Losing properties while multiplying or dividing ImageCollection Google Earth Engine

Question

I've just started working with Google Earth Engine and, while running some tests, I stumbled into this problem. If you initialize an imageCollection and print it, the console gives you something like this:

ImageCollection (200 elements)
type: ImageCollection
bands: []
features: List (200 elements)
  0: Image LANDSAT/LT05/C01/T1_SR/LT05_191028_20080526 (6 bands)
   type: Image
   id: LANDSAT/LT05/C01/T1_SR/LT05_191028_20080526
   version: 1521654404763968
   bands: List (6 elements)
   properties: Object (22 properties)
  1: Image LANDSAT/LT05/C01/T1_SR/LT05_191028_20080611 (6 bands)
  2: Image LANDSAT/LT05/C01/T1_SR/LT05_191028_20080627 (6 bands)
  3: Image LANDSAT/LT05/C01/T1_SR/LT05_191028_20080713 (6 bands)
  4: Image LANDSAT/LT05/C01/T1_SR/LT05_191028_20080729 (6 bands)
  .
  .

and so on and so forth. However, if I try to map a .multiply() or a .divide() function over the collection and print it, I lose all informations on the images. Let's say I want to rescale the collection pixel values, my code will be:

var scaledCollection = collection.map(
    function(img) {
      return img.multiply(0.0001);
    });

print(scaledCollection);

The output in the console becomes:

ImageCollection (200 elements)
type: ImageCollection
bands: []
features: List (200 elements)
  0: Image (6 bands)
   type: Image
   bands: List (6 elements)
   properties: Object (19 properties)
  1: Image (6 bands)
  2: Image (6 bands)
  3: Image (6 bands)
  4: Image (6 bands)
  .
  .

Among other things, the properties lost are system:asset_size, system:footprint and system:time_start. Since the object structure remains the same (ImageCollection), why do some properties disappear if the only thing that I'm doing is iterate a map algebra operation over a collection?

Thanks in advance.

Answer 1

You can copy them to the new image using:

newImg.copyProperties(oldImg, img.propertyNames())

Applied to your example:

var scaledCollection = collection.map(
    function(img) {
      return img.multiply(0.0001).copyProperties(img, img.propertyNames());
    });

print(scaledCollection);

I can only speculate that properties are not maintained because it cannot be assumed that they still apply or are correct. Suppose you multiply two Landsat Images from adjacent paths together - the time, footprint, and other metadata regarding sun angle, etc are no longer correct. I understand your case is a simple scalar operation, but the .multiply() image method does not know that (it could be the former case).