The map shows a great visualization of phone calls in UK from a study made by researchers at University College London, MIT, and Cornell University, with the help of BT Group.

The map depicts bidirectional traffic of around 12 billion phone calls over a period of one month. Each of the 3042 pixels represents an area of 9.5 x 9.5 km with the color intensity indicating the length of the call time. Figure 1 from 10.1371/journal.pone.0014248 Read more about the research, on the paper published at PLoS, Redrawing the Map of Great Britain from a Network of Human Interactions.