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Using ScriptProcessor to calculate the Normalized Difference Vegetation Index (NDVI) using Sentinel-2 satellite data

The following code samples show you how to calculate the normalized difference vegetation index of a specific geographical area using the purpose-built geospatial image within a Studio Classic notebook and run a large-scale workload with Amazon SageMaker Processing using ScriptProcessor from the SageMaker Python SDK.

This demo also uses an Amazon SageMaker Studio Classic notebook instance that uses the geospatial kernel and instance type. To learn how to create a Studio Classic geospatial notebook instance, see Create an Amazon SageMaker Studio Classic notebook using the geospatial image.

You can follow along with this demo in your own notebook instance by copying and pasting the following code snippets:

Query the Sentinel-2 raster data collection using SearchRasterDataCollection

With search_raster_data_collection you can query supported raster data collections. This example uses data that's pulled from Sentinel-2 satellites. The area of interest (AreaOfInterest) specified is rural northern Iowa, and the time range (TimeRangeFilter) is January 1, 2022 to December 30, 2022. To see the available raster data collections in your Amazon Web Services Region use list_raster_data_collections. To see a code example using this API, see ListRasterDataCollections in the Amazon SageMaker Developer Guide.

In following code examples you use the ARN associated with Sentinel-2 raster data collection, arn:aws:sagemaker-geospatial:us-west-2:378778860802:raster-data-collection/public/nmqj48dcu3g7ayw8.

A search_raster_data_collection API request requires two parameters:

The following code example contains the necessary key-value pairs needed for the RasterDataCollectionQuery parameter saved to the search_rdc_query variable.

search_rdc_query = {
    "AreaOfInterest": {
        "AreaOfInterestGeometry": {
            "PolygonGeometry": {
                "Coordinates": [[
                    [
              -94.50938680498298,
              43.22487436936203
            ],
            [
              -94.50938680498298,
              42.843474642037194
            ],
            [
              -93.86520004156142,
              42.843474642037194
            ],
            [
              -93.86520004156142,
              43.22487436936203
            ],
            [
              -94.50938680498298,
              43.22487436936203
            ]
               ]]
            }
        }
    },
    "TimeRangeFilter": {"StartTime": "2022-01-01T00:00:00Z", "EndTime": "2022-12-30T23:59:59Z"}
}

To make the search_raster_data_collection request, you must specify the ARN of the Sentinel-2 raster data collection: arn:aws:sagemaker-geospatial:us-west-2:378778860802:raster-data-collection/public/nmqj48dcu3g7ayw8. You also must need to pass in the Python dictionary that was defined previously, which specifies query parameters.

## Creates a SageMaker Geospatial client instance 
sm_geo_client= session.create_client(service_name="sagemaker-geospatial")

search_rdc_response1 = sm_geo_client.search_raster_data_collection(
    Arn='arn:aws:sagemaker-geospatial:us-west-2:378778860802:raster-data-collection/public/nmqj48dcu3g7ayw8',
    RasterDataCollectionQuery=search_rdc_query
)

The results of this API can not be paginated. To collect all the satellite images returned by the search_raster_data_collection operation, you can implement a while loop. This checks forNextToken in the API response:

## Holds the list of API responses from search_raster_data_collection
items_list = []
while search_rdc_response1.get('NextToken') and search_rdc_response1['NextToken'] != None:
    items_list.extend(search_rdc_response1['Items'])
    
    search_rdc_response1 = sm_geo_client.search_raster_data_collection(
    	Arn='arn:aws:sagemaker-geospatial:us-west-2:378778860802:raster-data-collection/public/nmqj48dcu3g7ayw8',
        RasterDataCollectionQuery=search_rdc_query, 
        NextToken=search_rdc_response1['NextToken']
    )

The API response returns a list of URLs under the Assets key corresponding to specific image bands. The following is a truncated version of the API response. Some of the image bands were removed for clarity.

