The Observation Post is the Gunpowder Age upgrade of the Lookout and serves the same purpose: It lifts the fog of war within a reasonable long range, making it an important early warning system. It also detects invisible units (such as Spies) within its line of sight. Note, that the building's line of sight does not increase with the upgrade - instead, it increases with each level of File:Science.png Science technology researched by +3 tiles.