The Portland Plotter is the navigational tool of choice for most sailors. It is quick and easy to use. The errors due to variation and deviation and can be preset which eliminates error when quick calculations are needed. It has a long straight edge for use on most charts, it is strong and durable and has a proven track record. The Portland Plotter has all the advantages over parallel rules because you do not need to use the compass rose and transpose the bearing. All you need do is line up the grid lines on the chart with grid lines on the plotter, point to the direction of the bearing or course, make sure North on the plotter is pointing to North on the chart and away you go. Quick, easy and accurate.