Zhao H, Smith BK, Norman K, Plaisant C, Shneiderman B.  2005.  Interactive sonification of choropleth maps. IEEE Multimedia. 12(2):26-35.

Dang G, North C, Shneiderman B.  2001.  Dynamic queries and brushing on choropleth maps. Fifth International Conference on Information Visualisation, 2001. Proceedings. :757-764.