Cartographic Terminology

[Midgardsormr]

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.

[Hai-Etlik]

It's important to realize that rhumb lines usually aren't straight (on the globe), any that aren't in the four cardinal directions are spirals. The meridians and the equator are both rhumb lines are great circles ("Straight lines" in spherical geometry) The other parallels are all regular circles. Maps that show rhumb lines are either large scale, so they only show a short section of the spiral that doesn't show much curvature, or they are in the Mercator projection, which distorts things to make rhumb lines appear straight.

Rhumb lines and compass/wind roses are indications that the map is bearing preserving, and tools to aid in taking advantage of that property for navigation. Rhumb lines are mostly associated with "Dead Reckoning" navigation.

Rhumb lines are often drawn in a web of sorts, centring a cluster of lines at each vertex of a regular polygon so that the lines intersect the other verticies. The map you've linked to is doing this although you can only see a portion of the whole pattern. The whole thing looks like this:

portolan-web.png

[Vagabondage]

@Midgardsormr & Hai-Etlik -- Thank you so much for your answers. This explains a lot and is absolutely fascinating/beautiful!

Would wind-roses function the same way over large areas of land?