We present a model for describing the visibility of a polyhedral terrain from a fixed viewpoint, based on a collection of nested horizons. We briefly introduce the concepts of mathematical and digital terrain models, and some background notions for visibility problems on terrains. Then, we define horizons on a polyhedral terrain, and introduce a visibility model, that we call the horizon map. We present a construction algorithm and a data structure for encoding the horizon map, and show how it can be used for solving point visibility queries with respect to a fixed viewpoint.
IntroductionDescribing a terrain through visibility information has a variety of applications, such as geomorphology, navigation and terrain exploration. Problems which can be solved based on visibility are, for instance, the computation of the minimum number of observation points needed to view a region, or the computation of optimal locations for television transmitters (Cole and Sharir 1989, Goodchild and Lee 1989, Lee 1991.A terrain can be mathematically modelled as a surface described by a continuous function z =+(x, y), defined over a connected, not necessarily convex, domain D. Two points on a terrain are considered mutually visible when they can be joined by a straight-line segment lying above the terrain. In practice, a mathematical terrain model is approximated through a digital model, built on the basis of a finite set of points belonging to the terrain. Visibility algorithms developed in the computational geometry literature usually operate on polyhedral digital models, i.e., planar-faced approximations of a terrain (Agarwal and Sharir 1986, Mulmuley 1989, De Berg et al. 1991, Katz et al. 1991, Overmars and Sharir 1992, Reif and Sen 1988. Visibility algorithms operating on gridded DTMs are also available in commercial GIs. Such algorithms, though simple in structure and implementation, are not necessarily efficient and accurate-the visibility information they produce is not generally accurate because it is computed by using the grid cells as visibility units and by assuming arbitrary simplifications for the surface defined over each cell (Lee 1991); see also (Fisher 1993) for a discussion of uncertainty in information obtained by existing grid visibility algorithms.In this paper, we address the problem of representing the visibility of a polyhedral terrain from a fixed viewpoint and efficiently solving point visibility queries, i.e.,