-9 instead of the days 5-9?

.filter(ee.Filter.calendarRange(5,9))

This by default takes the calendar Range from the 5th of January to the 9th of January which generally have temperatures below freezing. If you change it to:

.filter(ee.Filter.calendarRange(5, 9, "month"))

You will get the summer months you probably wanted.

Related Solutions

[GIS] Image collection to Single Image in Google Earth Engine

// UPDATE: The easiest way is now toBands().
var merged = MOD1.toBands();

// For completeness, this is the iterate() way:
var mergeBands = function(image, previous) {
  return ee.Image(previous).addBands(image);
};

var merged = MOD1.iterate(mergeBands, ee.Image([]));

Google Earth Engine – Understanding Image Expression in Google Earth Engine

Assuming your affirmation that the remote sensing reflectance has to divide by pi, it is preferable to have three functions for mapping original and followings ImageCollections. First of them, it prepares selected bands for correct use (dividing by pi and it is also necessary a scale factor). Second function calculates band max between B1 and B2 and includes it in a new collections as 'B12_max' and finally, your desired function. They were incorporated in following code and one value, for pixel in point (-15.857348155192721, 28.47817439021581), arbitrarily selected in the ocean, was tested with respective formulas in LibreOffice. They matched perfectly.

var pt = ee.Geometry.Point([-15.857348155192721, 28.47817439021581]);

function prepareImages(image) {
  var time = image.get('system:time_start');
  return image.divide(Math.PI*10000)
    .set('system:time_start', time);
}

var dataset = ee.ImageCollection('COPERNICUS/S2_SR')
                  .filterDate('2020-01-01', '2020-01-03')
                  .filterBounds(pt)
                  .select('B1', 'B2', 'B3', 'B4')
                  .map(prepareImages);

var collBandMax = dataset.map(function (image) {
  var img_B1 = image
    .select('B1');

  var img_B2 = image
    .select('B2');

  var img_max_B1_B2 = img_B1.max(img_B2)
    .rename('B12_max');
    
  image = image.addBands([img_max_B1_B2]);

  var time = image.get('system:time_start');

  return image.set('system:time_start', time);

});

print("Collection with Band Max", collBandMax);

var chl_oc2 = collBandMax.map(function(image){

  var x = image.expression(
    'log10 ( Bmax / B3 )', {
      'Bmax': image.select('B12_max'),
      'B3': image.select('B3'),
  }).rename('x');
  
  var chl_conc = image.expression('10**(0.2389-1.9369*x+1.7627*x**2-3.077*x**3-0.1054*x**4)',
                             {'x':x} ).rename('chl_conc');

  var time = image.get('system:time_start');

  return chl_conc.set('system:time_start', time); });

print(chl_oc2);

var visualization = {
  min: 0.0020371832715762603,
  max: 0.018589297353133374,
  bands: ['B4', 'B3', 'B2'],
};

Map.setCenter(-15.6871, 28.647, 20);

Map.addLayer(dataset.mean(), visualization, 'RGB');
Map.addLayer(chl_oc2, {}, 'chl_oc2');

//Image (3 bands)
//B1: 0.009963099437553
//B2: 0.016074649252282
//B3: 0.011681972822945

After running above code, I got result of following image for corroborating pixel value in that point.

In following image it can be observed that this value matched with obtained in LibreOffice spreadsheet.

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