A tiled web map,slippy map or tile map is a map displayed in a browser by seamlessly joining dozens of individually requested image or vector data files over the Internet. It is 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. The first tiled web maps used raster tiles, before the emergence of vector tiles. There are several 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 parallelize. Also, displaying rendered images served from a web server is less computationally demanding than rendering images in the browser, a benefit over technologies such as WFS. While many map tiles are in raster format, the number of suppliers of vector tiles is growing. The advantage of vector tiles is that each client can apply a custom style to the map. In such a case the actual map tile pixels are rendered by the browser. An advantage of vector tiles is that the client can decide to rotate the map without all text labels becoming unreadable, as the labels don't rotate.
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.
Tile Map Service: an early standard supported by OpenLayers. One difference is the y axis is positive southwards in TMS, and northwards in OpenStreetMap.
TileJSON: a lightweight JSON description of all the parameters associated with a web map, created by Mapbox.
Bing Maps Tile System, using Quadkeys for addressing.
Client libraries
To display a tiled map in a browser usually requires the support of a web mappingframework. This framework handles the retrieval of tiles, display, caching, and user navigation. Popular frameworks for tiled maps include Google Maps API, OpenLayers and Leaflet.
