Portolan chart

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 . 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. == Specific features of portolan charts ==

Rhumb lines 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. Symbols Portolan charts use symbols to represent navigational hazards such as reefs, rocks, shoals, and sandbanks. In some early portolan charts, dated to the thirteenth century, black dots denoted rocks, and red dots sand or shoals. The symbol + is the earliest known cartographic symbol on nautical charts, representing submerged or barely visible rocks. This symbol is at times combined with others to represent a group of rocks or sandbanks. These names were often placed at the nearest coastal point to the hazard, but within the coastline. However, despite its many variations and additions throughout time and across different charts, the + symbol represents rocks/rocks awash even in today's nautical charts. As well as symbols for navigational hazards, some portolan charts also feature pictorial symbols representing features such as anchorage, lighthouses, beacons, and buoys. == Use ==

The portolan chart combined the exact notations of the text of the periplus or pilot book with the decorative illustrations of a medieval T and O map. In addition, the charts provided realistic depictions of shores. Many historians believe they were meant for practical use by mariners of the period. Portolans failed to take into account the curvature of the Earth; as a result, they were not helpful as navigational tools for crossing the open ocean, and were replaced by later Mercator projection charts. Portolan charts were also useful for navigation in smaller bodies of water, such as the Mediterranean, Black, or Red Seas. Additionally, in some instances the charts may have been used for travel inland by waterways for trade purposes, specifically in the context of Hungary, as several charts single out that country for special detail, such as showing many cities and riverways in Hungary. Scholars have also shown a dichotomy between portolan charts used for display purposes and ones used for navigation aboard ships, the latter of which largely do not survive to the modern day. Some of these scholars have argued that many or even most portolan charts were not created for practical navigational uses, but rather as an ornamental item to "show off the owner's worldliness."''' Examples of evidence that has led scholars to this theory are that some documented chart owners had no evident connection to seafaring, and that records show instances such as a merchant commissioning a chart depicting the parts of the Mediterranean he traveled after returning from his voyage, seemingly as a souvenir and not a guide. The true historical use of portolan charts remains a debate among historians of the subject, and many arguments have been made both for their use as navigational tools and as decorative objects. == Production ==

Most extant portolan charts from before 1500 are drawn on vellum, which is a high-quality type of parchment, made from calf skin. Single charts were normally rolled whereas those that formed part of atlases were pasted on wood or cardboard supports. The earliest surviving explanations of how to draw a portolan chart date from the 16th century, so the techniques used by medieval mapmakers can only be inferred. The instruments available in the Middle Ages are believed to have been a ruler, a pair of dividers, a pen, and inks of various colors. Drawing probably started with the windrose lines and then the mapmaker copied the coastal outlines from some earlier chart. Place names, geographic details and decoration were added in the end. Portolan charts, being entirely handcrafted before 1600, were initially replicated through tracing and therefore often contained errors and inaccuracies.''' However, over time the invention of printing technologies transformed chart production capabilities. These new printing capabilities allowed charts to be produced in much larger numbers and far more accurately, with copperplate printing especially useful in producing finer lines, smaller letters, and more detailed symbols. After their invention, both printing techniques remained in simultaneous use for the next century, with copperplate printing later becoming the preferred method. Portuguese The Portuguese portolan charts come from the Majorcan tradition, and as traditional portolan charts did not fulfill the requirements demanded by the expansion of the geographic horizons attained by Portuguese and Spaniards, they superposed the astronomical lines of the equator and tropics on top of the wind line network, and they continued being elaborated over the 16th and 17th centuries. Arab Three medieval portolan charts written in Arabic are preserved: • Map of Ahmed ibn Suleiman al-Tangi from 1413 to 1414. • Map of Ibrahim al-Tabib al-Mursi from 1461 • Map of western Europe, anonymous and undated, preserved in the Ambrosiana Library, dating from the 14th or 15th centuries. In addition there is a detailed description of a nautical Arab map of the Mediterranean in the Encyclopedia of the Egyptian Ibn Fadl Allah al-'Umari, written between 1330 and 1348. There are also descriptions limited to smaller geographic regions, in a work of Ibn Sa'id al Maghribi (13th century) and even in the work of Al-Idrisi (12th century). ==Theories of origin==

While the production dates of portolan charts are mainly clear and undisputed, the origin of the spatial data utilised in their creation remains scientifically unresolved, as no less accurate earlier mediaeval nautical charts have been uncovered, nor have late mediaeval cartographers documented precise information on how the data underlying their creations were initially observed. There have been many hypotheses about their origin, including creation by neolithic, Greek, Roman, and Egyptian civilizations,''' However, there are two principal scholarly hypotheses concerning their origin that have emerged out of the many competing ideas, which can broadly be lumped into two categories: the mediaeval origins hypothesis and the pre-medieval origins hypothesis. • The medieval origins hypothesis, which mainly stems from a traditional descriptive approach, states that a large body of bearing and distance observations, systematically observed by sailboat navigators throughout the Mediterranean, Black Sea, and East Atlantic during their dead reckoning navigations in the late medieval period, was compiled and presented to cartographers, who then transformed it into a coherent graphical representation known as a portolan chart. • The pre-medieval origins hypothesis, mainly built on the synthesis of cartometric analyses and known historical records, posits that portolan charts exhibit an accuracy that surpasses what could be expected from genuine mediaeval artefacts, indicating they are likely late medieval reproductions of some earlier-made maps or charts. One study concludes that portolans originated from earlier charts drawn on what is now called the Mercator projection, stating that portolans are mosaics of smaller charts, each with their own scale and orientation, and suggesting that the cartographic capabilities of whichever civilization produced the antecedent charts were more advanced than is currently acknowledged. This publication led certain researchers in the field to react to those ideas, but the criticism was not based on sufficiently strong arguments. To counter the hypothesis of the Mercator projection, some scholars have suggested that the Portolan charts were constructed using plane charting. Additionally, the one method used to make the earliest portolan charts was by "'averaging'" the data taken by many sailors over many years in order to increase the level of accuracy without using map projections. == Evolution ==

The earliest portolan charts focused on the Mediterranean and Black Sea coasts, with only partial and sometimes sketchy depiction of Atlantic coasts up to Scandinavia. In the 15th and 16th centuries, with the beginning of the Age of Discovery, the scope of portolan charts expanded south down to the Gulf of Guinea. Charts also started to be drawn by Portuguese and Spanish mapmakers for the newly explored seas in Africa, America, South Asia and the Pacific. Further, Portuguese cartographers used astronomical data to add latitude lines to the charts. As time progressed, the charts transitioned from being drawn on vellum to being drawn on paper. == See also ==