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Data Integration and Interoperability: ISO/OGC Standards for Geo-information

Tuesday, November 2nd 2004
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Summary:

Dr.Carl Reed helps readers understand current OGC efforts relating to data exchange between GIS and CAD systems, and what the future of will likely hold in terms of OGC specification work in this area…Data exchange and/or data integration has been always rather problematic between CAD systems as well as between CAD and GIS.Increasingly, however, there is a requirement to be able to seamlessly utilize both GIS and CAD data in a common application environment.Read more…

Geospatial content is stored in a variety of systems and formats.Different projects from AEC applications are maintained in software-dependent file formats.Data exchange and/or data integration has been always rather problematic between CAD systems as well as between CAD and GIS.Increasingly, however, there is a requirement to be able to seamlessly utilize both GIS and CAD data in a common application environment.

The membership of the Open GeoSpatial Consortium Inc.(formerly the Open GIS Consortium) is developing standards that enable open, vendor and format neutral geo-information discovery, access and sharing.Traditionally, the focus of the OGC has been on 2D GIS issues and less on the integration of GIS and CAD.However, during the last year, this view has changed.For example, the OGC's current abstract model for geometry incorporates many of the geometry types required in the AEC industry and the OGC Geography Markup Language (GML) allows encoding of these geometry types and to certain extent supports modeling of abstract 3D structures.

What is the OGC?
The Open GeoSpatial Consortium (OGC) is a global voluntary consensus standards organization that envisions "a world in which everyone benefits from geographic information and services made available across any network, application, or platform." Inherent in this vision is the requirement for geospatial standards and strategies to be an integral part of business process and enterprise architectures.

The OGC membership consists of geospatial technology providers, systems integrators, government agencies and universities all participating in a consensus process to develop, test, and document publicly available interface standards and encodings for the geospatial industry.

How Current OGC Standards Can be Used for CAD/GIS Integration
There are a number of OGC Specifications that can be used immediately to enhance the sharing and integration of CAD/GIS content and services.How these interface standards are used and what CAD/GIS interoperability issues they will solve is dependent on the requirements of the users and the applications.For example, if the requirement is to deploy a client application that allows the user to browse and view both CAD and GIS content as if it were stored in a single system and/or database, then implementing the OGC Web Map Service (WMS) Interface Specification may be the ideal solution.If the requirement is to share content, such as ingesting CAD data into a GIS, then perhaps a Geography Markup Language (GML) application profile is the way to go.This section discusses existing OGC standards that are of use for solving the CAD/GIS integration problem.

Seamless Visualization: Web Map Service Interface Specification
The OGC Web Map Service (WMS) Version 1.3 Interface Specification provides an excellent mechanism for quickly developing applications that allow a client to display CAD and GIS data without costly data conversion.The WMS utilizes an HTTP request structure that packages a request to one or more servers that understand a WMS request.These servers could provide access to a GIS database, a CAD database, a simulation database, an imagery database, a spatially enabled RDBMS and so forth. The server processes the WMS request and sends back a geo-registered picture to the client.The picture may be a PNG, a GIF, or a JPEG image.Since the WMS allows the client to specify a coordinate reference system, all picture images are returned to the client in the same reference system, allowing the pictures to be "displayed" one on top of the other.Figure 1 provides a simple example of the use of the WMS specification.


Figure 1: Example of a Client accessing multiple WMS enabled servers.

Returning CAD/GIS Content to the Client - The Web Feature Service Interface Specification
The purpose of the OGC Web Feature Service Interface Specification is to provide a well-known structure and mechanism for query and retrieval of geographic features.The OGC WFS interface is a collection of operations (implemented as messages carried over HTTP) for retrieving and manipulating geographic features.

The WFS requests are generated on the client and are posted to a WFS server using HTTP.A WFS request consists of a description of query or data transformation operations that are to be applied to one or more features in the target repositories.

The default payload encoding for transferring exposed features from the server to the client is the Geography Markup Language (GML).As the WFS service allows querying a feature repository, the features come back defined as GML.The features that are returned are selected by tests on values within the properties of a feature as specified in the query filter.The next section describes GML in greater detail.


Figure 2: A transactional WFS application being used to update transportation information collected via survey techniques.

Specifying Portrayal Rules
The importance of the visual portrayal of geographic and CAD data cannot be overemphasized.The skill that goes into portraying data (whether it be geographic or tabular) is what transforms raw information into an explanatory or decision-support tool.

The OGC Style Layer Descriptor (SLD) Specification addresses the need for geospatial consumers (either humans or machines) to control the visual portrayal of the data with which they work. Defining this language, called the StyledLayerDescriptor (SLD), is the main focus of this specification, and it can be used to portray the output of Web Map Servers, Web Feature Servers and Web Coverage Servers.In many cases, however, the client needs some information about the data residing on the remote server before he, she or it can make a sensible request.This led to the definition of new operations for the OGC services in addition to the definition of the styling language.

