I have a series of polygons that are the output of another GIS program - I need to merge the polygons into a single polygon. I have tried a couple of different methods with no luck. I have tried to use the "Dissolve" tool, using the Dissolve all - however this led to an essentially empty output (most probably because there isn't a common attribute between all the polygons). I have tried to use the calculator in the attribute table (there are around 350 000 polygons) to add a dummy column but it doesn't show in the Dissolve tool.
I am finding the merge tool does the job - to a point, but ultimately is extremely time consuming. Is there a way to merge all polygons in a layer without too many steps? Ultimately I just need the geographic area, the values/names etc are entirely irrelevant for what I'm currently doing. I have tried to select all within the attribute table by using the "invert selection" option (a recommendation on another site) but this doesn't appear to do anything. I have also read about a SelectPlus plugin, but no luck finding this one either.
Any suggestions greatly appreciated.
EDIT: Sorry it's a single file that I'm wanting to merge the polygons in (want the outline of all of them together, there are no gaps).
EDIT2: and while it's very slow, I've now found how to select all the polygons at once (using the radius selection option), which at least seems to be working if I do in stages.