Empire Total Footprint (manual)

You're empire's total footprint is a measure of how much land it would use if it was built on a perfectly flat surface. If your footprint exceeds your total land, that doesn't mean you're done building; look at any city on earth and it's readily apparent that land does not work that way. Instead, it means that the cost of adding new investments will start to increase. The penalty becomes progressively more oppressive so that, ultimately, a lack of land will strangle development.