A groundsheet is a large sheet of plastic or treated cloth that water cannot pass through. When people camp, they spread it on the earth so the damp in the grass cannot soak their tent floor or sleeping bag. Some tents have one built into the bottom; some campers carry a separate sheet and put it under the tent. It is thin and meant to stop water, not to make you comfortable. Though actually, people often mix it up with the sleeping mat — the thick pad you lie on. The groundsheet is the waterproof layer; the mat goes on top of that. You meet the word in camping English, more in Britain than in American shops, where people may just say tarp.

地布（露營用防水布，不是防潮墊那種睡墊）