A23a Checkpoint

Import libraries¶

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import ee
import geemap

import ee import geemap

Create an interactive map¶

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Map = geemap.Map(center=[40, -100], zoom=4)

Map = geemap.Map(center=[40, -100], zoom=4)

Add Earth Engine Python script¶

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# Add Earth Engine dataset
image = ee.Image("USGS/SRTMGL1_003")

#  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#  Chapter:      A2.3 Surface Water Mapping
#  Checkpoint:   A23a
#  Authors:      K. Markert, G. Donchyts, A. Haag
#  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# *** Section 1 ***

# Define a point in Cambodia to filter by location.
point = ee.Geometry.Point(104.9632, 11.7686)

Map.centerObject(point, 11)

# Get the Sentinel-1 collection and filter by space/time.
s1Collection = ee.ImageCollection('COPERNICUS/S1_GRD') \
    .filterBounds(point) \
    .filterDate('2019-10-05', '2019-10-06') \
    .filter(ee.Filter.eq('orbitProperties_pass', 'ASCENDING')) \
    .filter(ee.Filter.eq('instrumentMode', 'IW'))

# Grab the first image in the collection.
s1Image = s1Collection.first()

# Add the Sentinel-1 image to the map.
Map.addLayer(s1Image, {
    'min': -25,
    'max': 0,
    'bands': 'VV'
}, 'Sentinel-1 image')

# Specify band to use for Otsu thresholding.
band = 'VV'

# Define a reducer to calculate a histogram of values.
histogramReducer = ee.Reducer.histogram(255, 0.1)

# Reduce all of the image values.
globalHistogram = ee.Dictionary(
    s1Image.select(band).reduceRegion({
        'reducer': histogramReducer,
        'geometry': s1Image.geometry(),
        'scale': 90,
        'maxPixels': 1e10
    }).get(band)
)

# Extract out the histogram buckets and counts per bucket.
x = ee.List(globalHistogram.get('bucketMeans'))
y = ee.List(globalHistogram.get('histogram'))

# Define a list of values to plot.
dataCol = ee.Array.cat([x, y], 1).toList()

# Define the header information for data.
columnHeader = ee.List([
    [
    {
        'label': 'Backscatter',
        'role': 'domain',
        'type': 'number'
    },
    {
        'label': 'Values',
        'role': 'data',
        'type': 'number'
    }, ]
])

# Concat the header and data for plotting.
dataTable = columnHeader.cat(dataCol)

# Create plot using the ui.Chart function with the dataTable.
# Use 'evaluate' to transfer the server-side table to the client.
# Define the chart and print it to the console.
dataTable.evaluate(function(dataTableClient) {
    chart = ui.Chart(dataTableClient) \
        .setChartType('AreaChart') \
        .setOptions({
            'title': band + ' Global Histogram',
            'hAxis': {
                'title': 'Backscatter [dB]',
                'viewWindow': {
                    'min': -35,
                    'max': 15
                }
            },
            'vAxis': {
                'title': 'Count'
            }
        })
    print(chart)
})

# See:
# https:#medium.com/google-earth/otsus-method-for-image-segmentation-f5c48f405e
def otsu(histogram):
    # Make sure histogram is an ee.Dictionary object.
    histogram = ee.Dictionary(histogram)
    # Extract relevant values into arrays.
    counts = ee.Array(histogram.get('histogram'))
    means = ee.Array(histogram.get('bucketMeans'))
    # Calculate single statistics over arrays
    size = means.length().get([0])
    total = counts.reduce(ee.Reducer.sum(), [0]).get([0])
    sum = means.multiply(counts).reduce(ee.Reducer.sum(), [0]) \
        .get([0])
    mean = sum.divide(total)
    # Compute between sum of squares, where each mean partitions the data.
    indices = ee.List.sequence(1, size)

def func_rhf(i):
        aCounts = counts.slice(0, 0, i)
        aCount = aCounts.reduce(ee.Reducer.sum(), [0]) \
            .get([0])
        aMeans = means.slice(0, 0, i)
        aMean = aMeans.multiply(aCounts) \
            .reduce(ee.Reducer.sum(), [0]).get([0]) \
            .divide(aCount)
        bCount = total.subtract(aCount)
        bMean = sum.subtract(aCount.multiply(aMean)) \
            .divide(bCount)
        return aCount.multiply(aMean.subtract(mean).pow(2)) \
            .add(
                bCount.multiply(bMean.subtract(mean).pow(2)))

    bss = indices.map(func_rhf)















