If you are sensible and don’t use a tent, but instead sleep under a nice airy tarp, you may nonetheless want to protect your bottom and other expensive equipment with a ground sheet. As I have mentioned before a mylar space blanket (at about 50 grams) is good enough for this and does double duty by keeping you warm in an emergency – also good for your day pack when you go for a stroll away from your camp to check out that waterfall, rare orchid or monstrous stag, etc.
Another leading contender is a Polycro ground sheet. This is much tougher stuff than ‘normal’ plastic film such as painter’s drop cloths, etc. You can buy them from Gossamer Gear here: https://www.gossamergear.com/products/polycryo-tent-footprint-ground-cloths for US$8.46 for two (Nov 2017) or you can make your own with supplies from eg Bunnings, such as this: https://www.bunnings.co.nz/3m-window-insulator-kit-5-windows_p00167658 or this: 3M Indoor Window Insulator Kit, 5-Window You get a heap of sticky tape too which might come in handy for repairs of some sort.
As you can see Milo and Spot like it too!
Gossamer Gear’s Specs (above) are as follows: Size 40 x 96 in / 102 x 244 cm 46 grams each. You can even make a transparent fly tarp (which might be cool) or tent out of this stuff if you want, as eg here: http://www.theultralighthiker.com/transparent-tarp-instructions/ For a groundsheet the .75 mil material is adequate (and lighter). I like the see-through9Ness) of it actually. It really helps with finding those ‘lost’ small objects you dropped just now!
Still: Perhaps one of the best features of a Polycryo groundsheet is that in an emergency it can be used as a still. There is always water to be extracted from soil or vegetation. Simply dig a hole, place your billy in the centre, cover the hole with your groundsheet (after filling it with succulent vegetation), and place a pebble in the centre to direct the condensation into your billy.
An equivalent piece of silpoly or silnylon, say the 1 oz/yd2 stuff would be under 4 ounces, say110 grams but would last much longer. I have been using a piece .8 oz/yd2 spinnaker cloth for a number of years, something like this: https://www.gossamergear.com/collections/shelters/products/groundsheet which weighs 2,7 ounces or 76 grams. Diminsions: 90″ / 228.5 cm long by 40″ / 101.5 cm wide and costs US$40 (Nov 2017).
You could purchase 2.1 metres of silpoly material (that’s enough for a double sheet ie 7′ x 5′ – 2.1 x 1.5 metres). It does not need hemming (that’s why it’s called ‘Ripstop’) and will last you many years. The ‘dark Olive’ is a nice colour. http://www.tiergear.com.au/11/products/xenon-sil-11 It costs $A11.95/metre – so your groundsheet will cost $A25
PS: This is the fabric my Pocket Poncho tent is made from. It blends in very nicely in the woods, I think. Even the deer did not notice it! See: http://www.theultralighthiker.com/pack-rafting-the-remote-wonnangatta/