The origin problem of nautical cartography: the importance of evidence and method

Figures & data

Figure 1. Tracing of the portolan chart of Angelino Dulceti (1339).

Table 1. Portolan charts analysed.

Name / cartographer	Date	Chart owner
*Carte Pisane*	∼1270	Bibliothèque nationale de France (BnF), Cartes et Plans, Paris
Cortona chart	∼1300	Biblioteca dell'Accademia Etrusca di Cortona
*Riccardiana*	1300–1325	Biblioteca Riccardiana, Florence
Angelo Dalorto	1325	Privately owned, Florence
*Angelino Dulceti*	1339	BnF, Cartes et Plans, Paris
*Ristow-Skelton #3*	1325–1350	Library of Congress, Washington, USA
Pizzigani brothers	1367	Biblioteca Palatina, Parma
Francesco Beccari	1403	Beinecke Rare Book and Manuscript Library, New Haven, USA
Mecia de Viladestes	1413	BnF, Cartes et Plans, Paris
*Petrus Roselli*	1466	James Ford Bell Library, Minneapolis, USA

Figure 2. Boundary between the Atlantic South (inside the dashed ellipse) and the Atlantic Mid sub-charts on Dulceti’s chart, of which the coastline tracing is shown.

Figure 3. Anticlockwise tilt angle of sub-charts in degrees. The mean magnetic declination for the year 1250 of the sub-charts is shown as small red circles. A proper representation of the spatial variation of the magnetic declination is provided in Figure 10.

Figure 4. Scale differences of sub-charts, relative to the western Mediterranean on the same portolan chart.

Figure 5. Stretch (or compression) of sub-charts in east–west direction.

Figure 6. Degree-of-fit (planimetric accuracy) of sub-charts in kilometres. In the rightmost column (grey) the degree-of-fit is shown of the entire Mediterranean and Black Sea, excluding the Atlantic coasts.

Figure 7. Length of the portolan mile in kilometres, derived from the scale bars on 10 charts and the numerical data in the Compasso de Navegare and Liber de existencia riveriarum. Scale bars on the same chart are grouped and shown in the same colour. The mile values were corrected for deformation of the parchment and apply to the mid-latitude (40°N) of the western Mediterranean.

Figure 8. Coverage areas of sub-charts

Figure 9. Composite of rectified sub-charts of Angelino Dulceti’s portolan chart. Its coastal outlines have been traced in blue for better visibility; the coastlines on the Mercator reference chart are shown in red.

Figure 10. Lines of equal magnetic declination in the Mediterranean of 1250.

Figure 11. Plane-charted network of rhumb lines in the western Mediterranean, used for simulating ‘medieval’ charting.

Figure 12. Bearing errors as a function of longitude in the Liber de existencia riveriarum in degrees. The horizontal red line indicates the mean error of 0.7°.

Figure 13. Bearing errors in the Liber de existencia against the background of bearing errors of tracks, scaled from the Carte Pisane.

Table 2. Comparison of the accuracy (standard deviations) of the data in the Liber de existencia riveriarum and the Compasso de Navegare.

	Liber	Compasso
bearings	22.4°	21.9°
distances	31%	32%