|
A tiled web map, slippy map (in OpenStreetMap terminology) or raster tile map is a map displayed in a browser by seamlessly joining dozens of individually requested image files over the internet. It is currently the most popular way to display and navigate maps, replacing other methods such as WMS which typically display a single large image, with arrow buttons to navigate to nearby areas. Google Maps was one of the first major mapping sites to use this technique. Tiled web maps may in turn be replaced by vector tiles as the standard. There are several major advantages to tiled maps. Each time the user pans, most of the tiles are still relevant, and can be kept displayed, while new tiles are fetched. This greatly improves the user experience, compared to fetching a single map image for the whole viewport. It also allows individual tiles to be pre-computed, a task easy to parallelise. Also, displaying rendered images served from a web server is much less computationally demanding than rendering images in the browser, a benefit over technologies such as WFS. == Defining a tiled web map == Properties of tiled web maps that require convention or standards include the size of tiles, the numbering of zoom levels, the projection to use, the way individual tiles are numbered or otherwise identified, and the method for requesting them. Most tiled web maps follow certain Google Maps conventions: * Tiles are 256x256 pixels * At the outer most zoom level, 0, the entire world can be rendered in a single map tile. * Each zoom level doubles in both dimensions, so a single tile is replaced by 4 tiles when zooming in. This means that about 22 zoom levels are sufficient for most practical purposes. * The Web Mercator projection is used, with latitude limits of around 85 degrees. The de facto OpenStreetMap standard, known as Slippy Map Tilenames or XYZ, follows these and adds more: * An X and Y numbering scheme * PNG images for tiles * Images are served through a REST API, with a URL like 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tiled web map」の詳細全文を読む スポンサード リンク
|