  Population migration is valuable information which leads to proper decision
in urban-planning strategy, massive investment, and many other fields. For
instance, inter-city migration is a posterior evidence to see if the
government's constrain of population works, and inter-community immigration
might be a prior evidence of real estate price hike. With timely data, it is
also impossible to compare which city is more favorable for the people, suppose
the cities release different new regulations, we could also compare the
customers of different real estate development groups, where they come from,
where they probably will go. Unfortunately these data was not available.
  In this paper, leveraging the data generated by positioning team in Didi, we
propose a novel approach that timely monitoring population migration from
community scale to provincial scale. Migration can be detected as soon as in a
week. It could be faster, the setting of a week is for statistical purpose. A
monitoring system is developed, then applied nation wide in China, some
observations derived from the system will be presented in this paper.
  This new method of migration perception is origin from the insight that
nowadays people mostly moving with their personal Access Point (AP), also known
as WiFi hotspot. Assume that the ratio of AP moving to the migration of
population is constant, analysis of comparative population migration would be
feasible. More exact quantitative research would also be done with few sample
research and model regression.
  The procedures of processing data includes many steps: eliminating the impact
of pseudo-migration AP, for instance pocket WiFi, and second-hand traded
router; distinguishing moving of population with moving of companies;
identifying shifting of AP by the finger print clusters, etc..
