Weems & Plath Compute-A-Course is a compact, multi-purpose plotting tool, an ideal addition to general boating or commercial navigational tool sets. The Compute-A-Course navigation wheel can be used for position fixing, course plotting, and distance measurements. Using the log scale located in the instructions on the back of the vinyl storage sleeve makes time and distance calculations a breeze. Best used for course plotting, distance measuring & position fixing. Instructions & logarithmic speed scale for solving time / speed / distance located on back of storage sleeve.