Join Landsat and MODIS Image Collections in Earth Engine?

Question

I'm going to join Landsat-8 and MODIS image collections using the below code but returns nothing. Does anyone know about this?

code link: https://code.earthengine.google.com/2504c97ec795f4f458f7966795110846

Map.centerObject(geometry);

var landsatNDWI = function(img){
  var bands = img.select('SR_B[1-7]').multiply(2.75e-05).add(-0.2);
  var index = bands.normalizedDifference(['SR_B3','SR_B5']).rename('ndwi');
  return index
  .copyProperties(img,['system:time_start','system:time_end']);
};

var modisNDWI = function(img){
  var bands = img.select('sur_refl_b0[1-7]').multiply(0.0001);
  var index = bands.normalizedDifference(['sur_refl_b04','sur_refl_b02']).rename('ndwi');
  return index
  .copyProperties(img,['system:time_start','system:time_end']);
};

var landsat = ee.ImageCollection("LANDSAT/LC08/C02/T1_L2")
.filterDate('2018','2020')
.filterBounds(geometry)
.map(landsatNDWI);

var modis = ee.ImageCollection("MODIS/006/MOD09GA")
.filterDate('2018','2020')
.filterBounds(geometry)
.map(modisNDWI);




var filter = ee.Filter.equals({
  leftField: 'system:time_start',
  rightField: 'system:time_start'
});

// Create the join.
var simpleJoin = ee.Join.inner();

// Inner join
var innerJoin = ee.ImageCollection(simpleJoin.apply(landsat, modis, filter));

var joined = innerJoin.map(function(feature) {
  return ee.Image.cat(feature.get('primary'), feature.get('secondary'));
});

print('Joined', joined);

Answer 1

The trouble is that the dates of the Landsat images have a slightly different date format compared to the Modis images. The Landsat collection include the date and time of the images, Modis only the date:

print(landsat.first().date())
>>>   Date (2018-01-13 07:32:29)
print(modis.first().date())
>>>   Date (2018-01-01 00:00:00)

As the system:time_start property is in Unix time, the join will not return anything if there's a slight offset. To fix this, you'll need to strip the hours/minutes/seconds from the Landsat images. Here's a solution using ee.Date().update():

var landsatNDWI = function(img){
  var bands = img.select('SR_B[1-7]').multiply(2.75e-05).add(-0.2);
  var index = bands.normalizedDifference(['SR_B3','SR_B5']).rename('ndwi');
  return index
  .copyProperties(img,['system:time_start','system:time_end']);
};

var modisNDWI = function(img){
  var bands = img.select('sur_refl_b0[1-7]').multiply(0.0001);
  var index = bands.normalizedDifference(['sur_refl_b04','sur_refl_b02']).rename('ndwi');
  return index
  .copyProperties(img,['system:time_start','system:time_end']);
};

var landsat = ee.ImageCollection("LANDSAT/LC08/C02/T1_L2")
.filterDate('2018','2020')
.filterBounds(geometry)
.map(landsatNDWI)
.map(function(img){return img.set('system:time_start', ee.Date(img.get('system:time_start')).update({hour:0, minute:0, second:0}).millis())})

var modis = ee.ImageCollection("MODIS/006/MOD09GA")
.filterDate('2018','2020')
.filterBounds(geometry)
.map(modisNDWI);

var filter = ee.Filter.equals({
  leftField: 'system:time_start',
  rightField: 'system:time_start'
});

// Create the join.
var simpleJoin = ee.Join.inner();

// Inner join
var innerJoin = ee.ImageCollection(simpleJoin.apply(landsat, modis, filter));

var joined = innerJoin.map(function(feature) {
  return ee.Image.cat(feature.get('primary'), feature.get('secondary'));
});

print('Joined', joined);
>>>  ImageCollection LANDSAT/LC08/C02/T1_L2 (88 elements)