Export an Image as a Geotiff that is not aligned with Lat/Long Grid

Hi @Richard,

Thank you for your detailed question. Based on your description, here’s how you can handle your rotated image and alpha channel in MATLAB:

1. Exporting a Rotated Image with `geotiffwrite`

MATLAB's `geotiffwrite` function assumes that the image is aligned with the north-south-east-west axes. It does not natively support exporting images that are rotated relative to the latitude/longitude grid. However, you can work around this limitation by embedding your rotated image into a larger, axis-aligned image with transparency. This approach allows you to maintain the spatial reference while accommodating the rotation.

2. Adding a Transparent Alpha Channel

While `geotiffwrite` does not natively support alpha channels, you can add transparency by setting the RGB values of the transparent areas to `[NaN NaN NaN]`. When viewed in MATLAB, these NaN values are interpreted as transparent. However, please note that other software may not recognize this transparency unless they specifically support it.

3. Practical Workflow

Here’s a MATLAB-based workflow to achieve your goal:

1. Embed the Rotated Image:

   % Create a larger, axis-aligned image
   largeImage = NaN(height, width, 3);  % Adjust dimensions as needed

   % Place your rotated image into the center
   largeImage(yOffset:yOffset+rotatedHeight-1, 
   xOffset:xOffset+rotatedWidth-1, :) = rotatedImage;

2. Set Transparent Areas:

   % Define transparent areas (e.g., where the image is NaN)
   transparentAreas = isnan(largeImage(:,:,1));

   % Set RGB values of transparent areas to NaN
   largeImage(repmat(transparentAreas, [1 1 3])) = NaN;

3. Save as GeoTIFF:

     % Define spatial referencing object R
     R = georefcells(latlim, lonlim, size(largeImage(:,:,1)));

     % Write the image to a GeoTIFF
     geotiffwrite('output.tif', largeImage, R);

This workflow embeds your rotated image into a larger, axis-aligned image and sets transparent areas to NaN, allowing you to export it as a GeoTIFF.

4. Alternative: Using GDAL for True Rotated GeoTIFF

For true rotated georeferencing, you may need to use external tools like GDAL. GDAL allows you to assign specific corner coordinates to your image, enabling accurate georeferencing even for rotated images.

If you need further assistance with any of these steps or have additional questions, feel free to ask.