Conformal Cylindrical Properties of Adriatic Sea Basin Renderings on Portolan Charts

ABSTRACT

The geometry of Adriatic Sea basin renderings on 12 portolan charts made from the late thirteenth to the late sixteenth century was compared to a modern map by applying Helmert transformation in order to preserve their initial geometry and by using the same 40 identical points per chart. In the first stage of the research, the geometry of the portolan charts was compared to six selected map projections in order to determine the geometrical ‘best fit'. In the second stage, networks of lines plotted on charts were divided regionally and functionally and compared to the ‘best fit’ map projection to determine their geometric patterns in more detail. The research results showed that Adriatic Sea basin renderings on portolan charts were probably the result of a deliberately applied map projection which was geometrically most similar to the modern conformal cylindrical (Mercator) projection.

KEYWORDS:

• Portolan charts
• mapping accuracy
• conformal cylindrical projection
• Mercator projection
• Adriatic Sea

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes on the contributor

Tome Marelić is a research assistant at the Department of Geography, University of Zadar, Croatia. His prime research interest is quantitative (cartometric) analysis of historical maps and charts with the application of GIS and statistical software. His current research is focused on geometric properties of Adriatic Sea basin coastline renderings on late medieval and modern navigational charts.

Notes

1 The oldest known map on which the graticule of latitudes was rendered, from which it could be concluded that the map was deliberately projected in a normal equidistant cylindrical projection, was the Cantino planisphere chart dated 1502 (Gaspar, 2013: 69–70), now kept in the Biblioteca Estense, Modena, Italy (Call No: c.g.a.2).

2 This map projection (EPSG code: 32662) is labelled plate carrée (French for flat square) only if the equator is selected as the standard parallel (the parallel of true scale), although any other pair of parallels (of the same latitude) can be used as standard parallels (φ₀>0°). However, in such cases, the graticule grid does not have a square appearance (Snyder and Voxland, 1989: 22–25, 219; Snyder, 1993: 5).

3 The distance unit used for portolan chart scale bars was the portolan mile (also known as the millaria, miglia, or mia in abbreviation). The connection between markings on the scale bars and the portolan mile is evident on the 1512 Vesconte de Maiolo chart and on an earlier chart by Giovanni da Carignano, made in late thirteenth or early fourteenth century (it was kept in the Archivio di Stato, Florence, Italy, until it was destroyed in an Allied airstrike in 1943). According to these documents, the distance between two dots on a scale bar corresponded to ten portolan miles (Nordenskiöld, 1897: 22; Winter, 1956: 43–44; Campbell, 1987: 380).

4 It is difficult to tell with certainty how exactly these authors came to this conclusion since they do not support it with solid arguments. It might be that they had thought that the cartographers who made the preserved portolan charts were oblivious to the methodology of projecting Earth’s spherical geometry onto a plane (or unaware of it), or that they had not considered it as a necessity in order to create accurate nautical charts.

5 For latitude (φ) values; ‘+’ represents north (N) direction, while, ‘−’ represents south (S) direction. For the longitude (λ) values; ‘+’ represents east (E) direction, while ‘−’ represents west (W) direction.

6 The reference map in a shapefile format (.shp) was downloaded from http://www.marineregions.org/downloads, (Claus et al., 2017) on 20 October 2017.

7 The Adriatic Sea basin vector shapefile area used in this research as a reference map is 139,654 km² when projected in GNOM. When projected in MERC43, its surface area is 139,731 km², with a latitude extent of 646 km, and a longitude extent of 602 km (roughly a square of 625 km × 625 km).

8 Latitude φ₀ = 42.7° (N) is the mid-latitude for the Adriatic Sea basin with its southern limits declared by IHO (International Hydrographic Organization), as used in this research (International Hydrographic Organization, 1953: 17).

9 Normal portolano is Nordenskiöld’s hypothetical portolan chart prototype made in the late medieval period, upon which all other portolan charts were based (Nordenskiöld, 1897: 10).

10 It was named the Pisan Chart because it was previously owned by a family from Pisa, but it was probably of Genoese production (Crone, 1953: 30).

11 On the Carte Pisane (CP 1290), which contains two wind roses with (invisible) rhumb circles, only the western rhumb circle (on which the Adriatic Sea is rendered) was geometrically corrected.

12 The higher the φ₀ value, the smaller the perimeter of its parallel compared to the meridian length. Since on a normal equidistant cylindrical projection map the meridians must be depicted in the same scale as the φ₀ parallel, by increasing the φ₀, the graticule network on a normal equidistant cylindrical projection becomes more ‘compressed’ along its X-axis (or ‘extended’ along its Y-axis).

13 A one half of one compass point for 32-wind compass rose.

14 See note 6.

15 See note 6.