Check out this all-in-one Compass, Circle Ruler, and Protractor from Circle Perfect Tools. Use as a compass for drawing perfect circles/arcs, or a ruler for measuring existing circles, as well as a protractor/goniometer for visualizing and measuring angles.
All-in-one compass, ruler, protractor
Pre-drilled holes for easy drawing of circles and marking of angles/measurements
Ruler increments: Every 1/8″ (4mm)
Angles: 36 degrees with marks every 1 degree