{
	'Assets': {
        'aot': {
            'Href': 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/15/T/UH/2022/12/S2A_15TUH_20221230_0_L2A/AOT.tif'
        },
        'blue': {
            'Href': 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/15/T/UH/2022/12/S2A_15TUH_20221230_0_L2A/B02.tif'
        },
        'swir22-jp2': {
            'Href': 's3://sentinel-s2-l2a/tiles/15/T/UH/2022/12/30/0/B12.jp2'
        },
        'visual-jp2': {
            'Href': 's3://sentinel-s2-l2a/tiles/15/T/UH/2022/12/30/0/TCI.jp2'
        },
        'wvp-jp2': {
            'Href': 's3://sentinel-s2-l2a/tiles/15/T/UH/2022/12/30/0/WVP.jp2'
        }
    },
    'DateTime': datetime.datetime(2022, 12, 30, 17, 21, 52, 469000, tzinfo = tzlocal()),
    'Geometry': {
        'Coordinates': [
            [
                [-95.46676936182894, 43.32623760511659],
                [-94.11293433656887, 43.347431265475954],
                [-94.09532154452742, 42.35884880571144],
                [-95.42776890002203, 42.3383710796791],
                [-95.46676936182894, 43.32623760511659]
            ]
        ],
        'Type': 'Polygon'
    },
    'Id': 'S2A_15TUH_20221230_0_L2A',
    'Properties': {
        'EoCloudCover': 62.384969,
        'Platform': 'sentinel-2a'
    }
}

In the next section, you create a manifest file using the 'Id' key from the API response.

Create an input manifest file using the Id key from the search_raster_data_collection API response

When you run a processing job, you must specify a data input from Amazon S3. The input data type can either be a manifest file, which then points to the individual data files. You can also add a prefix to each file that you want processed. The following code example defines the folder where your manifest files will be generated.

Use SDK for Python (Boto3) to get the default bucket and the ARN of the execution role that is associated with your Studio Classic notebook instance:

sm_session = sagemaker.session.Session()
s3 = boto3.resource('s3')
# Gets the default excution role associated with the notebook
execution_role_arn = sagemaker.get_execution_role() 

# Gets the default bucket associated with the notebook
s3_bucket = sm_session.default_bucket() 

# Can be replaced with any name
s3_folder = "script-processor-input-manifest"

Next, you create a manifest file. It will hold the URLs of the satellite images that you wanted processed when you run your processing job later in step 4.

# Format of a manifest file
manifest_prefix = {}
manifest_prefix['prefix'] = 's3://' + s3_bucket + '/' + s3_folder + '/'
manifest = [manifest_prefix]

print(manifest)

The following code sample returns the S3 URI where your manifest files will be created.

[{'prefix': 's3://sagemaker-us-west-2-111122223333/script-processor-input-manifest/'}]

All the response elements from the search_raster_data_collection response are not needed to run the processing job.

The following code snippet removes the unnecessary elements 'Properties', 'Geometry', and 'DateTime'. The 'Id' key-value pair, 'Id': 'S2A_15TUH_20221230_0_L2A', contains the year and the month. The following code example parses that data to create new keys in the Python dictionary dict_month_items. The values are the assets that are returned from the SearchRasterDataCollection query.

# For each response get the month and year, and then remove the metadata not related to the satelite images.
dict_month_items = {}
for item in items_list:
    # Example ID being split: 'S2A_15TUH_20221230_0_L2A' 
    yyyymm = item['Id'].split("_")[2][:6]
    if yyyymm not in dict_month_items:
        dict_month_items[yyyymm] = []
    
    # Removes uneeded metadata elements for this demo 
    item.pop('Properties', None)
    item.pop('Geometry', None)
    item.pop('DateTime', None)

    # Appends the response from search_raster_data_collection to newly created key above
    dict_month_items[yyyymm].append(item)

This code example uploads the dict_month_items to Amazon S3 as a JSON object using the .upload_file() API operation:

## key_ is the yyyymm timestamp formatted above
## value_ is the reference to all the satellite images collected via our searchRDC query 
for key_, value_ in dict_month_items.items():
    filename = f'manifest_{key_}.json'
    with open(filename, 'w') as fp:
        json.dump(value_, fp)
    s3.meta.client.upload_file(filename, s3_bucket, s3_folder + '/' + filename)
    manifest.append(filename)
    os.remove(filename)

This code example uploads a parent manifest.json file that points to all the other manifests uploaded to Amazon S3. It also saves the path to a local variable: s3_manifest_uri. You'll use that variable again to specify the source of the input data when you run the processing job in step 4.

with open('manifest.json', 'w') as fp:
    json.dump(manifest, fp)
s3.meta.client.upload_file('manifest.json', s3_bucket, s3_folder + '/' + 'manifest.json')
os.remove('manifest.json')

s3_manifest_uri = f's3://{s3_bucket}/{s3_folder}/manifest.json'

Now that you created the input manifest files and uploaded them, you can write a script that processes your data in the processing job. It processes the data from the satellite images, calculates the NDVI, and then returns the results to a different Amazon S3 location.

