Hi Peskypeeza
Any picnic rug would be great over a ground sheet. It protects the GS from wear and adds to your comfort.
As to the footprint question I put a tarp under the whole of the SIG as then any moisture or dirt from the ground will go on this and it can be folded up at the end of the holiday and carried home in a bin bag and cleaned off later. SIG is still clean and tent can be folded and packed without to much trouble.
If you tent only has a SIG for the bedrooms I would just get one to go under that space.
It is also helpful for figuring out where to place your tent when camping as you can put down you footprint/tarp and move it around a bit till you are happy with the location, then put up your tent!
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