Medieval Maritime Cartography: The Rise and Development of Portolan Charts

Introduction

Towards the end of the thirteenth century a new type of chart appeared in Western Europe that marked a major advance in medieval cartography.

These charts broke with earlier practice by relying on direct observation aided by the new instrument, the mariner’s compass.

The coasts of the Black Sea, the Mediterranean and south‑west Europe were mapped with notable accuracy, establishing a standard that endured for centuries until astronomical position‑fixing became widespread in the eighteenth century.

Terminology and Form

These charts are commonly called “portolans”, though strictly a portolano is a written set of sailing directions, a distinction that can cause confusion.

Some prefer the neutral term medieval sea charts, while the compromise label portolan chart identifies the characteristic type of the fourteenth and fifteenth centuries.

Survival and Atlases

Portolan charts survive either singly or in atlases (standard charts divided into sections, sometimes bound with a calendar, world map or astronomical data).

Few examples survive from the fourteenth century—probably fewer than twenty—and only seven draughtsmen can be identified with certainty: Petrus Vesconte, Angellino de Dalorto (Dalcert), Johannes de Carignano, Perrinus Vesconte, Francesco Pizigano, Angellino Dulcert and Guillelmus Soleri.

The earlier charts were produced mainly by Italian draughtsmen; three related ‘world’ maps also date from this period.

Majorcan Output and the Catalan Atlas

The best‑known surviving work is the great Catalan Atlas of 1375, attributed to Cresques the Jew and now in the Bibliothèque Nationale, Paris.

Many extant charts are Majorcan or carry Catalan inscriptions, indicating Majorca’s emergence as a cartographic centre as the century progressed.

Materials and Appearance

Charts were drawn on single skins of parchment, often preserving the skin’s natural outline. Sizes ranged roughly from 36 × 18 inches to 56 × 30 inches. The coastline is shown in black (often faded) and emphasised by numerous port and coastal names written perpendicular to it.

Names are in black, with important harbours in red. Small islands and deltas are filled in red or gold; rocks and shoals are indicated by small crosses or dots. Inland detail on the typical or “normal” portolan is minimal—occasional rivers, mountain ranges and small vignettes of major cities with banners.

Later examples are richly drawn and brightly coloured, suggesting production for wealthy shipowners or merchants; working charts used at sea were more likely to be lost or discarded.

Representative Charts

• Carte Pisane (probably Genoese, late thirteenth century) extends from the Black Sea to southern England; crude in parts, includes named winds and a scale, with two series of lines radiating from centres near Smyrna and west of Sardinia.
• Atlas of Petrus Vesconte (1318) divided into nine sections and more carefully drawn.
• Perinetto Vesconte (1327) resembles the atlas but improves the southern English coastline and adds inland vignettes.
• Angellino de Dalorto (c.1325) is very precise and finely coloured; extends toward the Baltic, adds rivers (the Rhine, Elbe, Danube), green mountain ranges and many cities.

Scales and Units

All these charts show scales subdivided into fifths by dots, but the unit of length is never stated.

Professor Wagner’s measurements suggest two different units: about 4,100 feet in the eastern Mediterranean (roughly two‑thirds of a modern nautical mile) and about 5,000 feet for the Atlantic coast. This discrepancy causes the Atlantic coastline to appear contracted.

Geographic Coverage and Accuracy

The charts typically cover the Mediterranean and Black Seas and parts of Europe’s Atlantic coast. South of the Strait of Gibraltar the coast is charted a short distance beyond the Atlas Mountains.

To the north, the coasts of Spain, France, southern England, and the Low Countries are depicted less accurately; attempts at the Baltic are sketchy.

The most accurate charts correspond to regions where Genoese and Venetian trade was extensive—Venice dominated Black Sea trade, Genoa the eastern Mediterranean after 1298—and both city‑states traded in North Africa and as far as the Low Countries.

System of Direction Lines and Compass Roses

A striking feature is the system of radiating lines. From two points in the western and eastern Mediterranean, sixteen or thirty‑two lines radiate; subsidiary centres on the circles’ circumferences are equally spaced to cover the chart systematically.

Later charts show these lines emanating from compass roses, representing lines of direction (bearings).

On earlier charts the radiating lines are not always explicitly tied to a drawn compass or wind rose and cardinal points sometimes appear only at the margins. Petrus Vesconte’s chart of 1311 uses symbols to indicate cardinal points, and Dalorto’s chart of 1325 shows a circle with an eight‑point star denoting the main points.

