Beyer, Klaus G.
Berlin : Aufbau-Verlag Berlin und Weimar, 1984,
Buku Referensi
BAKOSURTANAL, 2008
Buku Teks
Getis, Arthur, author
McGraw-Hill Higher Education, 2009
Buku Teks
by Les Rowntree ... [and others]
Prentice-Hall, 2011
Buku Teks
Trewartha, Glenn T., author
McGraw-Hill, 1967
Buku Teks