Its nice to know that someone knows the reason. I would suggest  
 The only coherent, though often deficient source for knowlege of the (British) Islands that has come down to us from the most flourishing period of the Empire, is the map of Ptolemy, the result of a combination of the lines of roads and of the the coasting expeditions during the first century of Roman occupation. One great fault, however, has crept into the map by his having made use of a totally different source, namely the astronomical fixations of lattitude executed by Pytheas 
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