  Despite the services of sophisticated search engines like Google, there are a
number of interesting information sources which are useful but largely
inaccessible to current Web users. These information sources are often ad-hoc,
location-specific and only useful for users over short periods of time, or
relate to tacit knowledge of users or implicit knowledge in crowds. The
solution presented in this paper addresses these problems by introducing an
integrated concept of "location" and "presence" across the physical and virtual
worlds enabling ad-hoc socializing of users interested in, or looking for
similar information. While the definition of presence in the physical world is
straightforward - through a spatial location and vicinity at a certain point in
time - their definitions in the virtual world are neither obvious nor trivial.
Based on a detailed analysis we provide an integrated spatial model spanning
both worlds which enables us to define presence of users in a unified way. This
integrated model allows us to enable ad-hoc socializing of users browsing the
Web with users in the physical world specific to their joint information needs
and allows us to unlock the untapped information sources mentioned above. We
describe a proof-of-concept implementation of our model and provide an
empirical analysis based on real-world experiments.
