Jim Gray Alexander S. Szalay Aniruddha R. Thakar Gyorgy Fekete William O'Mullane María A. Nieto-Santisteban Gerd Heber Arnold H. Rots There Goes the Neighborhood: Relational Algebra for Spatial Data Search http://arxiv.org/abs/cs.DB/0408031 2004 CoRR cs.DB/0408031 db/journals/corr/corr0408.html#cs-DB-0408031