t32 = class(nvxtot)

      vtxbds     Vertex bounds. This is an array of six values giving
                 the minimum and maximum values of each component of
                 the vertex set. Units are km.

                 [2,3] = size(vtxbds); double = class(vtxbds)

      voxsiz     Fine grid voxel size.  DSK voxels are cubes; the edge
                 length of each cube is given by the voxel size.  This
                 size applies to the fine voxel grid.  Units are km.

                 [1,1] = size