The rhumb line is a line of constant bearing across the earth's surface, and it is an aid to navigation that permits navigation without complex tools or detailed knowledge. The locations of the roses are rallying points, which can be used as starting positions or waypoints along your route. Leave port, sail in a straight line until you reach the rallying point, then turn and sail in a straight line until you reach the next, then turn and sail in a straight line to your destination port. You can thus connect almost any two ports on this map using only two waypoints, and you don't have to risk your ship on the shoals by coasting.