Adding time as 'year' band to image collection

Question

I have uploaded 35 images (annual maximum NDVI) to GEE and put them manually (drag and drop) together to an image collection. There is no time band, but for the ee.Reducer.linearFit() function, I need to add the year of each image as a time band to each image.

I found this image.addBands(image.metadata('system:time_start') but since there is no system:time_start, this can not be used.

This is another way of doing it:

var Maxyears = ee.List.sequence(1981, 2015);
var NDVIMax35 = ee.ImageCollection.fromImages(
  ee.List.sequence(0, NDVI_Max_Stack.bandNames().length().subtract(1))
    .map(function(i) {
      return NDVI_Max_Stack.select(ee.Number(i))
        .rename('Max35')
        .set('year', Maxyears.get(i));
    })
  );

But I have the image collection already, so I need a different function than fromImages()

https://code.earthengine.google.com/?scriptPath=users%2Fwelp1992%2FMaster%3AClimate%20Data

Answer 1

You can edit the metadata to specify a start time when you upload an image:

Or you can edit it after upload, but after you open the asset info dialog, make sure to click the “Edit” toggle button to enable editing. You must do this before putting them in the collection as they are not editable within the collection.

Then the images will have a system:time_start property as you're familiar with and you can use any of the usual techniques to turn that into a band.

If you are uploading from the command line with earthengine upload then use the --time_start option. If you are uploading using manifests use the start_time field.

---

If you have an existing collection that does not have standard time metadata, but has the information somewhere else such as the image's ID, then you can map over the collection to copy it into system:time_start, such as in this example where I delete the dates from a standard collection and reconstitute them:

var imageCollection = ee.ImageCollection("LANDSAT/LO08/C01/T1");
print(imageCollection.first().date().format());

var noDates = imageCollection.map(function (image) {
  return image.set('system:time_start', null);
});
print(noDates.first().date().format());  // will error

var withDatesAgain = noDates.map(function (image) {
  return image.set('system:time_start', 
    ee.Date.parse(
      "YYYYMMdd",
      image.id().split("_").get(2)
    ).millis());
});
print(withDatesAgain.first().date().format());

https://code.earthengine.google.com/22303aa6f4be4738e46acce8d0f0dda5

Answer 2

This is another method in the cases of uploading a new collection.

1. You can use copyProperties method to copy 'system:time_start' when you calculate NDVI.

    var NDVI = sentinel.map(
    function(img) {
         return img.normalizedDifference(['B8','B4'])
                  .rename('NDVI')
                  .copyProperties(img, ['system:time_start']);
    });

2. Transform 'system:time_start' to 'year' and then add 'year' band to the image collection.

   var dataset = ee.ImageCollection('LANDSAT/LT05/C01/T1_SR')
                 .map(function(img) {
                         var date = ee.Image(img).date();
                         var Year = date.get('year');
                         
                         var B1 = img.select(['B1']);
                         var zones = ee.Image(0).where(B1.gt(-5000), Year);
                         var zones2 = zones.mask(zones).rename('year');
                         
                         return img.addBands(zones2)
                      });