I have a Shapefile with Polygon features representing the world's land areas (continents, islands). Is it possible to invert these, and get a geometry that represents the world's oceans and seas? So the original land Polygons need to become rings in a single global Polygon that covers the planet.
Basically, I'm looking for something like Select > Inverse in Photoshop. A Python, command line or QGIS solution is preferred.