The complete compass rose appears integrally in the Catalan map of 1375. Given the consistent arrangement of the lines across charts, it is reasonable to conclude they depict compass bearings.

Magnetic Variation and Chart Orientation

Compared with modern charts, the central axis of the Mediterranean on portolans is rotated about 10° to the left.

It is believed the magnetic variation in the Mediterranean at that time was roughly ten degrees east, suggesting the charts were drawn with magnetic north vertical.

Using the Lines for Navigation

No contemporary manual survives explaining the direction‑line system, but the sixteenth‑century John Rotz describes a method using dividers to find the ray nearest the course between two points on the chart and then reading the bearing from the nearest compass rose.

Later navigators used a parallel ruler. Lines were often drawn in alternating colours to reduce mistakes.

This system allowed courses to be plotted quickly over long distances, contrasting with coastwise navigation using written portolanos, and marked a fundamental difference between charts and sailing‑direction books.

Projection and the Earth’s Sphericity

Portolan charts lack a grid of parallels and meridians; they treat the mapped area as a plane and ignore meridian convergence. For the limited latitudinal range covered this was not a serious defect: the direction lines approximate loxodromes (lines of constant bearing).

Only in the early sixteenth century did charts begin to include a scale of latitudes.

As oceanic navigation expanded, latitude observations became important to verify dead reckoning and the Earth’s sphericity could no longer be ignored, leading ultimately to the Mercator projection, which represents lines of constant bearing as straight lines.

Origins, the Compass, and Dating

Portolan charts were closely linked to the compass, whose introduction made their construction practicable.

Some scholars (e.g. Professor Wagner) argue for earlier origins based on scale comparisons with ancient units, proposing incorporation of older sailing directions.

Surviving medieval portolani include brief directional lists (for example Adam of Bremen’s twelfth‑century reference), but such summaries alone probably could not produce the detailed charts; the direction‑line method depended on the compass.

A primitive floating‑needle compass probably existed in the twelfth century, with an improved needle balanced on a pin around 1250; later the compass card was added, allowing bearings to be taken quickly.

The earliest physical chart likely dates to the third quarter of the thirteenth century: the Carte Pisane is usually assigned to the late thirteenth century, the first dated chart is from 1311, and literary references to charts appear from about 1250–75.

Recorded evidence indicates charts were in use by 1270 (King Louis IX’s fleet and pilots identifying Cagliari on a chart), and Raymond Lull mentioned charts among seamen’s instruments. Thus the portolan chart probably originated around 1250–75.

Adoption and Early Use

Adoption by Mediterranean navigators was gradual. In 1354 King Peter of Aragon ordered each war galley to carry two sailing charts, a measure that likely stimulated Catalan chart production.

Some planners adopted charts early: Marino Sanudo included charts by Petrus Vesconte in his crusading plans.

Vesconte combined new chart information with older sources, while Dalorto incorporated small medieval world‑map motifs (a T‑O map), stock phrases like “Europa incipit ad Gallicia” and vignettes such as the Tower of Babel, indicating a transitional mindset between traditions.

Updating Regional Detail: The British Isles

Progress in updating regional detail is visible in successive depictions of the British Isles. On the Carte Pisane, Britain is crudely shown and lies outside the main chart.

From about 1325 fuller representations are attempted, but knowledge of Scotland is limited and Ireland is oversized relative to England. Perino del Vaga’s 1327 outline shows accurate knowledge largely confined to southern England (from the Bristol Channel to the Thames); northern and western coasts remain poorly detailed.

This suggests relatively accurate local surveys were appended to older, more generalized outlines. Improvements to the southern English coast appear by the early fifteenth century (for example G. Pasqualini, Venice, 1408), likely tied to growing commercial links between northern Italy and western Europe: Venice’s “Flanders galleys” (first mentioned 1317) called at Southampton, Sandwich and London, and Vesconte may have obtained coastal observations from their commanders.

Possible Origins of the Surveys

Some have proposed a coordinated Genoese survey under Admiral Benedetto Zaccaria in the late thirteenth century to explain the rapid charting of coasts, though direct evidence is lacking.

Conclusion

In summary, portolan charts emerged in the second half of the thirteenth century, closely tied to the mariner’s compass, and were principally developed by northern Italian navigators and cartographers—especially from Genoa and Venice.

They arose to meet the needs of Italian commercial communities expanding maritime communications.

The thirteenth‑century chartmakers’ achievement was a major practical advance in geographic knowledge that remained unsurpassed for centuries.

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