The App "Berlin – Divided City" "shows the course of the Berlin Wall and the former sectors of the city on a map.
If your are in Berlin, the app determines in which of the former sectors you are and displays your location on a map from 1989 and for comparison on a current map (OpenStreetMap).
The map which dates from the year 1989 and originates from the CIA (Central Intelligence Agency) is in the public domain. It shows the border line after the last exchange of territories between the GDR and West Berlin on 1 July 1988 and the reunification on 3 October 1989.
The basis for the boundaries of the sectors are the "Geometrien der Ortsteile von Berlin" (http://daten.berlin.de/datensaetze/geometrien-der-ortsteile-von-berlin-juli-2012, Statistical Office Berlin-Brandenburg, License: CC -BY 3.0 Attribution). At locations where these boundaries did not comply with the limits of 1988/89, they have been adjusted to the best knowledge.
The actual course of the wall differed partly from the legal boundary.