How to filter ground w/ Octomap leaf iterator?

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Hello,

I know that it is possible to filter ground using the RANSAC method, but I need to filter ground in some outdoor environments where the ground is hardly fit to a plane.

I want to know if there is a fast way of looking for the lowest (min z) voxel at each x,y coordinate.

Right now, I am using a leaf_iterator and if a voxel is occupied, I check all of the nodes below. If they are free or null, then it is ground. But this is a rather expensive operation and I might check for ground for several voxels at the same x,y, even if I already found ground in that coordinate.

Thank you in advance.