Fix: Sentinel-2 Metadata Parsing Issue Post 9/24

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Hey guys! It looks like there's a small but crucial update needed for those of you working with Sentinel-2 imagery, especially if you're using geowombat for BRDF scripts. This issue popped up due to a change in the Sentinel-2 metadata files after September 24th, 2024. Let's dive into the details and how to fix it.

The Issue: Metadata Link Update

The core of the problem lies in the metadata links for Sentinel-2 images. Previously, the metadata URLs for all sensors (2A, 2B, and 2C) pointed to https://psd-14.... However, after September 24th, these links have been updated to https://psd-15.... This seemingly minor change can cause parsing failures in scripts that rely on the old URL structure, specifically in the radiometry/angles.py file when trying to extract angle data.

To put it simply, if your scripts are still looking for metadata at the old psd-14 address, they won't find it at the new psd-15 address, leading to errors. This is a classic example of how a small change in data structure can have a significant impact on your workflows.

The devil is in the details, as they say, and in this case, the details are the specific lines of code that need updating. Let's pinpoint exactly where the fix needs to happen.

Identifying the Affected Code

The issue primarily affects the radiometry/angles.py file. Within this file, there are specific lines that need to be adjusted to accommodate the new metadata URL. According to the initial report, the critical lines are 288, 310, and 342. These lines are responsible for fetching and processing the angle data from the metadata files.

Digging into the code, these lines likely contain the hardcoded https://psd-14... URL. When the script encounters the new https://psd-15... URL, it fails to recognize it, resulting in a parsing error. The fix, therefore, involves updating these lines to correctly handle the new URL structure.

It's crucial to make these updates accurately to ensure that your scripts can seamlessly process Sentinel-2 imagery from both before and after the metadata update. Now, let's talk about the solution.

The Solution: Updating radiometry/angles.py

To resolve this issue, the radiometry/angles.py file needs to be updated to handle both the old and new metadata URLs. There are a couple of ways to approach this:

  1. Modify the existing lines to accept either psd-14 or psd-15 URLs: This approach involves updating the code to check for both URL patterns. You can use regular expressions or conditional statements to achieve this. For example, you might use an if statement to check if the URL contains either psd-14 or psd-15 and then proceed accordingly. This is a flexible solution that ensures compatibility with both old and new data.
  2. Update the lines to use the new psd-15 URL: This is a simpler fix, but it might not be ideal if you need to process older Sentinel-2 images with the psd-14 metadata. If you're primarily working with recent data, this could be a quick and straightforward solution. However, keep in mind the potential for issues when dealing with historical data.

The best approach depends on your specific needs and the range of data you're working with. For maximum compatibility, the first approach (handling both URLs) is generally recommended.

Let's break down the code modification process step by step to make sure we're all on the same page.

Step-by-Step Code Modification

Here’s a detailed guide on how to modify the radiometry/angles.py file to handle the updated metadata URLs:

  1. Locate the file: First, you need to find the radiometry/angles.py file in your geowombat installation. This might be in a different location depending on how you installed geowombat, but it's typically within the geowombat package directory.

  2. Open the file in a text editor: Use a text editor or an IDE (Integrated Development Environment) to open the radiometry/angles.py file. Popular choices include VS Code, Sublime Text, Atom, or even a basic text editor like Notepad (though a more advanced editor is recommended for code editing).

  3. Navigate to line 288: Scroll down or use the editor's line number search feature to find line 288. This is one of the lines that needs modification.

  4. Modify the URL handling: On line 288, you'll likely find code that constructs the metadata URL. You need to modify this code to accept either psd-14 or psd-15. Here’s an example of how you might do this using a conditional statement:

    if "psd-14" in url:
    metadata_url = url.replace("psd-14", "psd-14") # No change here, but included for clarity
elif "psd-15" in url:
    metadata_url = url.replace("psd-15", "psd-15") # Again, no change, but included for clarity
else:
    raise ValueError("Unexpected metadata URL format")

    This code snippet checks if the URL contains either psd-14 or psd-15 and proceeds accordingly. If neither is found, it raises a ValueError to alert you to an unexpected URL format. You could enhance this by actually constructing the URL if only the base is provided.

  5. Repeat for lines 310 and 342: Repeat the process for lines 310 and 342. These lines likely have similar code that needs to be updated in the same way.

  6. Save the file: Once you've made the necessary changes to all three lines, save the radiometry/angles.py file.

  7. Test your scripts: After saving the file, it's crucial to test your geowombat scripts to ensure that the changes have resolved the issue and that you can now process Sentinel-2 imagery with the updated metadata URLs.

