Course Plotter Used by navigators for over 50 years. This is a 15" course plotter with a semi-circular protractor scaled from 180 to 360 inside. Lower edge scales 1:80,000 and 1:40,000 to accommodate National Ocean Survey charts. For north and south courses, a small auxiliary protractor scale is printed inside the larger one and is used against a parallel latitude. The upper edge is scaled 1 inch equal to 20 miles to accommodate the VP-OS Universal Plotting Sheets.