It seems surprising that an academic living along the Nile had such detailed knowledge of northern Europe -- and it's certain that Ptolemy never took his own measurements in the Germanic lands. Instead, researchers believe he drew on Roman traders' travel itineraries, analyzed seafarers' notes and consulted maps used by Roman legions operating to the north.
So basically, Ptolemy was doing what these researchers in public health were doing:
Culotta and two student assistants analyzed more than 500 million Twitter messages over the eight-month period of August 2009 to May 2010, collected using Twitter’s application programming interface (API). By using a small number of keywords to track rates of influenza-related messages on Twitter, the team was able to forecast future influenza rates.
What was once old is new again!