Write a script that calculates the NDVI

Amazon SageMaker Studio Classic supports the use of the %%writefile cell magic command. After running a cell with this command, its contents will be saved to your local Studio Classic directory. This is code specific to calculating NDVI. However, the following can be useful when you write your own script for a processing job:

%%writefile compute_ndvi.py

import os
import pickle
import sys
import subprocess
import json
import rioxarray

if __name__ == "__main__":
    print("Starting processing")
    
    input_data_path = '/opt/ml/processing/input_data/'
    input_files = []
    
    for current_path, sub_dirs, files in os.walk(input_data_path):
        for file in files:
            if file.endswith(".json"):
                input_files.append(os.path.join(current_path, file))
    
    print("Received {} input_files: {}".format(len(input_files), input_files))

    items = []
    for input_file in input_files:
        full_file_path = os.path.join(input_data_path, input_file)
        print(full_file_path)
        with open(full_file_path, 'r') as f:
            items.append(json.load(f))
            
    items = [item for sub_items in items for item in sub_items]

    for item in items:
        red_uri = item["Assets"]["red"]["Href"]
        nir_uri = item["Assets"]["nir"]["Href"]

        red = rioxarray.open_rasterio(red_uri, masked=True)
        nir = rioxarray.open_rasterio(nir_uri, masked=True)

        ndvi = (nir - red)/ (nir + red)
        
        file_name = 'ndvi_' + item["Id"] + '.tif'
        output_path = '/opt/ml/processing/output_data'
        output_file_path = f"{output_path}/{file_name}"
        
        ndvi.rio.to_raster(output_file_path)
        print("Written output:", output_file_path)

You now have a script that can calculate the NDVI. Next, you can create an instance of the ScriptProcessor and run your Processing job.

Creating an instance of the ScriptProcessor class

This demo uses the ScriptProcessor class that is available via the Amazon SageMaker Python SDK. First, you need to create an instance of the class, and then you can start your Processing job by using the .run() method.

from sagemaker.processing import ScriptProcessor, ProcessingInput, ProcessingOutput

image_uri = '081189585635.dkr.ecr.us-west-2.amazonaws.com/sagemaker-geospatial-v1-0:latest'

processor = ScriptProcessor(
	command=['python3'],
	image_uri=image_uri,
	role=execution_role_arn,
	instance_count=4,
	instance_type='ml.m5.4xlarge',
	sagemaker_session=sm_session
)

print('Starting processing job.')

When you start your Processing job, you need to specify a ProcessingInput object. In that object, you specify the following:

s3_output_prefix_url = f"s3://{s3_bucket}/{s3_folder}/output"

processor.run(
    code='compute_ndvi.py',
    inputs=[
        ProcessingInput(
            source=s3_manifest_uri,
            destination='/opt/ml/processing/input_data/',
            s3_data_type="ManifestFile",
            s3_data_distribution_type="ShardedByS3Key"
        ),
    ],
    outputs=[
        ProcessingOutput(
            source='/opt/ml/processing/output_data/',
            destination=s3_output_prefix_url,
            s3_upload_mode="Continuous"
        )
    ]
)

The following code example uses the .describe() method to get details of your Processing job.

preprocessing_job_descriptor = processor.jobs[-1].describe()
s3_output_uri = preprocessing_job_descriptor["ProcessingOutputConfig"]["Outputs"][0]["S3Output"]["S3Uri"]
print(s3_output_uri)

Visualizing your results using matplotlib

With the Matplotlib Python library, you can plot raster data. Before you plot the data, you need to calculate the NDVI using sample images from the Sentinel-2 satellites. The following code example opens the image arrays using the .open_rasterio() API operation, and then calculates the NDVI using the nir and red image bands from the Sentinel-2 satellite data.

# Opens the python arrays 
import rioxarray

red_uri = items[25]["Assets"]["red"]["Href"]
nir_uri = items[25]["Assets"]["nir"]["Href"]

red = rioxarray.open_rasterio(red_uri, masked=True)
nir = rioxarray.open_rasterio(nir_uri, masked=True)

# Calculates the NDVI
ndvi = (nir - red)/ (nir + red)

# Common plotting library in Python 
import matplotlib.pyplot as plt

f, ax = plt.subplots(figsize=(18, 18))
ndvi.plot(cmap='viridis', ax=ax)
ax.set_title("NDVI for {}".format(items[25]["Id"]))
ax.set_axis_off()
plt.show()

The output of the preceding code example is a satellite image with the NDVI values overlaid on it. An NDVI value near 1 indicates lots of vegetation is present, and values near 0 indicate no vegetation is presentation.

This completes the demo of using ScriptProcessor.