OGC and KML - The Path Forward

By Carl Reed III

There is considerable community interest in Google's recent submission of KML into the OGC consensus standards process. There are also many questions concerning the "whys" and "hows" of this process, as well as how KML fits with the existing OGC standards baseline. This column provides the background, context and current status of this activity. The content in this column is derived from various OGC meeting notes, blogs and collaboration between OGC staff and OGC members, including Google.

At the December 2006 OGC Technical Committee meetings in San Diego, Michael Jones, John Hanke and Brian McClendon from Google spoke to the OGC Technical Committee in a plenary session. One of the topics they discussed was a proposal to submit KML into the OGC standardization process. The next day at the OGC Planning Committee (PC) meeting, the PC members in attendance had a very open and frank discussion regarding Google's proposal. We covered such topics as how best (and to what extent) to harmonize KML with other OGC standards, the standardization timeline, intellectual property and copyright, backwards compatibility issues, how to make sure that the current (and future) KML developer community can remain engaged in the process without being OGC members, and so forth.

The recommendation approved by the OGC members was that the KML 2.1 Reference Manual be submitted into the OGC process in time for the April 2007 meetings. The manual, reformatted into the OGC Standards document template, was discussed and approved for release as an OGC Best Practices Paper. Part of the decision to release the document was to develop a "preamble" that properly positions KML relative to other OGC standards and also describes in general terms the way forward for KML becoming an OGC standard. The following is from the preamble.

Google submitted KML (formerly Keyhole Markup Language) to the Open Geospatial Consortium (OGC) to be evolved within the OGC consensus process with the following goal: KML Version 3.0 will be an adopted OpenGIS implementation specification that will have been harmonized with relevant OpenGIS specifications that comprise the OGC standards baseline. There are four objectives for this standards work:
  • That there be one international standard language for expressing geographic annotation and visualization on existing or future Web-based online maps (2d) and earth browsers (3d)
  • That KML be aligned with international best practices and standards, thereby enabling greater uptake and interoperability of earth browser implementations
  • That the OGC and Google will work collaboratively to ensure that the KML implementer community is properly engaged in the process and that the KML community is kept informed of progress and issues
  • That the OGC process will be used to ensure proper life-cycle management of the KML candidate specification, including such issues as backwards compatibility
The OGC has developed a broad Standards Baseline. Google and the OGC believe that having KML fit within that family will encourage broader implementation and greater interoperability and sharing of earth browser content and context.

At what information sharing space is KML targeted? KML is an XML language focused on geographic visualization, including annotation of maps and images. Geographic visualization includes not only the presentation of graphical data on the globe, but also control of the user's navigation in the sense of where to go and where to look.

From this perspective, KML is complementary to most of the existing OGC specifications including key standards such as GML (Geography Markup Language), WFS (Web Feature Service) and WMS (Web Map Service). Currently, KML (v2.1) utilizes certain geometry elements derived from GML (version 2.1.2). These elements include point, line-string, linear-ring and polygon.

The OGC and Google have agreed that there can be additional harmonization of KML with GML (e.g. to use the same geometry representation) in the future. The Mass Market Geo Working Group in the OGC will define additional harmonization activities. Other OGC specifications such as Context and SLD will also be considered as part of the harmonization activity. An important work item in the OGC's new OWS-5 testbed activity will be to develop enhancements to the KML language.

We at the OGC are excited by the opportunity that the KML standards work represents. The ability of a "traditional" standards organization to interact and collaborate with the dynamic "Web 2.0/Where 2.0" community provides a very positive and dramatic shift in how geospatial standards for the mass market world can be developed.

Published Friday, June 1st, 2007

Written by Carl Reed III

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