In order to interactively investigate large-scale 3D data sets, we
propose an improved hierarchical data structure for structured grids
and an original hierarchical data structure for unstructured grids.
These multi-tiered implementations allow the user to interactively
control both the local and global density of the mesh.  Therefore, the
user can interactively refine areas of interest and decimate
peripheral regions.  By controlling the density of the mesh throughout
the volume, the user controls where computational cycles are spent and
gains a deeper insight into the geometric structure of the mesh.