I have a shapefile containing land-polygons of the whole world (so the shapefile contains the coastlines as polygons). But as I said these are land polygons.
What I need for a special purpose is to transform these land polygons in water polygons. Something like loading the shapefile into a software, drawing a "rectangle" polygon covering the whole area and then saying something like "crop land polygons" so the water polygons remain.
I hope my explanation is quite clear. If you have any suggestions what tool is capable of doing this I'd be happy about a small "tutorial" on how to do this.
Thanks in advance,
Yes, what you need to find or create is a shape file representing the whole world. The you can union it using the shape file that contains the continents. After the union, you will need to delete the portion that came from continents. In ArcGIS, use ArcToolbox > Analysis Tools > Overlay > Union.
If you have ESRI software, you have a feature class called WORLD30.shp that you can use for the entire world. You also have a feature class called continent that you can use for the continents.