Not just old marine maps -- modern marine charts have the same. They mark radio towers, grain silos, etc. because although GPS is widely-used, the old navigation techniques are still important.
Re:Paper Map Sales Are Booming
by Turing_Machine on Monday January 23, 2023 @06:03PM
(#34496360)
I believe the U.S. Naval Academy actually stopped teaching old school celestial navigation for a while, then someone with a bit of common sense realized that there were scenarios (particularly in a war situation) where the GPS constellation might not be available, and started teaching it again.
You can't just pull over at a gas station in the middle of the Pacific Ocean and ask for directions. :-)
Old marine maps had various landmarks on them, along with their height. This allowed you to get angle and distance, giving you the opportunity to calculate position with pen and paper even if you only had a single landmark in view.