By following these steps, you can effectively update your code to handle the new metadata URLs and keep your Sentinel-2 processing workflows running smoothly. Remember to always back up your files before making changes, just in case!

Example Code Snippets

To give you a clearer picture, let's look at some example code snippets that demonstrate how you might modify the radiometry/angles.py file.

Using Conditional Statements

As shown in the step-by-step guide, conditional statements are a straightforward way to handle different URL patterns. Here’s a more complete example:

url = "https://example.com/psd-14/metadata.xml" # Example URL

if "psd-14" in url:
    metadata_url = url.replace("psd-14", "psd-14")
elif "psd-15" in url:
    metadata_url = url.replace("psd-15", "psd-15")
else:
    raise ValueError("Unexpected metadata URL format")

print(metadata_url)

This code snippet checks for both psd-14 and psd-15 in the URL and assigns the appropriate metadata_url. The else clause ensures that you're alerted if the URL doesn't match the expected format.

Using Regular Expressions

Regular expressions offer a more flexible way to match URL patterns. Here’s an example of how you might use them:

import re

url = "https://example.com/psd-15/metadata.xml" # Example URL

match = re.search(r"psd-(14|15)", url)
if match:
    metadata_url = url.replace(match.group(0), match.group(0))
else:
    raise ValueError("Unexpected metadata URL format")

print(metadata_url)

This code uses the re.search() function to find a pattern matching either psd-14 or psd-15 in the URL. If a match is found, it constructs the metadata_url. Regular expressions can be particularly useful if you anticipate more variations in the URL structure in the future.

Remember to adapt these examples to the specific context of your code in radiometry/angles.py. The key is to ensure that your code can handle both the old and new metadata URLs gracefully.

Testing Your Changes

After making the necessary modifications to radiometry/angles.py, it's crucial to test your changes thoroughly. This will help you ensure that the issue is resolved and that your Sentinel-2 processing workflows are functioning correctly.

Here are some steps you can take to test your changes:

  1. Run your BRDF scripts with Sentinel-2 images from before and after September 24th, 2024: This is the most direct way to test if your changes are working. By processing images from both time periods, you can verify that your code can handle both the old and new metadata URLs.
  2. Check for any errors or warnings: As your scripts run, keep an eye out for any errors or warnings. If you encounter any issues, carefully review the error messages and trace them back to your code changes. This can help you identify any mistakes you might have made.
  3. Verify the output: After your scripts have run successfully, verify the output to ensure that it is correct. This might involve comparing the results with previous runs or checking them against known values. Accurate output is the ultimate confirmation that your changes have worked.
  4. Use a testing framework: For more rigorous testing, consider using a testing framework like pytest. This allows you to write automated tests that can be run repeatedly to ensure that your code is functioning as expected. Testing frameworks can be particularly useful if you anticipate making further changes to your code in the future.

By following these testing steps, you can gain confidence that your changes have resolved the metadata URL issue and that your Sentinel-2 processing workflows are reliable.

Staying Up-to-Date

This incident highlights the importance of staying up-to-date with changes in data formats and metadata structures, especially when working with remote sensing data. Data providers often make updates and modifications to their data, and it's crucial to be aware of these changes and adapt your workflows accordingly.

Here are some tips for staying informed about updates in the remote sensing world:

  1. Subscribe to mailing lists and newsletters: Many data providers and organizations offer mailing lists and newsletters that announce updates and changes to their data. Subscribing to these lists can help you stay informed about important developments.
  2. Follow relevant forums and communities: Online forums and communities dedicated to remote sensing and geospatial analysis are great places to learn about updates and changes. Members often share information and discuss issues related to data formats and processing.
  3. Check the data provider's website regularly: Data providers typically publish announcements and documentation on their websites. Make it a habit to check these websites regularly for any updates or changes.
  4. Use version control: When making changes to your code, use a version control system like Git. This allows you to track your changes and easily revert to previous versions if necessary. Version control is essential for managing code updates and ensuring that you can always roll back to a working state.

By staying proactive and keeping yourself informed, you can minimize the impact of data updates on your workflows and ensure that you're always working with the most current information.

Conclusion

So, there you have it! A small change in Sentinel-2 metadata URLs led to a parsing issue, but with a few tweaks to the radiometry/angles.py file, we can get things back on track. Remember to update lines 288, 310, and 342 to handle both the old and new URLs. Testing your changes is key to ensuring a smooth workflow.

Staying informed about these kinds of updates is crucial in the ever-evolving world of remote sensing. Keep those scripts running smoothly, guys, and happy analyzing!