Throughout previous texts I have shown that the inner workings of Portolan Charts and Atlases indicate several distorting factors which have the effect of slewing the chart anti-clockwise from c3 to c11 degrees. That difference can be evaluated as the accuracy of the charts author in his draughtsmanship. But a study of the Iberian Peninsula showed that the basic slewing was in fact c12 degrees and emanated from one single fact; the draughtsman had used two distinct measurements for a degree of latitude which are in fact the same measurement in different forms but had used a single scale bar to measure them, thus totally ruining any accuracy that may have been possible. Therefore the Iberian Peninsula has the in-built potential to totally slew the whole chart by c12 degrees.
But that appeared to be a rather curious situation akin to the tail wagging the dog.
Thus I considered that the problem is in fact the opposite; that is, the chart was first drawn with a 12 degree slew anti-clockwise, then the Iberian Peninsula was tacked onto the chart, which would be correct if it was, as I have suggested previously a Roman Map. As the Roman World expanded the chart grew to encompass all of their lands and thus was not necessarily at first a full Mediterranean Sea basin map, but evolved from several forms.
Marcus Vipsanius Agrippa produced a “world” map in c11BCE following a “world survey” instigated by Julius Caesar in c54BCE, and that map was displayed in Rome.
Thus using extant Roman Texts this paper sets down the scenario for the 12 degree slewing of one of the original Roman Charts and how it developed into a Portolan Chart.
I have added to this text the work by Joan Blaeu, not for his charts but for his written words which discuss many of the problems I have uncovered.
This paper contains 19, A4 pages and 17, A4 diagrams.