Vision range, expressed in hexes radius, varies based on the abilities of the unit and whether it is on the surface layer or the underground layer. The table below summarizes the different base vision range and the various bonus that apply to it.
|Ability||Surface vision range||Underground vision range|
|True Sight (reveals concealed units in vision range of the unit)||+0||+0|
|Vision Range Upgrade (stackable)||+1||+1|
|Explorer||+1 to boats (and cities)||+1 to boats (and cities)|
|Embarked (malus canceled by the Mariner ability and the Advanced Seafaring tech)||-1||-1|
|Shrine to the Fickle Mermaid||+2||+0|
|Dark Sun Cosmic Event||-2||-2|
Units have a minimum vision range of 1
Several items provide Vision Range Upgrade.
Several abilities allow units to be concealed. Concealed units will not be visible even if they are inside your vision range, except if you are directly adjacent to them or if your units have True Sight.
True Sight reveals concealed units only in the vision range of the unit possessing the ability. A Dwarf Forge Priest in a surface metropolis for instance will reveal concealed units only on a 3 hexes radius around him, not over the whole metropolis' domain.
More details on how the AI deals with concealment can be found on the page dedicated to concealment.