1. A method, comprising: receiving, by one or more processors, from a three-dimensional vision sensor of a robot, a group of data points sensed by the three-dimensional vision sensor; identifying, by the one or more processors, one or more voxels of a three-dimensional voxel model that are occupied by the group of data points; determining, by the one or more processors, for each occupied voxel, data representing the occupied voxel, wherein the data includes statistics about how a plurality of data points of the group of data points are spatially distributed within the occupied voxel; identifying, by the one or more processors, for each occupied voxel, a column of the three-dimensional voxel model that contains the occupied voxel; and indexing in memory, by the one or more processors the data representing voxels of the three-dimensional voxel model, the indexing comprising indexing, in the memory, the data representing occupied voxels contained in each column by elevation without indexing one or more unoccupied voxels by elevation.