http://dx.doi.org/10.1007/3-540-48482-5_7 Christian Böhm Gerald Klump Hans-Peter Kriegel XZ-Ordering: A Space-Filling Curve for Objects with Spatial Extension. 75-90 SSD/1999/16510075.pdf 1999 conf/ssd/99 SSD http://link.springer.de/link/service/series/0558/bibs/1651/16510075.htm db/conf/ssd/ssd99.html#BohmKK99