Map View Distance/fog

I’m making a map like harbor2ocean but it is more military and modern with a larger harbors, anyways i am wondering how the creator of Harbor2Ocean made it so you couldn’t see across the whole map. Is this fog? and how do i make this happen on my map?

env_fog_controller entity.

A big map like that will need a hell of a lot of optimizing.

I found this guide here to be a lot of help.