Conclusions and Future Work

In this paper we have described VARIANT, a prototype system for processing and visualizing terrains at variable resolution based on a special multiresolution model, called the Multi-Triangulation. We have briefly presented the MT, we have illustrated the architecture of the system and described the various components of the system itself.

We are currently implementing the following modules that will become part of the system:

We have developed a technique for the management of Multi-Triangulations in secondary memory. This problem is relevant because multiresolution deals with huge amounts of data, likely leading to structures too big to fit in memory. In GISs, often terrain datasets have such a size that a single-resolution terrain model built from such sets would be too big to be loaded into memory. The same problem arises even more likely for an MT, which represents the terrain at a wide range of resolutions including the maximum one.

TAGLIATO CORTO Data structures and algorithms for the management of MTs on secondary storage can be designed for solving two different problems:

  1. given a large MT on disk, load just a part of it, and then apply query algorithms in main memory to the loaded part;
  2. define and apply query algorithms having a direct access to a large MT resident on disk.
For Problem 1, in [#!Mag99!#] we have presented a method based on splitting an MT into parts which are still MTs and which can be loaded separately, as well as algorithms for selecting the parts of the split MT which must be loaded for answering a given local query. Problem 2 requires methods that allow the efficient retrieval of the disk page containing a part of an MT, when access to that portion is required. We are currently studying an indexing technique based on the two spatial coordinates x and y plus a third coordinate representing the depth in the DAG. FINE TAGLIO

The technique we have developed solves the following problem: given a large MT on disk, load just a part of it, and then apply query algorithms in main memory to the loaded part. The method is based on splitting an MT into parts which are still MTs and which can be loaded separately. Algorithms have been designed for selecting the parts of the splitted MT which must be loaded for answering a given local query [#!Mag99!#].

For its modular structure, VARIANT is an extensible system and allows the addition of further modules as soon as they are developed. In the future, we are planning to add MT-clients performing specific computations such as computation of optimal paths.

Programs that compute drainage, and simulate behavior of water flow can take advantage of the possibility to use higher accuracy only in the proximity of most important flow channels (rivers), and/or of critical regions (e.g., regions that might be subject to floods). Also in this case, it is possible to improve performance by adopting suitable threshold functions, increasing with distance from regions of interest. For instance, if the drainage network related to a given river must be extracted, the threshold function can be increasing with distance from the river, while the region of interest might be limited to the slopes around its valley.

Paths can be defined on a terrain according to various optimality criteria, e.g., paths of minimum length, paths with minimum variation in height, with minimum slope along the path, paths avoiding or preferring certain types of soil (see [#!BerKre97!#,#!LanMahSac97!#,#!Kre94PATH!#] for various characterizations of optimal paths). Multiresolution can help optimization tasks since it gives the possibility of reasoning on a terrain model at increasingly high accuracy, while progressively refining a rough candidate solution.