Jim Gray Alexander S. Szalay Tamas Budavari Robert Lupton María A. Nieto-Santisteban Ani Thakar Cross-Matching Multiple Spatial Observations and Dealing with Missing Data http://arxiv.org/abs/cs/0701172 2007 CoRR abs/cs/0701172 db/journals/corr/corr0701.html#abs-cs-0701172 informal publication