    # Return the mean value corresponding to the maximum BSS.
    return means.sort(bss).get([-1])


# Apply otsu thresholding.
globalThreshold = otsu(globalHistogram)
print('Global threshold value:', globalThreshold)

# Create list of empty strings that will be used for annotation.
thresholdCol = ee.List.repeat('', x.length())
# Find the index where the bucketMean equals the threshold.
threshIndex = x.indexOf(globalThreshold)
# Set the index to the annotation text.
thresholdCol = thresholdCol.set(threshIndex, 'Otsu Threshold')

# Redefine the column header information with annotation column.
columnHeader = ee.List([
    [
    {
        'label': 'Backscatter',
        'role': 'domain',
        'type': 'number'
    },
    {
        'label': 'Values',
        'role': 'data',
        'type': 'number'
    },
    {
        'label': 'Threshold',
        'role': 'annotation',
        'type': 'string'
    }]
])

# Loop through the data rows and add the annotation column.
i) {
    i = ee.Number(i)
    row = ee.List(dataCol.get(i))
    return row.add(ee.String(thresholdCol.get(i)))
})

# Concat the header and data for plotting.
dataTable = columnHeader.cat(dataCol)

# Create plot using the ui.Chart function with the dataTable.
# Use 'evaluate' to transfer the server-side table to the client.
# Define the chart and print it to the console.
dataTable.evaluate(function(dataTableClient) {
    # loop through the client-side table and set empty strings to None
    for i in range(0, dataTableClient.length, 1):
        if (dataTableClient[i][2] === '') {
            dataTableClient[i][2] = None
        }

    chart = ui.Chart(dataTableClient) \
        .setChartType('AreaChart') \
        .setOptions({
            'title': band + \
                ' Global Histogram with Threshold annotation',
            'hAxis': {
                'title': 'Backscatter [dB]',
                'viewWindow': {
                    'min': -35,
                    'max': 15
                }
            },
            'vAxis': {
                'title': 'Count'
            },
            'annotations': {
                'style': 'line'
            }
        })
    print(chart)
})

# Apply the threshold on the image to extract water.
globalWater = s1Image.select(band).lt(globalThreshold)

# Add the water image to the map and mask 0 (no-water) values.
Map.addLayer(globalWater.selfMask(),
    {
        'palette': 'blue'
    },
    'Water (global threshold)')

#  -----------------------------------------------------------------------
#  CHECKPOINT
#  -----------------------------------------------------------------------

