AEC From the Ground Up-AEC CAD Data Standards

31 Aug, 2005 By: AIA ,H. Edward Goldberg

Standards keep all project members on the same page.

In technical use, a standard is a concrete example of an item or a specification against which all others may be measured. For example, laboratories and standards organizations keep "primary standards" for length, mass and other units of measure. De facto standards emerge when they are adopted by a majority of users. Once this happens, for good or not, it's very difficult to change. Standards also exist to bring together diverse parties (figure 1). Given the billions of dollars in construction that takes place every year, and using my assumption that two million or so people use CAD technology in this market, practicing without using standards can become very costly.

 Figure 1. Adobe's depiction of how its PDF format fits into a building remodeling project illustrates the scope of collaboration required by AEC processes and hence the need for document standards.
Figure 1. Adobe's depiction of how its PDF format fits into a building remodeling project illustrates the scope of collaboration required by AEC processes and hence the need for document standards.

The AEC CAD arena uses many types of standards. These include file format standards, file publishing standards, and user standards such as layers and object naming. This article is not in defense of any standard, but rather an examination of what, in my opinion, are the de facto CAD data standards.

National CAD Standards

The latest version of NCS (National CAD Standards), v3.1, was published in January 2005. The NCS codifies information for the building design and construction industry by classifying electronic building design data consistently. It prescribes layer names, drawing set organization, drafting conventions and notations. This standard is a compilation and modification of three existing documents published by members of the FIC (Facility Information Council):

  • 1. CAD Layer Guidelines, American Institute of Architects
  • 2. Uniform Drawing System Modules 1–8, the Construction Specification Institute
  • 3. Plotting Guidelines, U.S. Coast Guard

These three organizations are offering software vendors the opportunity for a nonexclusive NCS license to encourage them to integrate NCS v3.1 into their programs through drawing templates, symbols libraries or other methods.

Figure 2. Compressed formats usually support redlining and markup tools so reviewers can make notes but not change drawings, as shown in this sample DWF file.
Figure 2. Compressed formats usually support redlining and markup tools so reviewers can make notes but not change drawings, as shown in this sample DWF file.

A copy of the NCS can be purchased for $250 by members of the three originating groups. Nonmembers pay $350. The next NCS revision cycle was set to open last month. Users of NCS v3.1 can participate on the NCS Project Committee. Details on this committee and on the NCS can be found at

IFCs and BIM

The IFC (industry foundation classes) system is a data representation standard and file format for defining architectural and constructional CAD graphic data as 3D real-world objects, mainly so that architectural CAD users can transfer design data between different software products. Last month's column discussed IFCs in more detail (

The National Institute of Building Sciences through FIC kicked off development of a National BIM Standard on August 29 of this year. Its goal is to produce v1.0 of the standard within a year. Deak Smith, an architect with the office of the Secretary of Defense and chair of the committee developing the standard, expects that the BIM standard will be based on the IFC representation standard.

Smith, whose background includes developing digital methods for overseeing the needs of the multitudes of buildings owned by the military, sees the BIM (or virtual model) as a method of coordinating infrastructure from design through maintenance. The integration of BIM and GIS (geographic information systems) is an essential direction that this technology must take. Smith also sees the need for a new breed of "digital architects" capable of understanding construction methods as well as computer technology.


The DWG file format is probably the most commonly used CAD file format today, primarily because it's the native format of Autodesk's AutoCAD-based products. However, Autodesk does not freely publish this format. Its open formats for data exchange are DXF and DWF.

Autodesk recently launched its RealDWG program, which makes the DWG format more accessible to selected companies. RealDWG is a licensed software library that allows C++ and .NET developers to read and write DWG and DXF files. Autodesk RealDWG is also used internally by Autodesk to provide DWG support in nonAutoCAD-based products such as VIZ, Revit and Inventor. Software applications that link with the Autodesk RealDWG libraries are called host applications and do not require the presence of an AutoCAD application.


Unlike typical standards-setting bodies, the Open Design Alliance focuses on the practical matter of developing component software libraries that enable its members to develop applications capable of reading and writing popular CAD file formats. The alliance supports MicroStation's DGN format, which is open, and OpenDWG, which the alliance reverse engineers from Autodesk's DWG format.

Evan Yares, president of the alliance, contends that Autodesk's refusal to expose the DWG architecture jeopardizes the archiving of data. He suggests that at some time in the future, Autodesk could cease to license software that reads certain DWG versions. In this case, without an "open," known format, users might not be able to retrieve their data.

Though OpenDWG is designed to provide as near to perfect compatibility with DWG as is possible, it offers several advantages that, according to the alliance, make it a safe choice for even mission-critical applications. The format is documented, with no hidden encryption. It's supported by a dedicated team of technical professionals focused on fixing any problems that may crop up. The libraries are clean and updated quickly to reflect new versions of AutoCAD. They provide compatibility with all versions of DWG from 2.5 to 2006.

Formerly known as the OpenDWG Alliance, the Open Design Alliance is a nonprofit, membership-based consortium. Commercial software developers pay an annual membership fee to belong. Membership in the alliance is designed to provide a level playing field, with all members at the same level agreeing to the same membership terms. For its part, Autodesk contends that the alliance is not a neutral standards body, but rather a commercial allliance. Autodesk also says it would need to meet different membership criteria to join.

Compressed Document Formats

Both Adobe and Autodesk offer their own compressed document formats, and each has its own merits. Though Adobe's PDF is more widely accepted for document transfer, Autodesk's DWF format creates much smaller files for quicker transfer.

PDF (portable document format) is the file format developed by Adobe Systems for representing documents in a manner that is independent of the original application software, hardware and operating system used to create those documents. A PDF file can describe documents containing any combination of text, graphics and images. These documents can be one page or thousands of pages, simple or complex with a rich use of fonts, graphics, color and images. PDF is an open standard, and anyone may write applications that read or write PDFs.

Autodesk DWF (design Web format). Autodesk's portable document format has been around for almost nine years, but gained a higher profile with the release of AutoCAD 2004. At that time, Autodesk decided to no longer provide a free DWG viewer, but instead provide a free viewer for DWF only. The DWF file format is designed to share complex design information while maintaining its integrity, with file sizes that are often one-tenth the size of other file formats. DWFs can be published from Autodesk products (figure 2) or with the downloadable DWF Writer, which also works with other Windows-based CAD applications.

Standards in Sum

One of the best ways to ensure productivity is to work according to a set of standards. Doing so will speed projects to completion with greater accuracy and facilitate more effective data exchange with clients and collaborating firms.

H. Edward Goldberg, AIA, NCARB, is a practicing licensed architect, AEC industry analyst and author of Autodesk Architectural Desktop 2005: A Comprehensive Tutorial (Prentice Hall; Look for the 2006 edition this fall. Visit for more information, or e-mail

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