http://dx.doi.org/10.1007/3-540-52208-5_19 Jürg Nievergelt 7 ± 2 Criteria for Assessing and Comparing Spatial data Structures. 3-27 SSD/1989/P003.pdf 1989 conf/ssd/89 SSD db/conf/ssd/ssd89.html#Nievergelt89