Tile Service
A map service that delivers pre-rendered map tiles at fixed scales, so a viewer loads only the tiles it needs. OGC standardizes it as WMTS and OGC API - Tiles.
Detailed Definition
A tile service cuts a map into small, pre-made images at a fixed set of scales and sends only the pieces a viewer needs. Panning and zooming stay fast because the tiles already exist.
The standards
The Open Geospatial Consortium's Web Map Tile Service (WMTS), version 1.0.0 (OGC 07-057r7), "provides a standard based solution to serve digital maps using predefined image tiles." OGC says WMTS "complements the existing Web Map Service standard of the OGC" and offers "a standardized approach to declaring the images which a client can request from a server." See Web Map Service.
Its successor, OGC API - Tiles, "has been designed to support any form of tiled data," including "tiles of vector features" as well as coverages and maps. Both depend on a tile matrix set, which OGC defines as "a tiling scheme that enables an application to partition and index space based on a set of regular grids defined for multiple scales in a Coordinate Reference System (CRS)."
A working example
USGS describes its USGS Topo basemap as "a tile cache base map service that combines the most current data in The National Map (TNM), and other public-domain data, into a multi-scale topographic reference map." Its service page lists:
- tiles 256 by 256 pixels
- 24 levels of detail
- spatial reference 3857
- a WMTS interface offering GetCapabilities and GetTile
The page also gives the date its data was last refreshed. A cache is a snapshot taken at that date.
Tiles versus live maps
A WMS request is processed when it is made. A tile cache shows the data as of the last time the tiles were built. A map tile is also a picture: it cannot be queried for attributes or measured against another layer. See Raster Data.
Why it matters in mining claim research
Tiled basemaps are the backdrop of most web maps of mining claims: topography, imagery, and place names. They are good context and poor evidence. For anything that changes, such as mining claim status or land status, the question is how current the source is. A cached layer can lag the record, and the refresh date on the service, or the record itself, settles it. See Web Feature Service.
Related Terms
GIS
A geographic information system: software and data that store, analyze, and display information tied to locations on the earth.
Spatial Data
Data that identifies where something is on the earth and what it is like, stored as vector features, raster grids, or coordinates in records.
API
Application Programming Interface: a defined access point that lets one program request data or functions from another. Many government data services publish APIs.
Raster Data
GIS data stored as a grid of cells, each holding a value such as an elevation, a color, or a class. Used for imagery and continuous surfaces.
Web Map Service
An Open Geospatial Consortium (OGC) standard for requesting georeferenced map images from a server over HTTP. It returns pictures of data, not the data itself.