I'm wondering if they may be cousins. From the Baltimore City Directories that I have copied, and they are scattered years, there are not many Roder/Roeders in Baltimore. I'm guessing mine stayed in Baltimore, as well. Hartmann's daughter, Josephine Roeder married a fellow who was studying for the priesthood in Baltimore. He taught in the area (Bryantown, MD), and they then moved to Upper Marlboro, MD. I don't believe that her folks moved though.
Do you know where your Roeders are buried in Baltimore? That might help us connect, too.
Thanks for your help!