WMS 1.1.1 Final Recommendation

ESDS-RFC-006 Technical Working Group Final Report

The ESDS-RFC-006 Technical Working Group (TWG) has conducted a review of ESDS-RFC-006 with the following conclusion:

That the Standards Process Group should endorse ESDS-RFC-006 as a Recommended Standard.

Recommendation

The TWG bases its recommendation on an analysis of the following factors in a NASA context.

Strengths - WMS 1.1.1 is a mature specification. It was published in January, 2002 as an update to the original WMS 1.0 specification adopted by the Open Geospatial Consortium in April, 2000. There is a newer version, WMS 1.3, that is the topic of an Technical Note (ESDS-RFC-005) approved by the SPG. WMS 1.1.1 has many different client and server implementations. Some implementations are embodied in open source (MapServer, GeoServer, among others) and some are embodied in products sold commercially (ESRI, MapInfo, and others). WMS 1.1.1 has been deployed in a wide variety of settings - many US Government agencies, foreign government agencies, university and research organizations, and commercial organizations have operating WMS 1.1.1 servers. The TWG review centered on NASA implementations and the review comments regarding those implementations can be seen on the SPG web site.

Weaknesses - Based on the TWG review results, WMS 1.1.1's identified weaknesses could best be described as (a) not having enough features and (b) not being applicable for all aspects of Earth Science visualization. There were no specific deficiencies found in the specification itself. Thus, potential users of WMS 1.1.1 are encouraged to make note of the next two sections on Applicability and Limitations.

Applicability - WMS 1.1.1 is by and large a visualization specification. Thus it is applicable to the problem of generating renderings of georeferenced data and delivering those renderings to client applications. By virtue of the rendering request parameters, renderings made by independent and unassociated servers can be overlaid, yielding the ability to provide visual fusion of georeferenced data. WMS 1.1.1 does also provide the ability to request results in non-visual formats, and this ability can be put to good use by application designers. However, it is advisable to use WMS 1.1.1 in concert with other data delivery protocols when there is a need for sophisticated, non-rendered data transport.

Limitations - System designers seem to often attempt to employ WMS 1.1.1 in ways that it was not designed to be used, particularly as a data transport mechanism. WMS 1.1.1 also has developed a reputation for being a "slow" protocol when employed in very high performance applications. This tends to be a result of the specific implementations chosen rather than a limitation of the specification itself, however it is a common complaint. Techniques such as pre-rendering or caching can significantly improve the performance of WMS 1.1.1 based applications without needing to change the protocol itself.

Overall, WMS 1.1.1 is a widely implemented and widely deployed specification that provides a standard way of performing a specific set of tasks. The ESDS-RFC-006 TWG thus recommends its endorsement by the SPG as an Earth Science Data Systems Standard.

History

ESDS-RFC-006 was submitted by the Open Geospatial Consortium (OGC) following their submission of ESDS-RFC-005. RFC-005 proposed endorsement of the later, current version of the WMS specification, version 1.3. During the TWG review, it was determined that there was not widespread use of WMS 1.3 and that most users of WMS were using version 1.1.1. This determination was due to the fact that most of the technical reviews received by the TWG from reviewers were about version 1.1.1. The SPG decided to endorse RFC-005 as a Technical Note stating that version 1.3 was an emerging specification that should be considered by users of the WMS specification. Then OGC submitted RFC-006 with WMS version 1.1.1. The TWG contacted all previous reviewers to confirm that they would allow their reviews to be used for RFC-006. At that point, the review process continued.

Overall, the TWG conducted two reviews, a Technical Review designed to determine the technical validity of the specification within a NASA context, and an Operational Review designed to determine the operational readiness of the specification within a NASA context.

The review of the specification was completed over a period of approximately 18 months. Based on the responses received from two sets of survey questions and from additional research the TWG concludes that the WMS 1.1.1 specification demonstrates sufficient operational readiness to be endorsed by the SPG.

RFC Overview

ESDS-RFC-006 proposes the OpenGIS® Web Map Service Implementation Specification, Version 1.1.1 (WMS 1.1.1) as an ESDS Standard. WMS 1.1.1 is a specific version of the OpenGIS Web Map Service specifications. Its functionality is described in the excerpt below:

A Web Map Service (WMS) produces maps of georeferenced data. We define a "map" as a visual representation of geodata; a map is not the data itself. This specification defines three WMS operations: GetCapabilities returns service-level metadata, which is a description of the service's information content and acceptable request parameters; GetMap returns a map image whose geospatial and dimensional parameters are well-defined; GetFeatureInfo (optional) returns information about particular features shown on a map.

This specification defines a syntax for World Wide Web (WWW) Uniform Resource Locators (URLs) that invoke each of these operations. Also, an Extensible Markup Language (XML) encoding is defined for service-level metadata.

When requesting a map, a client may specify the information to be shown on the map (one or more "Layers"), possibly the "Styles" of those Layers, what portion of the Earth is to be mapped (a "Bounding Box"), the projected or geographic coordinate reference system to be used (the "Spatial Reference System," or SRS), the desired output format, the output size (Width and Height), and background transparency and color.

When two or more maps are produced with the same Bounding Box, Spatial Reference System, and output size, the results can be accurately layered to produce a composite map. The use of image formats that support transparent backgrounds allows the lower Layers to be visible. Furthermore, individual map Layers can be requested from different Servers. The WMS specification thus enables the creation of a network of distributed Map Servers from which Clients can build customized maps.

A particular WMS provider in a distributed WMS network need only be the steward of its own data collection. This stands in contrast to vertically-integrated web mapping sites that gather in one place all of the data to be made accessible by their own private interface.

• Material from OpenGIS® project document OGC 01-068r3.

Refer to pages 1-6 the specification document itself for a description of WMS.

The RFC and a copy of the specification can be downloaded here: http://www.esdswg.org/spg/rfc/ese-rfc-006

The specification can also be downloaded here: http://portal.opengeospatial.org/files/?artifact_id=1081&version=1&format=pdf