Gravity-type spatial interaction model fits for the mobile phone usage data.

<p>Gravity models were calibrated for each movement variable. A) The parameter values for are shown from the fit for various trip durations. Each parameter value from the census data is shown in the corresponding color as a dotted line. A gravity model was calibrated to fit the number of trips between counties lasting between 2 and 3 months. B) The actual data versus the gravity model fit is shown in the figure (Data/Fit). The ratio of true data to the results of the fitted model are shown broken down by C) population at the origin county, D) population at the destination and E) the distance (in kilometers) between the origin and destination. The model underestimates movements from low population counties (both as an origin and destination) and shorter trips.