The original map is not yours, but whatever you do to it is yours. I work on a orphan and even though it is technically mine I consider it credit due to the original author. My work is just to develop it further and take on from where the other left off.I was speaking about the getting an orphan map but people still said it would not be my map.
Edit: Unnecessarily long explanation, call it yours if you so desire, but credit the original author for sure.