  The spatial arrangement of urban hubs and centers and how individuals
interact with these centers is a crucial problem with many applications ranging
from urban planning to epidemiology. We utilize here in an unprecedented manner
the large scale, real-time 'Oyster' card database of individual person
movements in the London subway to reveal the structure and organization of the
city. We show that patterns of intraurban movement are strongly heterogeneous
in terms of volume, but not in terms of distance travelled, and that there is a
polycentric structure composed of large flows organized around a limited number
of activity centers. For smaller flows, the pattern of connections becomes
richer and more complex and is not strictly hierarchical since it mixes
different levels consisting of different orders of magnitude. This new
understanding can shed light on the impact of new urban projects on the
evolution of the polycentric configuration of a city and the dense structure of
its centers and it provides an initial approach to modeling flows in an urban
system.
