Fig. 10: Ptolemy’s map of Southern and South-Western England

The squares are towns on Ptolemy’s map; the circles are the places in their actual locations.

On the functioning of this unique map, see here. Distances are approximate but bearings from Londinium are remarkably precise, with an average deviation of only 1.8° for the south-west and 1.7° for the rest of the map. In fact, since the survey was based, not on degrees, but on right-angled triangles with whole-number sides, most of the deviations are consistent with the inevitable margin of error. The blatant exception is Rutupiae (Richborough), which deviates from the true bearing by 13.5°. Its coordinates may belong to the faulty coastal data (here) or, since this was the landing site of the invasion of AD 43, to military measurements using water clocks.

The coordinates of Corinnium (or Corinium) and Calcua (or Caleva) clearly define the positions of Gloucester and Dorchester-on-Thames. Bearings from these two places to the other towns are almost as accurate as they are from Londinium. The only evidence for the Roman name of Cirencester (probably a later, Anglo-Saxon name) is Ptolemy’s misinterpreted map and the muddled Ravenna Cosmography of the seventh century (Coates). Cirencester itself became an important Roman town only about twenty years after the production of the map.

The map of the far south-west uses a different graticule and takes Isca (Exeter) rather than Londinium as its focal point.

A section or panel of the original map appears to be missing. Logically, it would have included several other major sites at road junctions: Kenchester, Towcester, Water Newton, etc. The original may have been engraved on metal plates or on stone tablets, like the Roman cadastral map of Orange (second century AD). To fill the gap, Ptolemy or Marinus of Tyre shifted the upper section of the map, which reflects a slightly later stage of the Roman conquest, to the south and west. The seven places in this section are mutually coherent and share the same directional accuracy relative to Londinium.