So is this something you intend to publish yourself, or is it a project that will be published by somebody else? Now if you commission a cartographer to create the interior of your landshape, unless you pay for the "ownership" to fully own the copyright, what is the problem of the artist retaining copyright? The artist doesn't own the story that the map facilitates, nor presumably the labeled locations on the map, as those are content from your story as well. Allowing the artist to retain copyright after a year from publication is pretty common in the industry - among those publishers I still create maps on commission are under that agreement. Still, I cannot publish the map separately for a different story, for a different novel, for a different author. So what is the problem with allowing the artist to retain copyright? It won't affect ownership of your product, that remains yours.
For the most part, I'm done as freelance cartographer, though I have been one for almost 15 years. You might notice the title under my account name as "publisher". I became that in 2010, but back then, I was just releasing standalone map products usable in any RPG game, not for a specific story or adventure - taverns, castles, that kind of thing. However, I also published a Japanese horror setting, as an imprint under Rite Publishing - 15 guides, supplements and modules. Now I publish supplements and modules for Starfinder RPG, and I do so for 3 authors, besides myself.
Most publishers begin as author/game designers, not illustrators. However, that's how I got into the industry. As a small publisher now, I still create all my maps, but I also create all my own illustrations for all my products. The next thread following this one, right now, shows the 3D work I've been doing. I do my own art, because, as a small publisher, I don't really have an art budget. So if I want art in my publications, I do it myself. While my professional cartography is well known, I'm just now getting better at 3D.
I'm describing all this, because part of the reason, I create my own art, is to retain full copyright of everything I publish, and not worry about other's ownership in my products. With my author/game designers. whom I publish for, I split profits with them, and they retain authorship of the work, but I own the copyright. As artist/cartographer, I fully own the copyright of that too. That's my advantage being a publisher, with competent artistic ability. While I'm not "rich", I am successful, and have a big advantage over my peers among the publishing industry, because I create my own art. My only cost is time and effort, but I can include a dozen maps in a product, whereas most publishers cannot afford that.
GP