An effective synchronization method of GSI Tiles using a GSI Tile List

Abstract

“GSI Tiles” are tile-based geospatial information for web maps provided by the Geospatial Information Authority of Japan (GSI) under the standardized terms of use of the Japanese Government. GSI Tiles contain official topographic map data maintained under the Survey Act, and also more than 1,000 layers, covering topographic maps, aerial and satellite images, thematic maps, and disaster information. While these data are typically used for web maps such as the “GSI Maps” run by GSI, they are also available for download, and it has been decided that the same server which is used to run “GSI Maps” is to be used to provide these data for download. To keep the server highly available, an effective synchronization method for GSI Tiles has been developed. To fulfill this purpose, the concept of “GSI Tile Lists” was proposed. A GSI Tile List contains metadata for the files for a layer of GSI tiles. File paths, modification times (mtime), size, and MD5 message-digests are recorded in a GSI Tile List. The download and synchronization of the GSI Tiles can be made more effective by using a GSI Tile List. GST Tile Lists are updated weekly to reflect the rapid update of the topographic data of GSI. A reference implementation of the GSI Tiles downloader using GSI Tile Lists (qdltc) has been developed and published via a social coding site. A layer of topographic map data covering the whole area of Japan, consisting of 50 million tiles, can be downloaded within around a week. A weekly update of these topographic data, consisting of tens of thousands of tiles, can be downloaded within around 7 hours.