You do realize that you and an artist "could" agree upon an exclusive license for your use of the map(s), with a set price and time period limit for agreement of purchase of the copyright? Ultimately, this arrangement can protect both sides with proper negotiating. Typically, copyright lasts for pretty much a standard lifetime or two(this varies by country of course) which is between 70ish-150ish years. License means you and the artist agree to terms of who can use the work, for how long, and in what manner. There is no reason you cannot negotiate the license with a set price to purchase the copyright at the end of the license period. This would provide you with the ability to use the art work "now" and yet still protect the artist.... if you don't pay at the end of the license period(or periods if you can negotiate multiple checkpoints), you stop using the art.