A dome compass is a centre stake with a cord that guides the inside face of every bag course, and there are two ways to use it. A cord of fixed length pivoting from a point at the sphere’s centre traces a true spherical surface by itself, while a cord shortened by a set amount for each course needs the correct inward step calculated for every course.
Part of Earthbag Building
A compass guide and a form guide are simple setting-out tools that keep a dome’s curve or an arch’s shape accurate as bags are placed.
The Cord Marks Where The Inside Face Must Go
Bags are soft and shapeless until tamped, so something has to define the line. The compass is a stake driven at the dome’s centre with a cord attached. The parent article describes it as a stake at the centre, a cord equal to the radius, and a plumb bob or level at the cord’s end, with the cord shortened for each course. How much to shorten is the part that goes wrong.
What You Need
- A stake or post at the dome centre, firmly set.
- A cord that does not stretch, and a plumb bob or level.
- A tape measure and a calculator.
- A wooden form for an arch or a niche.
Method: Two Ways To Use The Cord
Method A: fixed-length pivot. Fix a cord of length equal to the dome’s inner radius R to a pole at the point that would be the centre of the sphere, at the height of the springing. Sweeping the cord around touches the inside surface of a sphere of radius R at every angle, so the inside face of each course is laid to touch the cord end. The cord needs no shortening, and it can only produce a sphere.
Method B: shortened cord (plan radius). Keep the cord at the stake and, for each course, shorten the cord to the horizontal radius of the inside face at that height. For a hemisphere this radius is the square root of (R2 minus z2), with z the height of the course, and the amount to shorten between courses is the difference between neighbouring radii, which you tabulate in advance; see the table in Corbelled Dome Technique.
The parent article’s instruction to shorten the cord by the bag width minus the corbel offset does not match this geometry, which shortens by the inward step alone. Use the tabulated steps.
Step 1: Check the cord after every course for stretch and for a stake that has moved.
Step 2: Check the inside face with a plumb line or level at several points around the circle.
Step 3: For arches, build a wooden form of the arch shape, set it on props and lay bags over it. Check the form is plumb and square before you load it; see Window and Door Framing.
Common Mistakes
- A stake that leans. All courses inherit the error.
- Stretchy cord. Use wire or a non-stretch line.
- Shortening by a constant amount. The step per course changes with height.
Safety
Stakes, forms and overhead bags
Do not stand inside a partly corbelled dome while bags are lifted over your head, and keep the arch form propped until the arch can stand alone.
What’s Next
- Learn next: Buttressing and Reinforcement — holding the base against the spread
- Combine with: Levels and Plumb Bobs — checking the face