See screenshot, from geojson.io, with a geojson file loaded:
So how do we make a polygon out of the blank area? Looking for a way which doesn't involve manually copying selected sequences of co-ordinates from each enclosing polygon's edge. But if we can automate that, awesome!
Obviously the solution would involve specifying a bounding box limit, so we don't polygon the planet.
Some kind of proximity detection could be a way to go : enlist lines (ie, pair of sequential co-ordinates) which don't have any close-by counterparts in the other polygons loaded. Just an idea.
Guess I'm wishing for an "invert selection" like feature ;)