[GIS] Filter image collection with a date list in Google Earth Engine

datefiltergoogle-earth-engine

I know how to filter with a date range in Google Earth Engine, but I would like to filter a list of concrete dates which aren't consecutive, single days, for example, ['2005-08-05','2009-09-01','2017-02-08'].

var collection = ee.ImageCollection("MODIS/MYD11A1").select('LST_Day_1km');

var reference = collection.filterDate('2016-01-01', '2016-12-31')

Best Answer

Make a list of millisecond dates, and then use filter inList:

var collection = ee.ImageCollection("MODIS/MYD11A1").select('LST_Day_1km');

var reference = collection.filterDate('2016-01-01', '2016-12-31')

var datelist = ee.List(['2005-08-05','2009-09-01','2017-02-08'])
.map(function(date){return ee.Date(date).millis()})

var newcol = collection.filter(ee.Filter.inList("system:time_start", datelist))

print(newcol)

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[GIS] How to filter image collection by date, excluding a date from a date range in Google Earth Engine

You can remove images from an image collection by creating a date filter for the 'bad/missing' data and then remove it using ee.Filter.not(). For example:

var collection = ee.ImageCollection('NCEP_RE/surface_temp');
var AST2015 = collection.filterDate('2015-01-01', '2016-01-01');

var start_bad_data = '2015-06-18T00:00:00';
var end_bad_data = '2015-06-20T00:00:00';
var bad_data_filter = ee.Filter.date(start_bad_data, end_bad_data);

// Select the bad data.
var AST2015_bad_data = AST2015.filter(bad_data_filter);
// Select the good data.
var AST2015_good_data = AST2015.filter(bad_data_filter.not());

Map.addLayer(AST2015, {}, 'AST2015');
Map.addLayer(AST2015_bad_data, {}, 'AST2015_bad_data');
Map.addLayer(AST2015_good_data, {}, 'AST2015_good_data');

[GIS] Returning image for each iteration using Google Earth Engine

Furthermore it appears only 3 items are stored in the list which makes me assume the iteration is more complex than a literal iteration where all images are calculated one by one?

You've got some of the idea.

In Earth Engine, your JavaScript code is, by default, not at all involved in the substantial computation. Instead, it is executed to construct a data structure representing the desired computation (in JSON), which is then sent to the Earth Engine servers to be run.

Therefore, any client-side function in map or iterate is not called for every item in the collection, but once to get that representation. (Or in this case, three times because you have three separate server calls: two prints and a Map.addLayer.)

The documentation for map mentions this, but I see the documentation for iterate doesn't.

The mapped function is limited in the operations it can perform. Specifically, it can’t modify variables outside the function; it can’t print anything; it can’t use JavaScript ‘if’ or ‘for’ statements. However, you can use ee.Algorithms.If() to perform conditional operations in a mapped function.

So, you cannot expect the code you have written, which modifies a variable from inside the function supplied to iterate, to be useful. It does not collect the actual data, but only the placeholders that are meant to be used by iterate (hence names like _MAPPING_VAR_0_0).

Instead, what you must do is put the data you want to generate in the iterated value. In this case, this is easy to do — just return the accumulated list, and when you want the previous image take the last element of the list. So, I think this is what you wanted to do:

function drylength(current, previous){
  previous = ee.List(previous);  // cast to establish the client-side type
  var mask = current.remap([0], [1], 0,"precipitation").rename('precipitation');
  var sum = mask.add(previous.get(-1)).multiply(mask);

  return previous.add(sum);
}

var dataset = TRMM
.filterDate("2015-01-01","2015-02-01")
.sort('system:time_start:');

var list = ee.List(dataset.iterate(drylength, ee.List([
  dataset.max()
    .remap([0], [0], 0, "precipitation")
    .rename('precipitation')
])));
var totalPrecipitation = list.get(-1);

print(list);
print(totalPrecipitation);
Map.addLayer(ee.Image(totalPrecipitation), imageVisParam);

(Disclaimer: I have only modified the code so that it runs successfully, not checked whether the outputs are scientifically reasonable.)

Best Answer

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[GIS] How to filter image collection by date, excluding a date from a date range in Google Earth Engine

[GIS] Returning image for each iteration using Google Earth Engine

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