Well, it certainly makes sense to camp off the ground in Australia, where there may be more poisonous reptiles per square yard than anyplace on the planet. About 100 Australian snakes are poisonous, including the feared Taipan, which has the most potent snake venom known.
ARB Simpson II Rooftop Touring Tent

In the U.S.? Well, I聮m not so sure. The ARB Simpson II Rooftop Touring Tent (approximately $1,500; arbusa.com) certainly is a clever piece of engineering. It mounts to the rooftop of your SUV, then unfurls and is supported off the ground by poles, with one end still attached to the roof rack. A ladder gives you access. It聮s waterproof, comes with foam mattresses, and can be buttoned up in bad weather or opened to the breeze when it聮s warm.
But for starters, the Rooftop Touring Tent costs about three times what high-end camping tents cost. It isn聮t all that big聴55″ X 94″ X 52″聴so compared with a typical car-camping tent, it may be a little cramped. And there聮s some question about how sturdy it is when not attached to expedition-style roof racks, which are bolted to the SUV聮s frame, not just the roof sheeting. Besides, when you聮re car-camping, it really isn聮t that hard to find a flat place, is it? Not in my experience.
So, I wouldn聮t call it a waste of money. In the right situation I can see the utility; and it does free you a bit from the need to find a tent site. But for most situations it might be overkill.
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