I have a map defined by two seperate reference systems: one is a standard grid coordinate system, the other a series of irregualar polygons. I want to find which polygon each grid square overlays, and to do this I think I need to add my grid as a polygon layer where each grid square is a polygon. How do I do this? I have a .csv file in which I have defined as seperate columns each vertex of each grid cell. I have also managed to import this into QGIS as a series of point vectors (both .csv and as a shapefile) and convert this into a grid of lines. But I can't work out from here how to turn either the grid, or the original .csv file, into polygons.