Make a rectangular image random shape
Hi, I am working on a detailed aerial map (produced from a drone) which was imported into GIMP from a Geotiff. It is a rectangular image, but the map is a 'random shape' with white borders round.
I want to reduce the image itself to ONLY the map, and then use Filters - Decor - Fuzzy Blur, but I cannot reduce the rectangular image to just the map, so the Fuzzy Blur blurs only the edges of the map.
I cam select the map itself using lasso tool, and have tried 'create new layer from selection', but it still creates a rectangular image, with white or transparent border (I can choose which).
I have tried selecting and then Selection - feather, but this only allows a 100px feather, and I want a 500px feather (as it's quite a large detailed map).
I have seen examples of Fuzzy Blur, but they somehow always start with a random image shape, not rectangular.
Any suggestions? Thanks in advance.
(File not attached as its 300MB+)