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.
It is a single file that I'm wanting to merge the polygons in (want the outline of all of them together, there are no gaps).
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.
Best Answer
Using QGIS, apart from the Vector -> Geprocessing Tools - Dissolve Menu entry, you can also use Processing (formerly named Sextante), which offers dissolving by GRASS and SAGA.