system:time_index falta Google Earth Engine

Question

Escribí el siguiente código donde una ImageCollection se multiplica por diferentes factores de escala (469 en total). He creado una lista con todos los factores y la he combinado con la ImageCollection para multiplicar cada mes con su correspondiente factor. Los resultados muestran una colección final con el factor de escala aplicado.

https://code.earthengine.google.com/203deb3d2f9c19afe9236f4e53efbd0c

Sin embargo, en la ImageCollection final falta la propiedad system:time_index. Estaba tratando de usar .CopyProperties pero honestamente, no sé dónde.

¿Alguna sugerencia?

var sum_month = ee.List.sequence(0, 39*12).map(function(n) { // .sequence: number of years from starting year to present
  var start = ee.Date('1981-01-01').advance(n, 'month'); // Starting date
  var end = start.advance(1, 'month'); // Step by each iteration

  return ee.ImageCollection("UCSB-CHG/CHIRPS/DAILY")
        .filterDate(start, end)
        .sum()
        .set('system:time_start', start.millis());
});

var collection = ee.ImageCollection(sum_month)

var list = ee.List([
  0.884748080310752,
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0.884748080310752,
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//Create an Image Collection, each image.constant(value of list)
var Image_Multiplication = function(collection,valuelist){
  var list_size = valuelist.size();
  var sequence = ee.List.sequence(0,list_size.subtract(ee.Number(1)));

  var value_collection = ee.ImageCollection(sequence.map(function(i){
    return ee.Image.constant(list.get(i));
  }));

  var value_collection_list = value_collection.toList(value_collection.size());
  var collection_list = collection.toList(collection.size());

  return ee.ImageCollection(sequence.map(function(i){
    var image = ee.Image(collection_list.get(ee.Number(i)));
    var value_image = ee.Image(value_collection_list.get(ee.Number(i)));
    var multiplication_image = image.multiply(value_image);
    return ee.Image(multiplication_image);
  }));

};

var value_collection = Image_Multiplication(collection,list);

print(value_collection);

Map.addLayer(collection.first(),{},'Original Images',false);
Map.addLayer(value_collection.first(),{},'Multiplied Image',false);

Answer 1

Tendrá que copiar las propiedades de la imagen dos veces. La primera vez, crea una imagen sumada a partir de varias imágenes entre start y end Por lo tanto, usted tendrá que elegir una imagen para copiar las propiedades de. para simplificar, elijo la primera imagen:

var sum_month = ee.List.sequence(0, 39*12).map(function(n) { 
  var start = ee.Date('1981-01-01').advance(n, 'month'); // Starting date
  var end = start.advance(1, 'month'); // Step by each iteration

  // get collection filtered
  var imgColTemp = ee.ImageCollection("UCSB-CHG/CHIRPS/DAILY").filterDate(start, end);
  // get properties of the first image between the start and end
  var props = imgColTemp.first().toDictionary(imgColTemp.first().propertyNames());

  return imgColTemp.sum()
        .set('system:time_start', start.millis())
        .set(props); // set the properties
});

A continuación, se crea una nueva imagen con multiply y las imágenes pierden sus propiedades. Por lo tanto, cópialas de nuevo al final de tu script:

    return ee.Image(multiplication_image)
          // copy image properties again
          .set(image.toDictionary(image.propertyNames()));

Ver código completo

Answer 2

Si utiliza .copyProperties() con argumentos por defecto, sólo se copiarán las propiedades ordinarias (es decir, las que no son del sistema).

Para copiar todas las propiedades, incluidas las del sistema, utilice .copyProperties(image, image.propertyNames()) o especifique una lista de propiedades que desee copiar.

Ver enlace al script .