Migratiomancy is the art of predicting future events by observing the commuters in transit. The same principles used in migratiomancy can be applied to the observation of animal herds of flocks of birds, but as it is a rather modern method it is chiefly concerned with commuters.
The principle is simple: the people traveling to and from their jobs are the life-blood of the city; by studying them the seer can divine the state of the innastarn (the spirit of the city) and through that predict future events.
What is important is to watch the flow of people passing by. Observing other commuters while en route to work will not yield the same degree of reliability as sitting still and watching people go by. It is in the continous flow of commuters that truths are hidden and they are easier to spot from a distance than while caught up among them.
Migratiomancy has proven to be reliable enough that most larger cities employ at least a few seers to practice it regularly, for the safety of the city. In much the same way large corporations, political parties and crimelords often rely on seers using migratiomancy to predict important future events.