From scales on the chart the navigator can measure directly, without computation, the distance that the ship has traveled. Traditionally, Mercator charts have been used at sea. Lambert charts may also be used for long sea voyages, though they were designed for air navigation. A navigator needs other basic instruments to determine a course and plot it on a chart. A compass indicates direction.
Dividers are useful in measuring distances on charts.
Parallel rulers usually two straightedges connected by pivoted arms are used to transfer lines of direction from one portion of a chart to another. A transparent plotter is a combination protractor and straightedge employed in the measurement of angles and distances and in drawing course lines on a chart. In directing a course by dead reckoning, a device which measures distance traveled is also essential to the navigator . In piloting, the navigator guides a ship largely by the bearings of landmarks. A bearing is the horizontal angle between an object and a reference point for example, true north is the reference point for true bearings.