Efficiently Generating Multiple Representations for Web Mapping

Abstract

Line simplification is the most commonly used operation in map generalisation. In many commercial Geographic Information Systems (GIS), line simplification is performed by applying the classical Ramer-Douglas-Peucker (RDP) algorithm. However, such an algorithm has the drawback of not guaranteeing the preservation of topological consistency. This requires a posteriori checks to ensure that unwanted intersections introduced by the application of the algorithm get rectified. To overcome this problem, Saalfeld proposed a modification of the classical RDP algorithm based on the fact that, while generalising a polyline, conflicts can only occur with vertices of other polylines that lie within its convex hull. In this paper we propose an improvement to Saalfeld's algorithm to detect possible self-intersections of a simplified polyline more efficiently. This improves the performance especially when generalising very large datasets. Nevertheless, the processing time is still not acceptable for real-time web mapping. Therefore, we have integrated our algorithm into a web mapping system that pre-computes a sequence of topologically consistent map representations, stores them on the server, and transmits them progressively upon request. We present experimental results of the performance of the algorithm as well as results of the transmission system.