Purpose and Scope
GML is short for Geography Mark-up Language. This is unfortunate in that it provides an equally valid approach to XML encoding for applications that have no geographic content. It claims to be an “XML-based mark-up language used to encode information about real-world objects [features].” These features can be concrete (tangible) like roads and bridges or abstract like property or jurisdictional boundaries. Features have properties. Geometry can be a property of a feature. In fact, a feature may have multiple geometrical representations of different types or levels of precision. Though developed at the Open Geospatial Consortium, GML has been submitted to ISO for acceptance as one of the TC211 191xx family of GIS related standards. GML is the format for request and response messages for OGC Web Feature Services (WFS).
We have done a preliminary evaluation of GML to determine whether we should use it as the foundation for TransXML. The advantages we discovered include:
· XML Schema compliance: a GML schema follows the XML schema standard so GML is XML;
· Emerging presence in the Geospatial market;
· Object/property orientation makes it applicable to enterprise wide application, transcends conflicting and limiting CAD/GIS graphic representations, and provides a consistent approach for non-graphic as well as graphic entities;
· Designed to be a foundation on which specific industries, like transportation, are invited to develop domain specific application schemas, with the resultant schemas more likely to insure interoperability within a single application area as well as between multiple areas;
· Providing a common foundation across all TransXML schemas would provide the desired consistency and would allow this initial project to achieve more than would otherwise be possible. For example, many base types are already defined for us, including features and feature relationships, linking, geometry, coordinate reference systems, topology, temporal, metadata, and presentation style;
· A GML encoding of LandXML has been accomplished and has already resolved many of issues of supporting a design based approach with GML; and
· GML provides a mechanism for helping to smooth the transition between CAD and GIS.
On the opposite side, we acknowledge the following disadvantages:
· The false perception that GML is “geospatial” only – its object approach with geometry as an optional property of objects make it appropriate for non-spatial applications as well;
· GML is not well known (yet) in the design market;
· GML files can be larger than straight XML files – the LandGML experiment has already made progress in reducing the difference and more can be done. There will be some overhead associated with the object requirement and the fact that GML prescribes elements whereas some XML encoders use attributes;
· Though GML covers a very broad scope of geometries, we need to be sure it can handle the spiral and vertical (parabolic) curve profiles common to roadway design alignments. OGC as well as the TC211 standards group who developed the geometry standards on which GML is based are interested in working with us to cover our engineering-specific requirements;
· GML has a longer learning curve than “plain” XML schema;
· GML therefore presents additional risk for the project which would need to be mitigated to insure successful, on-time completion.
Based on our evaluation, there are four possible options:
1. Ignore GML for this project, allowing for future exploration later on
2. Decide now to adopt GML for all new schemas that are developed
3. Use GML for some new schemas (e.g. Safety Highway Information)
4. Begin Phase II with a GML experiment to gain a greater understanding of the impact of using GML
It has been decided that we should move forward with option 4. The proposed experiment will include the development of parallel XML and GML schema, instance documents, and extraction programs for a candidate application area which includes spatial as well as non-spatial information, such as terrain surface modeling. From this experiment we should be able to evaluate:
· the difference in effort required to use GML vs. straight XML – is there added effort for using objects with GML and is this offset by savings in using standard, predefined GML types?
· the difference in resultant file size of the instance documents to see if this is significant and if so, to see if this can be remedied
· the difference in clarity of the schemas and instance documents to see if any added complexity makes them easier or harder to understand
· the difference in effort required to parse the two documents
· whether the use of a more structured GML approach reduces ambiguity
· can what GML provides be used across all of our proposed schemas?
Based on these factors, we will be able to draw conclusions as to how the use of GML would help or hinder TransXML’s goal of providing data interoperability, and how it would help or hinder the ability to gain broad acceptance of TransXML schema.
From the results of the experiment, the team will report our findings to the panel, including comments received from stakeholders and make a recommendation on whether or not to proceed with GML.
Base Schema/ Standards
Existing schema and/or standards that this will be based on include:
· LandXML 1.0
· LandGML
· ISO TC211 19136 Geographic Information – Geography Markup Language (GML)
Resource Documents
· Crews, N., E. Hall, and D. Rebolj, LandXML Schema Version 1.0 Reference, 2002.
· Crews, N., LandXML-1.0.xsd
· Burggraf, D., LandGML0.6.xsd
· ISO, ISO/TC 211/WG 4/PT 19136 Geographic information – Geography Markup Language (GML), ISO CD 19136, February 7, 2004.
· Ron Lake, David S. Burggraf, Milan Trninic, and Laurie Rae, Geography Mark-Up Language, John Wiley & Sons, Inc., San Francisco, CA, 2004.
Sample Applications
Proposed application to be developed to demonstrate the use of this schema:
· Develop an XSLT report of coordinate data from a surface model for use by machine control in human readable format. This would basically consist of x, y, z coordinates along an edge of pavement, for example.