Let's begin the excavation by locating the center of the dome. Drive a stake or post into the ground at that point. Attach a non-stretchy rope or light chain to the center post so that it can rotate easily. Make sure it is long enough to reach beyond the proposed radius. Mark the rope or chain at least two feet (0.6 m) beyond the interior radius to include the width of the wall (Fig. 12.5).
That is to say, if you are building a 12-foot diameter (3.6 m) interior dome, make a mark on the rope or chain at eight-foot radius (2.4 m). This gives
12.5: Upper drawing caption: A rope or chain attached to a stake in the ground delineates the center of the diameter. Lower drawing caption: Allow extra width for installation of waterproof membrane or insulation.
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