An improvement in a compass that marks large radius arcs is disclosed. The compass is for marking on flat surfaces like the ground and walls where higher degree of accuracy for the radius or circle is desired. More particularly, the present flat compass allows a person to create a virtually unlimited radius arc using a center point and a rigid arm with a marking tip. The flat compass uses a rigid flat ruler with an embeddable anchor, such as a nail, on one end and a marking tip on the other that keeps the rigid arm essentially parallel with the marking surface as the surface is being marked. The marking tip is movable along the length of the rigid flat ruler. Multiple rulers can be connected to increase the size of the arc that will be drawn.