An Efficient Mechanism for Organizing and Indexing Tile Caches on Map Server

With the increasing popularity of global on-line mapping Web applications (e.g. Google Maps, Microsoft Virtual Earth, Yahoo Maps), Tile caches technique is widely used. Tile caches are a set of map images that have been pre-generated based on geographic vector data and the adoption of tile caches technique in the developed system dramatically decreases the loading time consumed for high quality image visualization. This paper proposes an efficient mechanism for organizing and indexing tile caches on map server. We cached all the map tiles using the same matrix notion to give each tile a unique file name and all tiles are stored in hierarchical file system directories using the zoom level and layer as the directory name. With the number of tile files growing exponentially along with the increase of magnification level and operating system limitations for numbers of files in a single directory, we propose an efficient grid-index algorithm to re-organize the files and directories. This paper gives the implementation of this mechanism in Wuhan City, China. Finally, this paper analyses the shape deformation in specific geographical regions which caused by this algorithm and presents two ways to solve the problem by modifying the tile´s width-to-length ratio or changing the step-length of longitude and latitude that each tile covers.