Portolanos were charts made by and for seamen during the Middle Ages. These practical, utilitarian, and remarkably accurate atlases had no gradations, but used a network of rhumb lines – straight lines extending in the direction of the compass points. They provided sailing directions and the location of ports and showed various coastal features. They even adhered to a systematic rule for traditional coloring. The stamp designs shown are excellent reproductions of portolano charts.
The Timor stamp is a reproduction of a 16th century map with 32-point compass roses and seven horizontal lines representing lines of latitude. The red line is the Tropic of Capricorn. [The original map also had the equator running horizontally across the top.] All other lines are rhumb lines. Near the center of the stamp is an origin for rhumb lines, but no compass rose was included.
A 32-point compass rose and several 8-point roses were incorporated into the 1502 portolano chart that is the design for the Cape Verde stamp. The two horizontal red lines are the Tropic of Cancer and Tropic of Capricorn. The line between them is the equator. The vertical line labeled in red is the demarcation line (between Spanish and Portuguese-claimed areas) declared in 1494 under the Treaty of Tordesillas. All other lines are rhumb lines.