Chapter 2 Discussion Questions
3.4 OTHER EFFORTS SUPPORTING STANDARDIZATION
3.4.4 XML-Based Schemas
Extensible Markup Language (XML) provides alternative schema languages and transport mechanisms, especially suited for Web use. In the same way that Table 3–2 COBie2 Data Sections
Object Type Defi nitions
Meta Data Exchange fi le
Project Attributes, Units, Decomposition
Site Attributes, Address, Classifi cation, Base Quantities, Properties Building Attributes, Address, Classifi cation, Base Quantities, Properties Storey Attributes, Base Quantities, classifi cation, Properties
Spatial container Attributes, Classifi cation, Quantities, Properties, space boundaries, Space Boundary Doors, Windows, Bounding space
Covering Attributes, Type, Covering material, Classifi cation, Base Quantities Window Attributes, Type, Classifi cation, Material, Base Quantities, Properties Door Attributes, Type, Classifi cation, Material, Base Quantities, Properties Furnishing Attributes, Type, Material, Classifi cation, Properties,
MEP elements Attributes, Type, Material ,Classifi cation, Properties Proxy furniture, fi xture,
equipment
Attributes, Type, Material, Classifi cation, Properties Zone Attributes, Classifi cation, Properties, Spatial assignment System Attributes, Classifi cation, Properties, Component Assignment,
System Service Buildings
NOTE: Attribute, Type, Classifi cation attribute types vary by object type.
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XML Schemas in AEC Areas
OpenGIS®, the Geographic Objects (GO) Implementation Specifi cation has been developed by the OGC (Open Geospatial Consortium). It defi nes an open set of common, language- independent abstractions for describing, managing, rendering, and manipulating geo- metric and geographic objects within an application programming environment (OGC 2007).
gbXML (Green Building XML) is a schema developed to transfer information needed for preliminary energy analysis of building envelopes, zones, and mechanical equipment simulation (gbXML n.d.). Multiple platforms provide an interface.
ifcXML is a subset of the IFC schema mapped to XML, supported by IAI. It also relies on XML Schema, XSD, derived from the IFC EXPRESS release schema for its mapping. The lan- guage binding, for instance, the method of how to translate the IFC EXPRESS model into the ifcXML XSD model follows the international standard ISO 10303-28ed2 “XML repre- sentation of EXPRESS schemas and data.” The ISO/CD 10303-28ed2 version of 05-04- 2004 is used for the language binding.
aecXML is administered by FIATECH, a major construction industry consortium supporting AEC research, and the IAI. It initially developed an integration framework that attempted to harmonize ifcXML and aecXML, as an umbrella schema, that could support multiple subschemas. It relied on XML business technology developed by the United Nations Centre for Trade Facilitation and Electronic Business. The integration schema is called Common Object Schema (COS) that consists of level XML structures of names, addresses, amounts, and other base information units. aecXML was initiated to represent resources such as contract and project documents [Request for Proposal (RFP), Request for Quotation (RFQ), Request for Information (RFI), specifi cations, addenda, change orders, contracts, purchase orders], attributes, materials and parts, products, equipment; meta data such as organizations, professionals, participants; or activities such as proposals, projects, design, estimating, scheduling, and construction. It carries descriptions and specifi cations of build- ings and their components, but does not geometrically or analytically model them. Bentley was an early implementer of aecXML. Recent activity is unknown (FIATECH 2007).
agcXML The Associated General Contractors (AGC) developed agcXML in 2007, a schema that supports construction business processes, based on the COS master schema of the aecXML effort. Its schemas include the exchange of information commonly included in the following document types:
Request for Information Request for Pricing/Proposals Owner/Contractor Agreements
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Schedule of Values Change Order
Application for Payment Supplemental Instructions Change Directive
Bid, Payment, Performance, and Warranty Bonds Submittals
agcXML is free and can be downloaded from: http://iweb.agc.org/iweb/Purchase/
ProductDetail.aspx?Product_code⫽AGCXML. It has been implemented by a few compa- nies, including VICO and Newforma.
