Portolan chart

Portolan charts are the earliest known type of nautical charts, and the oldest known examples were made in the late 13th and early 14th centuries in the Mediterranean region, usually displaying the areas between the Atlantic coasts of Western Europe and Northwestern Africa to the west and the Black Sea to the east. Besides those showing the entire area on a single map field, there are also portolan charts that show smaller territorial extents, either as separate editions or as a series of charts that together form portolan atlases.

The word portolan comes from the Italian portolano, meaning "related to ports or harbors", and which since at least the 17th century designates "a collection of sailing directions".

Portolan charts are manuscript charts rendered using ink on vellum sheets and are easily recognizable by their distinct visual characteristics, such as a content focus on coastal regions, networks of colour-coded straight lines emanating from one or more centres in 32 directions, linear scale bars calibrated in so-called portolan miles (miglio), and place names inscribed perpendicular to the coastline contours. Their most perplexing features are the extremely realistic portrayal of coastlines and a complete historical lack of their evolutionary path because the oldest known samples have already been made to a highly developed stage, and later-made charts and atlases have not become more accurate over time.

The term "portolan chart" was coined in the 1890s because at the time it was assumed that these maps were related to portolani, medieval or early modern books of sailing directions. Other names that have been proposed include "rhumb line charts", "compass charts" or "loxodromic charts" whereas modern French scholars prefer to call them just "nautical charts" to avoid any relationship with portolani.

Several definitions of portolan chart coexist in the literature. A narrow definition includes only medieval or, at the latest, early modern sea charts (i.e. those that primarily focus on sea basins and coastlines, leaving the depictions of inland areas with little or no content) that include a network of rhumb lines and do not show any indication of the use of spherical coordinates, i.e. latitudes and longitudes . The geographic extent of these mostly medieval portolan charts is limited to the Mediterranean and Black Seas with possible extensions to West European coasts up to Scandinavia and West African coasts down to Guinea. Some authors further restrict the term "portolan chart" to single-sheet maps drawn on parchment, whereas late medieval and early modern manuscript bindings that contain several nautical charts are usually called "nautical atlases" or "portolan atlases". A broader definition of portolan chart includes any manuscript nautical chart or atlas that primarily depicts coastal areas, contains a network of rhumb lines, and has place names written perpendicularly to the coastline. This expanded definition encompasses charts of virtually any sea area and even maps of the entire world, often referred to as nautical planispheres, as long as they satisfy the aforementioned criteria. It also comprises nautical charts that depict latitude scales and have been referred to as "latitude charts" by other authors to distinguish them from typical portolan charts showing the Mediterranean, which some scholars believe were created upon a large body of shipborne bearing and distance data observed through dead reckoning navigation during the Late Middle Ages.

Portolan charts are characterized by their rhumbline networks, which emanate out from compass roses located at various points on the map. The lines in these networks are generated by compass observations to show lines of constant bearing. Though often called rhumbs, they are better called "windrose lines": As cartographic historian Leo Bagrow states, "…the word [loxodromic or rhumb chart] is wrongly applied to the sea-charts of this period, since a loxodrome gives an accurate course only when the chart is drawn on a suitable projection. Cartometric investigation has revealed that no projection was used in the early charts…".

The straight lines shown criss-crossing portolan charts represent the sixteen directions (or headings) of the mariner's compass from a given point, which became thirty-two directions from around 1450. The principal lines are oriented to the magnetic north pole. Thus the grid lines varied slightly for charts produced in different eras, due to the natural changes of the Earth's magnetic declination. These lines are similar to the compass rose displayed on later maps and charts. "All portolan charts have wind roses, though not necessarily complete with the full thirty-two points; the compass rose ... seems to have been a Catalan innovation".

Portolan charts also featured various symbols, primarily black crosses and red dots, which represented navigational hazards such as reefs and rocks.