Rhumb Lines

10e Chart of Portugal's Sagres Region stamp
A 16-point compass rose with a fleur-de-lis indicator for north and a cross on the east point (a tradition of the 16th-18th centuries). Rhumb lines cross the map, originating from compass roses outside the map area shown. Cardinal direction rhumb lines are indicated in a color.
3.50e Vasco da Gama's Route to India stamp
One 32-point compass rose and three 16-point compass roses. The principal rose is centered on the equator – labeled equinoctial. A fleur-de-lis identifies north. All lines originate from compass roses and are, therefore, rhumb lines; however, the map is drawn on a cylindrical projection making all latitude lines horizontal. Consequently, the rhumb lines which extend east and west, in this instance, correspond to lines of latitude.

The term ‘rhumb lines’ used in context with old maps, refers to lines of direction extending from the points of the compass. They were an aid to navigation added to maps during the age of exploration. The most common use of rhumb lines was on sea charts. They indicated to the navigator the direction between locations. By placing several roses on a map and extending rhumb lines from them, a pattern of intersecting lines was set up which enabled the navigator to make a visual estimate of bearing to any point on the map from nearly any other point. Rhumb lines bore no relationship to the construction of the map itself but were added after the drawing of the chart.

1p Spanish Map of Spain and the Americas stamp
A 16-point compass rose with extending rhumb lines. No equator is included. The size of Spain is greatly exaggerated and appears without neighboring Africa.
50f Jacques Cartier stamp
Two 16-point compass roses with North arrows pointing down (the north up tradition was only just evolving on early maps). Rhumb lines cross-cross the map, but none originate from the roses shown. This would not occur on an authentic map.