BIM Collaboration Format (BCF) is an XML format for person-to-person to go with other forms of exchange. It is called an Information Takeoff format. During design reviews, vari- ous action items are identifi ed. These are then acted upon by the various members of the project team. But how should these action items be transmitted? The answer comes from clash detection tools that identify a clash in 3D coordinates, associates an offset camera position to display the condition, then appends the action item to be taken, as identifi ed by the parties involved. Originally this capability was limited to the clash detection appli- cation, such as Navisworks. However, transmitted in XML, the action item can be imported into any BIM platform and displayed for the user to act on. The use can be much wider than clash detection; it can be used for any type of review, whether automated [such as generated by Solibri Model Checker (Solibri 2010)] or carried out manually through an in-person meeting or Web conference. The benefi t of BCF is that it directly loads and runs in the BIM design platform that generated the component of interest. BCF was pro- posed and defi ned by Tekla and Solibri, and has received commitments for support from Autodesk, DDS, Eurostep, Gehry Technologies, Kymdata, MAP, Progman, and QuickPen International.
CityGML is a common information model for the representation of 3D urban objects. It defi nes classes and relations for relevant topographic objects in cities and regional models with respect to their geometrical, topological, semantic, and appearance properties. Included are generalization hierarchies between thematic classes, aggrega- tions, relations between objects, and spatial properties. This thematic information goes beyond graphic exchange formats and supports virtual 3D city models for sophisticated analysis tasks in different application domains like simulation, urban data mining, facility management, and thematic inquiries. The underlying model differentiates fi ve levels of detail (LOD). CityGML fi les can (but don’t have to) contain multiple representa- tions for each object in different LOD simultaneously. For more information, see www .citygml.org/1523/.
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some exchange formats are strictly fi le-oriented, some of the new exchange formats are only XML-based. XML is an extension to HTML, the language used to send information over the Web. HTML has a fi xed set of tags (a tag tells what kind of data follows and is a primitive schema) that defi ne presenta- tion formats, different kinds of media, and other types of fi xed format Web data. XML expands upon HTML by providing user-defi ned tags to specify an intended meaning for data transmitted. XML has become very popular for exchange of information between Web applications, for example, to support e-commerce transactions or collect data.
There are multiple methods for defi ning custom tags, including Document Type Declarations (DTDs) that are developed for mathematical formulas, vector graphics, and business processes, among many others. There are mul- tiple ways to defi ne XML schemas, including XML Schema (www.w3.org/
XML/Schema), RDF (Resource Description Framework) (www.w3.org/
RDF/), and OWL Web Ontology Language (www.w3.org/TR/2004/REC-owl- features-20040210/). These are shown in Figure 3–2. Research is proceeding to develop even more powerful tools around XML and ever more powerful schemas, based on precise semantic defi nitions called ontologies. Practical results for these more advanced approaches have thus far been limited.
Using current readily available schema defi nition languages, some effec- tive XML schemas and processing methods have been developed in AEC areas.
Five of them are described in the previous box.
Each of these different XML schemas defi nes its own entities, attributes and relations, and rules. They work well to support work among a group of collaborating fi rms that implement a schema and develop applications around it. However, each of the XML schemas is different and incompatible.
ifcXML provides a global mapping to the IFC building data model, for cross- referencing. Efforts are underway to harmonize the OpenGIS schema with IFC. Translators do exist for mapping IFC models to CityGML. XML format- ting takes more space than, say, IFC clear text fi les (between 2 and 6 times more space). However, it can be processed signifi cantly faster than a text fi le and thus works more effectively than fi le exchanges in many cases. The longer- term issue is to harmonize the other XML schemas with equivalence mappings between them and with data model representations. The analogy is when the railroads in the United States all rapidly built tracks over the country, each with their own gage; they worked fi ne within their own community, but could not link up.
Two important XML formats for publishing building model data are DWF and 3D PDF. These provide lightweight mappings of building models for lim- ited uses. They are reviewed in Chapter 2.
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