The Astrolabe Project

In working with a compass and a straight-edge I’m finding that there are techniques that can be used to improve the accuracy of the drawing.

When drawing arcs to bisect lines or angles it is best to set the compass radius so that the construction arcs cross as close to 90 degrees as possible. The shallower the angle of incidence, the less defined the intersection, and the greater chance of the line drawn between the intersections being off by a (small, admittedly) amount.

When using the Compass to measure distances, it is easier to transfer large distances than small ones with accuracy. If I need to draw a line and mark a point 1/32 of the way along it, I can do this with good accuracy by repeatedly dividing the line in half by construction. But if I want to transfer that 1/32 distance, it is easier and more accurate to transfer the 31/32 measurement. The mark ends up in the same place, but adjusting the compass accurately is easier.

When dividing a circle it is better to work each stage in the division over the whole circle then move to the next. If I need to divide a circle into 24 sections, I should first quarter the whole circle, then trisect all four quarters, then bisect all twelve sections. I could try to save time by trisecting one quarter, and bisecting one section of that; then use the compass to copy the arc of that 1/24 section 24 times. But I find that small errors in measurement compound if you do it that way. Working the whole circle at once tends to cancel out small errors.