The MT-Generators

An MT-generator is a program that incrementally modifies a TIN through a sequence of local updates, and converts such updates into an MT by calling the building operations provided from the MT-manager.

The input data are the usual data available for a terrain: typically, a set of points sampled on the terrain surface, plus possibly a set of line segments representing line features of the terrain.

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Such data can be obtained either directly, through data acquisition techniques (e.g., photogrammetry), or by processing other data sources, such as a digital terrain model (DTM) at high resolution encoded as a raster, as a TIN, or as a contour map.

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A specific MT-generator is characterized by:

The approach followed in modifying an initial TIN, which can be either refinement (i.e., from coarse to high resolution) or simplification (i.e., from fine to coarse resolution);
The kind of local update used;
The criterion adopted to select updates to be performed, which is often a consequence of a specific measure of accuracy.

The specific characteristics of the update sequence affect the characteristics of the MT produced (e.g., size of the data structure, speed in extraction, adaptivity to terrain features, etc.). A comparative study of some techniques for building an MT, which are also provided as MT-generators in VARIANT, is contained in [De Floriani et al.1997]. In the following, we briefly address the three items above in order to clarify policies that can be adopted in writing an MT-generator, and how they can effect the result.

Subsections