Communicating and Transporting CAD/GIS Content in an Open, Interoperable Manner: Geography Markup Language (GML)
GML is an XML grammar written in XML Schema for the modeling, transport, and storage of geographic information.The key concepts used by GML to model the world are drawn from the OGC Abstract Specification (available online: http://www.opengeospatial.org/techno/abstract.htm) and relevant ISO TC 211 standards.

GML provides a variety of objects types for describing geography including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values.The GML specification defines the XML Schema syntax, mechanisms, and conventions that:
  • Provide an open, vendor-neutral framework for the definition of geospatial application schemas and objects;
  • Support the description of geospatial application schemas for specialized domains and information communities;
  • Support the storage and transport of application schemas and data sets;
  • Increase the ability of organizations to share geographic application schemas and the information they describe.
Implementers may decide to store geographic application schemas and information in GML, or they may decide to convert from some other storage format on demand and use GML only for schema and data transport.

LandGML <-- --> LandXML: An Example of CAD/Survey/GIS Content Sharing that Maintains Semantic Integrity
In late 2004, OGC members Autodesk, Galdos, and the US Corp of Engineers completed the LandXML/LandGML Interoperability Experiment (IE).OGC Interoperability Experiments are brief, focused, and low-overhead initiatives led and executed by OGC members to achieve specific technical objectives that further interoperability and the OGC Technical Baseline.

The member driven LandGML IE built and tested a GML 3.1 application schema for encoding LandXML 1.0 documents (LandGML) and provided a tool to transform LandXML 1.0 documents into LandGML documents and a tool to transform LandGML documents to LandXML 1.0 documents."

A Bit about LandXML
LandXML is an industry-driven, open XML data exchange standard that provides interoperability in applications serving the civil engineering, survey and transportation industries.LandXML is now broadly supported in online cadastral applications, GIS applications, survey field instruments, civil engineering desktop and CAD-based applications, instant 3D viewers and high-end 3D visualization rendering applications. LandXML XML Schemas provide support for a range of encodings, such as road centerline line, curve and spiral geometries, vertical alignment and cross sections, hydraulic pipes and structures, digital terrain models, COGO point element and raw survey raw data collection parameters and measurements.

The LandXML <-- --> LandGML Interoperability Experiment
The goal of this OGC Interoperability Experiment was to automate the flow of civil engineering and land survey data directly into GIS applications and back again using XML-based open standards, allowing land development, transportation and GIS professionals to exchange high precision design data throughout the entire lifecycle of a project.In order to insure the seamless, "loss-less" flow of data between CAD systems and CAD/GIS environments, a GML 3.1Application Schema for LandXML was defined.

Figure 3 captures the essence of the process.


Click image for larger view.

A Java/XSLT based transformation tool for the LandXML to LandGML transformation was built and tested.An online LandXML to LandGML transform demonstration was provided by Autodesk and is available at http://www.landxml.org. The LandGML to LandXML transform tool is an open source C++ console application based on Xerces C SAX parser.

LandXML <-- --> LandGML Demonstration
A key part of the Interoperability Experiment was a demonstration of a "roundtrip" LandXML/LandGML transformation.In the demonstration, survey data for road alignments and parcels were encoded into LandXML, transformed to LandGML and then from LandGML back into LandXML.The final file was then ingested into AutoCAD and rendered.The roundtrip was 100% error free.

Click image for larger view.

Putting a Standards Based CAD/GIS Workflow in Place
The following figure provides an example of how multiple OGC interface standards coupled with the LandXML/LandGML transformation tools can be integrated into a CAD/GIS workflow.


Click image for larger view.

In this workflow, survey information is encoded as LandXML.A client application makes a request via the WFS gateway to access the survey data and to return the survey data to the client as GML encoded using the LandGML Application Profile.Another WFS call to a GIS database is used to obtain the parcel fabric as GML for the same geographic area. The client now has both the survey information and the parcel fabric.Using styling rules (Style Layer Descriptor) coupled with the WMS interface, the application can now render an integrated presentation of the survey and GIS data.

Conclusion
While considerable progress has been made by the OGC membership in the area of CAD/GIS integration, there is still much work to be done on the standards front to enable the seamless use and integration of CADD, GIS, and 3D content and services.To provide a focused forum for the discussion of the requirements for CADD/GIS interoperability, the OGC is planning on forming and new Working Group.This groups' charter would be to address the increasingly complex issue of seamless integration of data and applications between CAD, GIS and 3D rendering.

Other installments published to date in this series:

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