Vector tiles

Vector tiles, tiled vectors or vectiles are packets of geographic data, packaged into pre-defined roughly-square shaped "tiles" for transfer over the web. This is an emerging method for delivering styled web maps, combining certain benefits of pre-rendered raster map tiles with vector map data. As with the widely used raster tiled web maps, map data is requested by a client as a set of "tiles" corresponding to square areas of land of a pre-defined size and location. Unlike raster tiled web maps, however, the server returns vector map data, which has been clipped to the boundaries of each tile, instead of a pre-rendered map image.

There are several major advantages of this hybrid approach. Compared to an un-tiled vector map, the data transfer is reduced, because only data within the current viewport, and at the current zoom level needs to be transferred. The GIS clipping operations can all be performed in advance, as the tile boundaries are pre-defined. This in turn means that tiled vector data can be packaged up and distributed, without needing any kind of GIS system available to serve data.

Compared to a tiled raster map, data transfer is also greatly reduced, as vector data is typically much smaller than a rendered bitmap. Also, styling can be applied later in the process, or even in the browser itself, allowing much greater flexibility in how data is presented. It is also easy to provide interactivity with map features, as their vector representation already exists within the client. Yet another benefit is that less centralised server processing power is required, since rasterisation can be performed directly in the client. This has been described as making "rendering ... a last-mile problem, with fast, high-quality GPU[s] in everyone’s pocket".

There is a long history of the use of vector tile schemes in GIS. A very early GIS (circa 1966), the Canada Geographic Information System (CGIS), used a vector tile storage scheme that allowed limited resource computers to efficiently access and process vector map data. CGIS used the term "frame" instead of vector tiles.

In 1975, the US Wildlife Service initiated a national program to map and digitize all the wetlands in the US. In 1976 they awarded a contract to Autometric to develop a software application that allowed stereo imagery to be digitized using an analytical stereo plotter. Later support for 2d digitization from single photos (aka single photo resection) on a digitizing table was added. The software was called WAMS (Wetlands Analytical Mapping System). The WAMS data store used a tiled vector structure. Each tile was called a “geounit”. A geounit corresponded to one of the USGS quadrangle scales, typically 1:24000. As the data were digitized, they were immediately stored in the tiled data store and topologically structured. Lines crossing geounit boundaries were automatically split. The storage Coordinate Reference System (CRS) was WGS 72 lat/long. WAMS is described in detail in “The wetlands analytical mapping system: WAMS" [Pywell and Niedzwiadek, 1980]

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