The navigation data in the Compasso de Navegare: Accuracy and provenance

The debate on whether or not portolan charts derive from portolans has been ongoing for some 150 years. This paper discusses a comprehensive analysis of
the Lo Compasso de Navigare, the oldest portolan containing a complete set of course and distance data covering the Mediterranean and Black Sea. The much-cited analysis by Lanman dates from 1987. Pflederer presented an analysis of a limited subset of data from the Compasso de Navegare during the First Workshop on the Origin and Evolution of Portolan Charts in Lisbon in 2016. Both studies were limited in their scope and based on data samples instead of the entire dataset. The study discussed in this paper is comprehensive in the sense that, as far as possible, all locations in the Mediterranean and the Black Sea mentioned in Lo Compasso de Navigare have been identified. That resulted in a dataset consisting of 1330 bearings and distance pairs, large enough to compute reliable statistics and enabled a comparison of nearly every bearing and distance pair in Lo Compasso de Navigare with their true values to be made. An important aspect of this, discussed in this paper, is the suggestion by James E. Kelley Jr. that “portolans may owe more to portolan charts than vice versa”, which translates into the question whether and if so, to what extent, Lo Compasso de Navigare may have been scaled from one or more pre-existing portolan charts.