# Add Earth Engine dataset image = ee.Image("USGS/SRTMGL1_003") # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ # Chapter: A2.3 Surface Water Mapping # Checkpoint: A23a # Authors: K. Markert, G. Donchyts, A. Haag # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ # *** Section 1 *** # Define a point in Cambodia to filter by location. point = ee.Geometry.Point(104.9632, 11.7686) Map.centerObject(point, 11) # Get the Sentinel-1 collection and filter by space/time. s1Collection = ee.ImageCollection('COPERNICUS/S1_GRD') \ .filterBounds(point) \ .filterDate('2019-10-05', '2019-10-06') \ .filter(ee.Filter.eq('orbitProperties_pass', 'ASCENDING')) \ .filter(ee.Filter.eq('instrumentMode', 'IW')) # Grab the first image in the collection. s1Image = s1Collection.first() # Add the Sentinel-1 image to the map. Map.addLayer(s1Image, { 'min': -25, 'max': 0, 'bands': 'VV' }, 'Sentinel-1 image') # Specify band to use for Otsu thresholding. band = 'VV' # Define a reducer to calculate a histogram of values. histogramReducer = ee.Reducer.histogram(255, 0.1) # Reduce all of the image values. globalHistogram = ee.Dictionary( s1Image.select(band).reduceRegion({ 'reducer': histogramReducer, 'geometry': s1Image.geometry(), 'scale': 90, 'maxPixels': 1e10 }).get(band) ) # Extract out the histogram buckets and counts per bucket. x = ee.List(globalHistogram.get('bucketMeans')) y = ee.List(globalHistogram.get('histogram')) # Define a list of values to plot. dataCol = ee.Array.cat([x, y], 1).toList() # Define the header information for data. columnHeader = ee.List([ [ { 'label': 'Backscatter', 'role': 'domain', 'type': 'number' }, { 'label': 'Values', 'role': 'data', 'type': 'number' }, ] ]) # Concat the header and data for plotting. dataTable = columnHeader.cat(dataCol) # Create plot using the ui.Chart function with the dataTable. # Use 'evaluate' to transfer the server-side table to the client. # Define the chart and print it to the console. dataTable.evaluate(function(dataTableClient) { chart = ui.Chart(dataTableClient) \ .setChartType('AreaChart') \ .setOptions({ 'title': band + ' Global Histogram', 'hAxis': { 'title': 'Backscatter [dB]', 'viewWindow': { 'min': -35, 'max': 15 } }, 'vAxis': { 'title': 'Count' } }) print(chart) }) # See: # https:#medium.com/google-earth/otsus-method-for-image-segmentation-f5c48f405e def otsu(histogram): # Make sure histogram is an ee.Dictionary object. histogram = ee.Dictionary(histogram) # Extract relevant values into arrays. counts = ee.Array(histogram.get('histogram')) means = ee.Array(histogram.get('bucketMeans')) # Calculate single statistics over arrays size = means.length().get([0]) total = counts.reduce(ee.Reducer.sum(), [0]).get([0]) sum = means.multiply(counts).reduce(ee.Reducer.sum(), [0]) \ .get([0]) mean = sum.divide(total) # Compute between sum of squares, where each mean partitions the data. indices = ee.List.sequence(1, size) def func_rhf(i): aCounts = counts.slice(0, 0, i) aCount = aCounts.reduce(ee.Reducer.sum(), [0]) \ .get([0]) aMeans = means.slice(0, 0, i) aMean = aMeans.multiply(aCounts) \ .reduce(ee.Reducer.sum(), [0]).get([0]) \ .divide(aCount) bCount = total.subtract(aCount) bMean = sum.subtract(aCount.multiply(aMean)) \ .divide(bCount) return aCount.multiply(aMean.subtract(mean).pow(2)) \ .add( bCount.multiply(bMean.subtract(mean).pow(2))) bss = indices.map(func_rhf) # Return the mean value corresponding to the maximum BSS. return means.sort(bss).get([-1]) # Apply otsu thresholding. globalThreshold = otsu(globalHistogram) print('Global threshold value:', globalThreshold) # Create list of empty strings that will be used for annotation. thresholdCol = ee.List.repeat('', x.length()) # Find the index where the bucketMean equals the threshold. threshIndex = x.indexOf(globalThreshold) # Set the index to the annotation text. thresholdCol = thresholdCol.set(threshIndex, 'Otsu Threshold') # Redefine the column header information with annotation column. columnHeader = ee.List([ [ { 'label': 'Backscatter', 'role': 'domain', 'type': 'number' }, { 'label': 'Values', 'role': 'data', 'type': 'number' }, { 'label': 'Threshold', 'role': 'annotation', 'type': 'string' }] ]) # Loop through the data rows and add the annotation column. i) { i = ee.Number(i) row = ee.List(dataCol.get(i)) return row.add(ee.String(thresholdCol.get(i))) }) # Concat the header and data for plotting. dataTable = columnHeader.cat(dataCol) # Create plot using the ui.Chart function with the dataTable. # Use 'evaluate' to transfer the server-side table to the client. # Define the chart and print it to the console. dataTable.evaluate(function(dataTableClient) { # loop through the client-side table and set empty strings to None for i in range(0, dataTableClient.length, 1): if (dataTableClient[i][2] === '') { dataTableClient[i][2] = None } chart = ui.Chart(dataTableClient) \ .setChartType('AreaChart') \ .setOptions({ 'title': band + \ ' Global Histogram with Threshold annotation', 'hAxis': { 'title': 'Backscatter [dB]', 'viewWindow': { 'min': -35, 'max': 15 } }, 'vAxis': { 'title': 'Count' }, 'annotations': { 'style': 'line' } }) print(chart) }) # Apply the threshold on the image to extract water. globalWater = s1Image.select(band).lt(globalThreshold) # Add the water image to the map and mask 0 (no-water) values. Map.addLayer(globalWater.selfMask(), { 'palette': 'blue' }, 'Water (global threshold)') # ----------------------------------------------------------------------- # CHECKPOINT # -----------------------------------------------------------------------

Display the interactive map¶

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Map

Map