1. One or more computer-readable storage media encoded with computer instructions that, when executed on a processor, direct the processor to perform operations comprising: constructing a data structure, the data structure comprising: a first part forming a partition axis for at least one node, the partition axis based at least in part on a binary combination of top coordinate axes for the at least one node ranked by variances; and a second part identifying the partition axis for the at least one node relative to a partition axis of an ancestor node.