Basic techniques for building an MT come directly from algorithms for
terrain generalization. There are two basic approaches:
- Refinement methods start from a coarse TIN and
improve resolution progressively by the iterative application
of updates that increase resolution locally.
Examples of such methods for terrain generalization were
proposed, e.g., in
[De Floriani et al.1985,Fowler and Little1979,Franklin1994,Rippa1992].
The initial coarse TIN can be built either by a triangulation
of a small set of terrain features extracted with some standard
method from input
data, as in [Fowler and Little1979], or simply by triangulation of the
polygon bounding the domain, as in [De Floriani et al.1985].
In this case, the initial TIN corresponds to the root of the corresponding
MT, while the
- Simplification methods start from a TIN
at the highest possible resolution, directly built on the
whole data set, and coarsen resolution progressively by the
iterative application of updates that reduce resolution
locally.
Examples of such methods were
proposed, e.g., in [de Berg and Dobrindt1995,Hoppe1996,Lee1989].
In this case, the TIN resulting at the end of the process coincides with
the root of the corresponding MT, while the sequence of updates provides
information for building the MT nodes from fine to coarse resolution.
In this sense, a simplification sequence can be also seen as a refinement
sequence played in reverse.
There is no clear superiority of one such approach with respect to the other.
In both cases, it can be convenient to apply a sequence of batches of
independent updates, rather than a greedy sequence of single local updates
[de Berg and Dobrindt1995].
This permits to improve the performance of the MT as a spatial index
(see [De Floriani et al.1997] for details).
In VARIANT, we provide a few MT-generators based on variants of both
approaches.