Begin with the Geogen surname-mapping search engine (christoph.stoepel.net/geogen/en/). This site allows you to search for data on any German surname, resulting in detailed maps showing both relative and absolute population densities per district. Geogen also includes a map search to help you find your surname's geographic namesake.

Look at the Homepage Diederich surname distribution maps (h-diedrich.com/namensverteilung). Though limited to a few dozen names, the site provides detailed maps, showing population data for each zip code. If you find your name among the few, the site can help you focus your research. Note the site is German-only, so you'll want to use Google Translate (translate.google.com/) if you don't know the language.

Read the detailed map analysis of your name found at the archive of Thomas Rueffer's German Surnames website (web.archive.org/web/20081218181927). The site contains maps for 140 common surnames less detailed than Geogen's, but with indepth discussion for each map. Also included is a free eBook on the history and origins of German surnames, together with an offer to map your family's name for only $2.