I think that you are in Mount Airy if your zip code is 19119. Although I believe that the area bleeds into other areas. I have always considered to border going all the way to Washington Lane(as opposed to Johnson Street) on the south side and up to Lowber Avenue(As opposed to Stenton Avenue) on the east side due to geographical boundaries(Graveyards,Roads ending etc.) But I see the Wissahickon as a "Hard" border as well as Creshiem Valley Drive.