Google EarthEngine: Time series of reduceRegion()

Question

I am using the Google EarthEngine Python API. I have an image collection (MODIS) and would like to extract a time series containing mean NDVI for a region for each time step.

Currently, I am iterating over single images and extract the values for each. Like

feature_geometry = {
    'type': 'MultiPolygon',
    'coordinates': [[[
        [-120, 35],
        [-120.001, 35],
        [-120.001, 35.001],
        [-120, 35.001],
        [-120, 35]
    ]]]
}
ee.Initialize()
feature = ee.Feature(feature_geometry)
collection = ee.ImageCollection(
    'MODIS/006/MOD13Q1').filterDate('2017-01-01', '2017-05-01')
images = [
    item.get('id') for item in collection.getInfo().get('features')]
for image in images:
    print(ee.Image(image).reduceRegion(
        ee.Reducer.mean(), feature.geometry()).getInfo()['NDVI'])

The question: Is there a way to get the same result in a single request to EarthEngine, since I tend to run into request limits.

Ideas so far: a) Is there a way to somehow use ImageCollection.reduce()?, b) Create a single image where bands are time steps and then run Image.reduceRegion() on it. — Falk Schuetzenmeister, Oct 27 '17 at 18:12
Would not call that a duplicate but it might be a start. Will evaluate later this week. Thank you. — Falk Schuetzenmeister, Oct 30 '17 at 18:18

score 6 · Accepted Answer · answered Oct 31 '17 at 01:46

6

Here's an example of what I think you're asking:

import ee
ee.Initialize()

feature_geometry = {
    'type': 'MultiPolygon',
    'coordinates': [[[
        [-120, 35],
        [-120.001, 35],
        [-120.001, 35.001],
        [-120, 35.001],
        [-120, 35]
    ]]]
}

collection = ee.ImageCollection(
    'MODIS/006/MOD13Q1').filterDate('2017-01-01', '2017-05-01')

def setProperty(image):
    dict = image.reduceRegion(ee.Reducer.mean(), feature_geometry)
    return image.set(dict)

withMean = collection.map(setProperty)

print withMean.aggregate_array('NDVI').getInfo()

answered Oct 31 '17 at 01:46

Nicholas Clinton

873
4
6

Thank so much, that works great. Is there a simple way to add the image id to the return? I am trying to figure that out right now. – Falk Schuetzenmeister Oct 31 '17 at 18:26
Ok, got it. I just do withMean.getInfo() and traverse the giant dictionary. – Falk Schuetzenmeister Oct 31 '17 at 18:47
Yes. You can get the array-of-whatever from the properties of the image collection. Note that `aggregate_array()` is a shortcut for `reduce(ee.Reducer.toList(),...)`, the latter resulting in a more explicit map-reduce idiom. – Nicholas Clinton Oct 31 '17 at 23:35

Google EarthEngine: Time series of reduceRegion()

1 Answers1