Recently wrote a short article for a trend report of urban company JCDecaux. The whole book is called Les Audiences dans la Ville (The Audiences of the City) and addresses the new practices of people in contemporary cities. Other writers include: Fabien, Adam Greenfield, Frederic Kaplan, Daniel Kaplan, Bruno Marzloff and lots of others interesting people from the media/design/urbanism community. It's in french though.

My contribution called "social navigation in urban space" starts by describing skateboarder as an interesting metaphor for making spatial decision in the city (i.e. choosing where to go, which area to visit and "do something there"). Skateboarder indeed rely on: looking for physical spots (implicit traces like grinded or waxed sidewalks/benches), active tagging (stickers which are explicit traces), social knowledge of spots or often maps found in magazines or on the Web (community traces). Looking for a spot is, in essence, looking at the physical environment, relying on one's community or finding resources. I then describe how technological devices (gps, cell phones, digital cameras...) allow the same functions as they produce electronic traces that we can read/analyse to make spatial decisions (through social navigation). We can use the same typology:

the implicit traces: the one that appeared because of people's activity (doing skateboard results in grinded benches, using your cell phone allow to provide digital traces to some companies).
the explicit traces: active tagging of the environment in its ecology (graffiti, yell arrow).
community traces: active tagging or reference to the environment on an external resource (web).

The article goes on by discussing what does that mean to have traces and what sort of urban services we can think about that can take advantage of them.