The accompanying illustration shows one method of finding the centre of a circular arc where it is possible to strike arcs for intersections on one side only. A, B, and C are any points on the curve. Then with any radius in the compass, and with centres at A and C, strike arcs that intersect at D. Similarly, with another radius, strike arcs that intersect at E. Similar intersecting arcs at F and G may also be drawn, using C and B as the centres; then aline joining E and D and produced, if necessary, will cut the line joining E and G produced, if necessary, in H, the centre required.

Determining Centre of Circular Arc.