10
Introduction
This Web Map Tile Service (WMTS) Implementation Standard provides a standard based
solution to serve digital maps using predefined image tiles. The service advertises the tiles it has
available through a standardized declaration in the ServiceMetadata document common to all
OGC web services. This declaration defines the tiles available in each layer (i.e. each type of
content), in each graphical representation style, in each format, in each coordinate reference
system, at each scale, and over each geographic fragment of the total covered area. The
ServiceMetadata document also declares the communication protocols and encodings through
which clients can interact with the server. Clients can interpret the ServiceMetadata document to
request specific tiles.
The WMTS standard complements the existing Web Map Service standard of the OGC. The
WMS standard focuses on flexibility in the client request enabling clients to obtain exactly the
final image they want. A WMS client can request that the server creates a map by overlaying an
arbitrary number of the map layers offered by the server, over an arbitrary geographic bound,
with an arbitrary background color at an arbitrary scale, in any supported coordinate reference
system. The client may also request that the map layers be rendered using a specific server
advertised style or even use a style provided by the client when the WMS server implements the
OGC Styled Layers Descriptor (SLD) standard. However, all this flexibility comes at a price:
server image processing must scale with the number of connected clients and there is only limited
potential to cache images between the server and client since most images are different.
As web service clients have become more powerful, it has become possible to consider an
alternative strategy which forces the clients to perform image overlays themselves and which
limits the clients to requesting map images which are not at exactly the right position thereby
forcing the clients to mosaic the tiles obtained from the server and clip the set of tiles into a final
image. This restriction of image requests to a fixed, predefined set allows for servers to scale
based on communication processing abilities rather than image processing abilities because
servers can prerender some or all of their images and can use image caching strategies. The fixed
set of images also enables network providers to cache images between the client and the server,
reducing latency and bandwidth use. Popular, non standardized, commercial implementations of
this approach, such as Google Maps, Microsoft Virtual Earth and Yahoo! Maps have already
shown that there are clear performance benefits to adopting this methodology.
Some WMS servers have already embarked on this road, developing their own tiling
structures built by constraining WMS GetMap requests to a fixed set and then advertising those
constraints in their service metadata. Although this mechanism enables those servers to scale as
just described, the tiling structure and the advertising and discovery mechanisms are not
standardized. That unfortunately limits interoperability and forces developers to build, for each
server, special clients that can understand the server advertised constraints and limit the WMS
GetMap requests issued by the client to exactly the requests understood by the particular server.
This WMTS standard offers a standardized approach to declaring the images which a client can
request from a server, enabling a single type of client to be developed for all servers. While
developing a profile of WMS was initially considered, limiting a WMS in the ways important to
allow efficient access to cacheable tiles proved awkward while forcing implementors to read both
a standard and a profile seemed less efficient than developing this stand alone specification.