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Given a FeatureCollection that contains multiple Features (each consisting of polygons), I would like to use the Python API to loop through them all, and batch export them to Google Cloud Storage.

However, I can't figure out how to make all the code server-side, and hence my code runs extremely slowly (using the major no-no of getInfo inside a loop).

for i in range(0, 1000):
  region = ee.Feature(myFeatCol.get(i)).geometry()

  task = ee.batch.Export.image.toCloudStorage(
    image = naip.clip(region),
    description = 'uniqueName'+str(i),
    fileNamePrefix = 'uniqueName'+str(i),
    bucket = 'myGoogleBucketName',
    scale = 1,
    region = region.getInfo()['coordinates'])

  task.start()

The objective here is to end up with 1000 images in the bucket. It works, but is painstakingly slow! Several-minutes-per-image slow.

How can I create an efficient server-side loop to export an entire FeatureCollection?

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Currently, there is no way to export a collection of images; an export task can only generate one image, and each export task requires a separate call to the server and, more importantly, will be scheduled and run separately.

(There is an export type that produces multiple images — Export.map — but it would be quite tedious and inconvenient to rearrange your data into fake map tiles, and it would end up with every image square and the same size, have a lot of extra images, incorrect metadata, and so on.)

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  • Thanks for the quick response Kevin! I have tried creating a function and using .map to apply it to the whole FeatureCollection, but it fails to create unique names (as you expected)! Another idea I had was to preprocess the entire FeatureCollection with a single getInfo(), then reshape it into an ee.Dictionary, which I could then have GEE .map() over, but for some reason I still can't figure out the syntax to let GEE read it on the server side. "ee.Dictionary(myDict).get(feature.id())" returns the error "TypeError: Object of type 'ComputedObject' is not JSON serializable". Any ideas?
    – curiouserand
    Commented Oct 3, 2019 at 16:43
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    @curiouserand This is simply not possible. Export is a single API call, and you can't use a server-side map() to make more API calls. Even if it worked, it would still schedule separate export tasks and they would take the same amount of time.
    – Kevin Reid
    Commented Oct 3, 2019 at 19:45

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