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The lean-to tarp shelter makes use of a ridge-line. This ridge-line is created by tying a length of rope tightly between two trees or other points of support. This will create a point that you can secure your tarp to and finish your shelter. Find two trees that are as far apart as your tarp is long. Tie one end of the rope around the trunk of a tree. Tie it at the height that you want the top of your shelter to be at. Tie the other end of the rope at the same level on the opposite tree. It's very important that the ridge-line is as tight as it can be. The lean-to tarp shelter requires you to fix one edge of your tarp to your ridge-line. You can attach the tarp to the ridge-line using cord or rope and many tarps will come with grommets or loops built in. Make sure you attach the tarp to the ridge-line tightly to build a strong tarp shelter. Your tarp may have holes pre-made along its edge. If so, you can thread the ridge-line through these for an easy way to join them together. Many tarps will have loops along the corners or edges that can be used to tie them to your ridge-line. Once the top edge has been tied to the ridge-line, you can secure the bottom edge of the lean-to tarp shelter. Pull the bottom edge away from the center until it is at an angle that you want. Place anchoring stakes in the ground at the corners of the tarp and tie the tarp to them. This will hold the bottom portion of your shelter securely in place. Most people recommend using a 45 degree angle for their lean-to. Try adjusting the angle to raise or lower the height of the shelter's "ceiling".
Find a good spot for the cord. Fix one edge of the tarp to the cord. Anchor the tarp to the ground.