## Other Compass Ideas

If a rigid pole the height of the wall or structure being built is impractical or not desired, an expandable sliding compass arm can be fixed at the height that the spring line begins, which is where the walls begin stepping in to create a dome.

Attach a wheel caster (with the wheel removed) to a post buried into the ground, and fasten an expanding arm to the caster (Fig. 3.45). The compass arm rotates and pivots, but since it is fixed on one end at the spring line height, it is always at an angle that changes with each row of bags. Also, as the angle increases, the length of the arm increases as well, hence the expanding compass (Fig. 3.46). A level attached to this angling arm would be ineffective, so a water level becomes a necessity (See Appendix for directions on how to build and use a water level).

This process becomes even more complex when using this compass to do plumb vertical walls. For vertically plumb round walls it's just easier and less time consuming to use the pole compass. The same holds true for corbelled domes, too.

3.46: Expandable compass fixed to center of diameter at springline determines shape by lengthening of the arm at a diagonal.

compass arm (pipe) attached to wheel caster screwed into 4" x 4"post compass arm (pipe) attached to wheel caster screwed into 4" x 4"post

3.45: Detail for a caster wheel attachment.

 x \ 5' i4' \ 3* ¿Irl \a* 3.47: Pole compass with adjustable length horizontal arm is used to determine radius width by shortening the length of the arm. For large dome projects, you can imagine that the expandable compass arm could become quite unwieldy, as the length would continue to expand to the eventual height of the dome. Thereby, a 20-foot (6 meter) wide dome would need a compass arm that expands to twenty feet (6 m) in length. A pole com pass arm, on the other hand, continues to shorten every row from the length of the radius. Having tried different methods ourselves, we prefer the pole compass for being simple, precise, and multi-functional (Fig. 3.47).