Georeferencing Corona imagery in QGIS

Now that we have downloaded our Corona image, we need to add it to QGIS!

Extracting our Corona imagery

Before we can work with our imagery, we need to get it to the right place and extract it.

• 1) Move your Corona download into the folder you are using for this course – it might be a good idea to make a “Corona” folder for it.

_{Our downloaded Corona image.}

Now we need to extract it.

_{This is an identical process to what we did with the Landsat imagery in Week 3.5}.

• 2) Right-click the image and select “7-Zip” > “Extract Here”.
• 3) Repeat this with the new “tar” file.

_{The four extracted tif images.}

• 4) Double-click each image in turn to select which one you want to work with in QGIS.
• 5) Your image may be upside down, if it is, you can rotate it by right-clicking it and selecting “rotate right”, then repeating this again.

_{You can rotate your image if you need to.}

Georeferencing in QGIS

Adding a modern satellite image and starting the georeferencer

_{You can look back at Week 2 or Week 3 if you need a reminder on how to do this.}

• 1) Start QGIS – you can either continue with a previous project or create a new one.
• 2) Add the modern satellite image of your area using “Layer” > “Add Layer” > “Add Raster Layer”.

_{You need to add some modern satellite imagery of your area to georeference the Corona.}

• 3) Open the georeferencer by navigating to “Raster” > “Georeferencer…” on the main menu.
• 4) In the georeferencer window navigate to “File” > “Open Raster…” and open your selected Corona image.

_{The georeferencer with a section of Corona imagery added.}

Setting up the georeferencer

• 1) Navigate to “Settings” > “Transformation Settings”.
• 2) Change “Transformation type” to “Polynomial 1”.
• 3) Make sure “Target SRS” is set to “EPSG:4326 – WGS84”, this should be the default.
• 4) In “Output raster” replace “modified” with “GR”– this just makes it clear that the new Corona image we produced has been georeferenced.
• 5) Change “Compression” to “LZW” – this will make our final file as small as possible.
• 6) Tick “Save GCP points” – this helps backup our hard work in case anything goes wrong.
• 7) Tick “Load in QGIS when done” – this just means that we won’t have to add our new image manually at the end of the process!
• 8) Click OK to return to the georeferencer.

_{Adjusting the transformation settings.}

Adding GCPs

• 1) Zoom in to the top-left of your Corona image and try to find a distinctive geographical feature.
• 2) Switch to the main QGIS window and try to locate this feature on your Sentinel-2 or Landsat image.

• 3) Once you are happy with your point, switch back to the georeferencer and click the “Add Point” button.
• 4) Click on your feature on your Corona image.
• 5) Select “From Map Canvas” in the new window and click the feature on the main QGIS window.
• 6) Click “OK”.

_{Adding our first GCP.}

_{Our red GCP is present on both of our images.}

• 7) Repeat this to create four GCPs in total – one in each corner of your Corona image.

_{We now have four GCPs in each corner of our Corona image.}

Adding more GCPs

• 1) Select “View” > “Link Georeferencer to QGIS” and “Link QGIS to Georeferencer”.

_{Around fifty, well-spaced GCPs have been added to the georeferencer.}

Improving the georeferencing

_{One point is clearly incorrect and should be deleted and redone, you can see the size of the error for each point in the “Residual (pixels)” column of the GCP table.}

• 1) Make a note of the current mean error.
• 2) Navigate to “Settings” > “Transformation Settings”.
• 3) Change “Polynomial 1” to “Polynomial 2”.
• 4) Check the new mean error – it should have drastically reduced.

• 5) Try changing to “Polynomial 3” and see how this changes the mean error.

_{In this example the error has dropped to a quarter of the size using a more complex transformation. Visually it is apparent that the red lines are much shorter!}

Running the georeferencer

• 1) Navigate to “File” > “Start Georeferencing”.

_{The georeferenced Corona image has been added to the map.}

• 2) Zoom in and check you are happy with the results

_{Do be aware that because of how the imagery was taken, the georeferencing will never be perfect. If nowhere on the image is more than 100m from where it should be, this is a great result!}

• 3) Once you are happy you can close the georeferencer.

_{The red GCPs disappear after closing the georeferencer.}

_{A falaj/qanat is visible in this Corona image from northern Oman.}

_{How does the Corona compare to Landsat or Sentinel-2 imagery in your area of interest?}