With the increasing popularity of web-based map browsers, remotely obtaining a high quality depiction of cartographic information has become commonplace. Most web mapping systems, however, rely on high-capacity servers transmitting pre-rendered tiled maps in raster format. That approach is capable of producing good quality renderings on the client side while using limited bandwidth and exploiting the browser's image cache. These goals are harder to achieve for maps in vector format. In this work, we present an alternative client-server architecture capable of progressively transmitting vector maps in levels-of-detail (LOD) by using techniques such as polygonal line simplification, spatial data structures and, most importantly, a customized memory management algorithm. A multiplatform implementation of this system is described, where the client application is written entirely in JavaScript and processed within the web browser, avoiding the need of external applications or plug-ins. Results of experiments aimed at gauging both the performance and the display quality obtained with the system are presented and explained. Extensions to the system are also discussed, including issues such as level-of-detail versus visual importance tradeoffs and the handling of closed polygonal lines.
vector maps; map browsers; progressively transmitting; level of detail; client-server architecture
