Mapping over image collection

Mapping over an ImageCollection

Reference: https://developers.google.com/earth-engine/guides/ic_mapping

To apply a function to every Image in an ImageCollection use imageCollection.map(). The only argument to map() is a function which takes one parameter: an ee.Image.

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import ee
import geemap

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Map = geemap.Map()

Define a function¶

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def minus1000(num):
    result = ee.Number(num).subtract(1000)
    return result

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result = minus1000(2000)
print(result.getInfo())

Apply the map function¶

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years = ee.List.sequence(2000, 2010)
print(years.getInfo())

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results = years.map(minus1000)
print(results.getInfo())

Use anonymous/lambda function¶

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years = ee.List.sequence(2000, 2010)

results = years.map(lambda x: ee.Number(x).subtract(1000))
print(results.getInfo())

Filter an ImageCollection¶

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roi = ee.Geometry.Point(-83.92, 35.96)  # Knoxville, Tennessee

collection = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR') \
    .filterBounds(roi) \
    .filterDate("2019-01-01", "2020-01-01") \
    .sort("CLOUD_COVER")

print(collection.size().getInfo())

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image = collection.first()

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# image.getInfo()

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geemap.image_props(image).getInfo()

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collection.aggregate_array("CLOUD_COVER").getInfo()

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vis_params = {
    'bands': ['B5', 'B4', 'B3'],
    'min': 0, 
    'max': 3000,
    'gamma': 1.0
}

Map.addLayer(image, vis_params, "First image")
Map.centerObject(image)
Map

Select the best image each year¶

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roi = ee.Geometry.Point(-83.92, 35.96)  # Knoxville, Tennessee


def best_image(year):
    start_date = ee.Date.fromYMD(year, 1, 1)
    end_date = start_date.advance(1, 'year')
    
    image = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR') \
        .filterBounds(roi) \
        .filterDate(start_date, end_date) \
        .sort("CLOUD_COVER") \
        .first()
    
    return image

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start_year = 2013
end_year = 2020
years = ee.List.sequence(start_year, end_year)
year_list = years.getInfo()
print(year_list)

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images = years.map(best_image)

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count = images.size().getInfo()
print(count)

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ee.ImageCollection(images).aggregate_array("CLOUD_COVER").getInfo()

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for index in range(0, count):
    image = ee.Image(images.get(index))
    layer_name = "Image " + str(year_list[index])
    Map.addLayer(image, vis_params, layer_name, False)

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Map

Set image properties¶

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collection = ee.ImageCollection(images)

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collection.aggregate_array("system:time_start").getInfo()

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collection = collection.map(lambda img: img.set({"DATE": ee.Date(img.get("system:time_start")).format("YYYY-MM-dd")}))

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collection.aggregate_array("DATE").getInfo()

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Last update: 2021-03-17