Sagitta Rule

Visualization of the sagitta. The red line marked with s is the sagitta of the arc.

From Wikipedia:

In geometry, the sagitta of a circular arc is the distance from the center of the arc to the center of its base. It is used extensively in architecture when calculating the arc necessary to span a certain height and distance.

In Live Steam a Sagitta Rule is used to directly measure the radius of a curved section of track.

• 130 Sagitta Rule for 10 Foot Rail (PDF)