Die Weltkarte des Juan de la Cosa (1500 n.Chr.)

Abstract

As an eye witness to the discovery of America by Columbus, Juan de Ia Cosa possessed excellent and authentic knowledge of geography. His map of the world of 1500 a.d. is the subject of this article, which documents the computer-supported cartometric study undertaken to determine the degree of the map's precision. Based on an analysis of the map's "external" geometry (paper distortion), its "internal" precision was investigated by means of the locational comparison of points on the old map with the corresponding areas on a modern map. This process begins with the definition and digitalisation of "identical" points (salient coastline landmarks, identifiable cities) on the historic map. Then the relevant geographical coordinates of these points are determined and projected on a freely selectable plane. The comparison of the Cosa map's historical depiction with that of the modern map is carried out by means of an adjusted coordinate transformation (Helmert transformation) of the identical points. The criterion for the quality of correspondence between the two projections is the size of the remaining errors of position with regard to the identical points. The comparison is carried out iteratively by changing the map projection with the goal of minimising the remaining errors. According to the numerical investigation of various regions, the Mediterranean area exhibits mean square error of ± 73 km. The analysis of the entire region comprising the Mediterranean, Northwest Africa and Cuba reveals a standard deviation of ± 1 81 km, corresponding to approximately 3% of the distance to America.