O. Balovnev, M. Breunig, and A. Cremers. Advances in Spatial Databases, volume 1262 of Lecture Notes in Computer Science, Springer, Berlin / Heidelberg, (1997)
M. Trzaska, and K. Subieta. Proc. of the Eighth East-European Conference on Advances in Databases and Information Systems (ADBIS'2004, page 228--240. (2004)