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This work presents a high-performance algorithm to compute the horizon in very large high-resolution DEMs. We used Stewart's algorithm as the core of our implementation and considered that the horizon has three components: the ground, near, and far horizons. To eliminate the edge-effect, we introduced a multi-resolution halo method. Moreover, we used a new data partition approach, to substantially increase the parallelism in the algorithm. In addition, several optimizations have been applied to considerably reduce the number of arithmetical operations in the core of the algorithm. The experimental results have demonstrated that by applying the above-described contributions, the proposed algorithm is more than twice faster than Stewart's algorithm while maintaining the same accuracy. IntroductionIn the last decade, new larger Digital Elevation Models (DEMs) of higher resolution are being created. For example, a DEM of the entire world of size 15 · 10 15 points and precision 3 arc seconds (,90 · 90 m 2 at the equator) is now available in SRTM (Shuttle Radar Topography Mission) Database (CGIAR-consortium for spatial information 2007), in addition to a large number of DEMs of many regions of the world with resolution greater than 1 arc second (30 · 30 m 2 ). Moreover, in the near future, new global products of higher resolution are expected, as is the case for the DEM of the United States of resolution 10 · 10 m 2 . This fact is generating a huge need for new efficient high-performance algorithms.Knowledge of the visible parts of a terrain from a given point is important in many fields, in shading and visibility applications, in the development of geographic information systems, and particularly in solar irradiation models. For example, the computation of the horizon at all the points of a terrain is essential for an accurate calculation of direct and diffuse irradiation in all advanced solar radiation models (Dubayah, and Rich 1995, Fu and Rich 2007). In the case of ESRA (European Solar Radiation Atlas) models (Schamer and Greif 2000), the principal mathematical reference for several solar irradiation softwares (Marcel and Jaroslav 2004, Romero et al. 2008), the horizon computation is the most costly part of these models even for small areas . Moreover, running these

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