Norman measures to see if we are keeping the inside dimensions of our underground dry stone chamber in a correct circular orientation.
It's amazing how useful string is in dry stone walling. Not only is a single string line a fundamental measuring tool for determining a straight wall, setting two string lines parallel to one another on the batter frame creates a guide for the batter of the wall too.Tie a stone to a piece of string and you have a simple too for measuring plumb.
Tie string to a pole and you have the inside and outside dimensions for a circular hut. Slide the string up and down the the pole as a guide to keeping the wall's vertical component. Shorten the string proportionately to the height as you go up and you have your batter. Fasten the string at the top of your pole and the other end gives you the exact limits of a dome ceiling. Tie it at the apex of your opening and you have the arc for building a dry stone arch entrance. Hold both ends and let it hang loose in a U and you have the curved line of an inverted catenary arch (the strongest of the arch shapes).
Just about the only thing string wont do is give you a reference for horizontal or tell you weather. Or then again, maybe it can do that too.