Review: OpenGIS Web Map Tiling Service (WMTS) Interface Specification
OGC Requests Comments on Candidate Web Map Tiling Standard
February 27, 2009. Wayland, MA, USA.
The membership of the Open Geospatial Consortium, Inc. (OGC) is requesting comments from the public on the candidate OpenGIS Web Map Tiling Service (WMTS) Interface Standard.
The candidate WMTS Interface Standard is much like the OGC's popular Web Map Server (WMS) Interface Standard, but it enables better server performance in applications that involve many simultaneous requests. To improve performance, instead of creating a new image for each request, it returns small pre-generated images (e.g., PNG or JPEG) or reuses identical previous requests that follow a discrete set of tile matrices. This proposed standard provides support for multiple architectural patterns — KVP, REST and SOAP.
The 30 day public comment period for this document begins February 27, 2009. Comments are due by March 29, 2009. After the OGC's WMS Standards Working Group has addressed comments received in response to the WMTS RFC the draft documents will be submitted to the OGC Technical Committee and Planning Committee for their review and possible approval as an adopted OGC Standard.
The WMTS RFC can be downloaded from:
The OGC is an international consortium of more than 370 companies, government agencies, research organizations, and universities participating in a consensus process to develop publicly available geospatial standards. OpenGIS Standards support interoperable solutions that "geo-enable" the Web, wireless and location-based services, and mainstream IT. OGC Standards empower technology developers to make geospatial information and services accessible and useful with any application that needs to be geospatially enabled. Visit the OGC website at http://www.opengeospatial.org/.
Candidate OpenGIS Web Map Tiling Service Implementation Standard
Category: Candidate OpenGIS Implementation Standard
Edited by Joan Masó, Keith Pomakis, and Núria Julià
This standard is an evolution of Tile Map Service Specification elaborated by OSGeo and TileWMS elaborated by OnEarth, and is deeply related to them. As such, this is the first OGC standard to include a RESTful approach in addition to the usual OGC encodings. It is the first OGC standard to include RESTful approach in an effort to harmonize it to OSGeo proposal. It is intended to be the natural way to evolve WMS services into a more constrained but more scalable and faster service. It has been written to be read alone (relying on OGC 06-121r3 as a base document) but shares a lot of concepts with the WMS 1.3.0 standard.
Introduction: The Web Map Server (WMS) specification was developed to allow cartographic maps to be served over the internet in an interoperable manner. One of the primary goals of the MS specification (other than interoperability) was flexibility. And in this respect it has been very successful. A WMS client is able to request the overlay of an arbitrary number of map layers in an arbitrary bounding window with an arbitrary background color at an arbitrary scale in a number of coordinate systems with a number of styles (and in some cases, with arbitrary user-defined styles).
However, with this flexibility comes a price. Since a WMS server is required to generate each requested map image on the fly, it does not scale well. It is theoretically possible for a WMS to cache the map images that it generates, but in practice, due to the unbounded nature of map requests, this doesn't aid performance much if at all because cache hits will be rare. Many implementations have demonstrated that it is possible to serve most cartographic layer requests within a few seconds, but this is generally in a single-request-at-a-time environment. By the very nature of what a WMS server is required to do, it is unreasonable to expect a WMS server to be able to handle the load of dozens or hundreds of simultaneous requests. Therefore, WMS servers will never be able to achieve the popularity or ubiquity of what the industry is beginning to demand.
In an attempt to tackle this issue, this paper defines an alternative type of WMS service, called a Web Map Tiling Service (WMTS) that uses a tiling model. A tiling model divides the space in a fixed tile matrix. The map that the client wants to show is divided into tiles that are requested individually. By limiting the rendering parameters in the request to a discreet set of values, it becomes possible for a WMTS server to serve pre-rendered images (tiles) that require no processing whatsoever and take advantage of the caching mechanisms present on the internet. The WMTS server can simply return the appropriate pre-generated image (e.g., PNG or JPEG) file or reuse an identical previous request. This mechanism is highly scalable (as similar systems such as Google Maps illustrates). This standard specifies an abstract specification web service and common procedure oriented architecture style than is developed in KVP and SOAP/WSDL and also introduces a resource oriented architecture style that it is encoded in RESTful URLs.
An advantage of the resource oriented architecture style is that since RESTful implementations rely on hyperreferences to resources, they can be set up without any specific web service application using a standard web server serving a static XML capabilities document and a set of prerendered images. This could be useful because many internet service providers (especially the free ones) allow web pages and static content to be set up on a server but do NOT allow using ASP or CGI applications for security reasons. Tiles and RESTful implementations can be installed in such an environment without any assistance and without any special knowledge of web services since they are only files with standard MIME types.
This paper describes the details of the proposed WMTS standard, which consists of GetCapabilities, GetTile and GetFeatureInfo operations in the procedure oriented architecture style and the capability of retrieving a ServiceMetadata document, Tile images and FeatureInfo documents in the resource oriented architecture style...
- Candidate OpenGIS Web Map Tiling Service Implementation Standard. (PDF)
- Candidate OpenGIS Web Map Tiling Service Implementation Standard. (.DOC)
- Complete Candidate OpenGIS Web Map Tiling Service Implementation Standard and Associated Schemas (ZIP)
Executive Director, Outreach and Community Adoption
Open Geospatial Consortium, Inc
35 Main Street, Suite 5, Wayland, MA 01778 USA
Prepared by Robin Cover for The XML Cover Pages archive.