Untangling the Knot of CAD Preservation

<a href="http://commons.wikimedia.org/wiki/File:T-FLEX-CAD-12-Rus.png">T-FLEX-CAD-12-Rus</a> from Wikimedia Commons

T-FLEX-CAD-12-Rus from Wikimedia Commons.

At the 2014 Society of American Archivists meeting, the CAD/BIM Taskforce held a session titled “Frameworks for the Discussion of Architectural Digital Data” to consider the daunting matter of archiving computer-aided design and Building Information Modelling files. This was the latest evidence that — despite some progress in standards and file exchange — the digital preservationist all over the world are trying to get a firm grasp on the slippery topic of preserving Computer Aided Design files.

CAD is actually a suite of design software for 3-D modelling, simulation and testing. It is used in architecture, geographic information systems, archaeology, survey data, geophysics, 3-D printing, engineering, gaming, animation and just about any situation that requires a 3-D virtual model. It comprises geometry, intricate calculations, vector graphics and text.

The data in CAD files resides in structurally complex inter-related layers that are capable of much more than displaying visual models.  For example, engineers can calculate stress and load, volume and weight for specific materials, the center of gravity and visualize cause-and-effect.  Individual CAD files often relate and link to other CAD files to form a greater whole, such as parts of a machine or components in a building. Revisions are quick in CAD’s virtual environment, compared to paper-based designs, so CAD has eclipsed paper as the tool of choice for 3-D modelling.

CAD files — particularly as used by scientists, engineers and architects — can contain vital information, depending on the importance of the object or objects that result from the designs. CAD files are fragile, though, and subject to the same risk that threatens all digital files: failure of accessibility — being stuck on obsolete storage media or dependent on a specific version of specific software on a specific operating system. In particular, the complexity and range of specifications and formats for CAD files make them even more challenging to access and display than many other kinds of born-digital materials.


Skylab from NASA.

CAD software commerce thrives on rapid technological change, new versions of software and newer and more innovative software companies. This is the natural driver for commercial technology. But each new version and type of CAD software increases the risk of inaccessibility for CAD files created in older versions of software. Vendors, of course, do not have to care about that; the business of business is business — though, in fairness, businesses may continually surpass customer needs and expectations by creating newer and better software features. That said, many CAD customers have long realized that it is important — and may someday be crucial — to be able to archive and access older CAD files.

Design for a Flying Machine by Leonardo da Vinci

Design for a Flying Machine by Leonardo da Vinci

Building Information Modelling files and Project Lifecycle Management files also require a digital-preservation solution. BIM and PLM integrate all the information related to a major project, not only the CAD files but also the financial, legal, email and other ancillary files.

Part of a digital preservation workflow is compatibility and portability between systems. So one of the most significant standards for the exchange of product manufacturing information of CAD files is ISO 10303, known as the “Standard for the Exchange of Product model data” or STEP. Michael J. Pratt, of the National Institute of Standards and Technology, wrote in 2001 (pdf), “the development of STEP has been one of the largest efforts ever undertaken by ISO.” CAD, computer-aided engineering and computer-aided manufacturing are three types of systems that use STEP.

Some simple preservation advice that comes up repeatedly is to save the original CAD file in its original format. Save the hardware, software and system that runs it too, if you can. Save any metadata or documentation and document a one-to-one relationship with each CAD file’s plotted sheet. This will all be helpful down the road when someone tackles the file display.

The usual digital-preservation practice applies, which is to organize the files, backup the files to a few different storage devices and put one in a geographically remote location in case of disaster, and every seven years or so migrate to a current storage medium to keep the files accessible.

But what should you preserve? And why? Given the complexity of these files, and recognizing that, at its heart, digital preservation is an attempt to hedge our bets about mitigating a range of potential risks, it is advisable to try to generate a range of derivative files which are likely to be more viable in the future. That is, keep the originals, and try to also export or save the originals to other formats that might be opened in the future.

The final report from the FACADE project makes this recommendation: “For 3-D CAD models we identified the need for four versions with distinct formats to insure long-term preservation. These are:

1. Original (the originally submitted version of the CAD model)
2. Display (an easily viewable format to present to users, normally 3D PDF)
3. Standard (full representation in preservable standard format, normally IFC or STEP)
4. Dessicated (simple geometry in a preservable standard format, normally IGES)”

CAD files now join paper files — such as drawings, plans, elevations, blueprints, images, correspondence and project records — in institutional archives and firms’ libraries. In addition to the ongoing international work on standards and preservation, there must be a dialog with the design-software industry to work toward creating archival CAD files in an open-preservation format.

Finally, trained professionals need to make sense of the CAD files to better archive them and possibly get them up and running again for production, academic, legal or other professional purposes. That requires knowledge of CAD software, file construction and digital preservation methods.

Either CAD users need better digital curatorial skills to manage their CAD archives or digital archivists need better CAD skills to curate the archives of CAD users. Or both.


  1. Henk Vanstappen
    August 28, 2014 at 6:22 am

    Thanks for a clear explananation of a complex problem, and a practical list of resources on the topic.

    A few years ago I did some research on this topic for CVAa, an expertise center on architectural archives in Belgium. Basically, the idea was to find out how different CAD file formats behave when migrated to formats that are suitable for archival preservation (report on http://cvaa.be/sites/default/files/projecten/bijlagen/geheugen_van_de_architect.pdf; in dutch, but has some nice drawings ;-)

    The overall conclusion was – not surprisingly – that there is no turnkey solution: none of the formats (including the formats mentioned above) in our test were capable of preserving every aspect of an architectural drawing. This is due to the intrinsic properties of the file formats, but also practical considerations (such as the availability of suitable software tools, validation tools etc.)
    More interesting was the overall conclusion that selecting a suitable file format depends in the very first instance on the question ‘What do you want to preserve, and for whom.’ (e.g. an art historian studying late 20th century architecture has different expectations than an engineer repairing a B52 engine…) Answering this question will lead to a clear definition of the significant features that need to be preserved (look & feel, geometry, behavior etc.) Only then it is possible to select a suitable archival file format.

  2. Mike Ashenfelder
    August 28, 2014 at 9:05 am

    Thanks for writing, Henk, and for your comments. “What do you want to preserve, and for whom?” is indeed a significant question.

  3. Dana K. Smith, FAIA
    August 28, 2014 at 10:51 am

    I am happy to see that this work is occurring. Having been the one to initiate both the United States National CAD standard and the National BIM Standard-United States and a retiree from the Department of Defense, working in the Office of the Deputy Under Secretary of Defense for Installations and Environment who is responsible for the 570,000 facilities in DoD as well as being a former Director of Engineering Systems for the Naval Facilities Engineering Command, I have to say I have learned from the school of hard knocks. From that experience I see three facets to archiving BIM. 1) Data – IFC as indicated, 2) Geometry – PDF/A, and 3) Media – Cloud based data center to ensure the media is refreshed and accessible. I would love to have feedback from this group about their feeling about this approach and any gaps we need to fill.

  4. Patricia Galloway
    August 28, 2014 at 12:40 pm

    Students in two of my classes in digital preservation wrestled with the CAD problem last year and it turned out that the architecture school faculty and students at UT-Austin themselves generally save the CAD materials they want to keep from their coursework as PDFs. This is a special shame because it means that architecture students are very much disadvantaged in keeping their own work than are, for example, film students or art students using far less closely-held software.

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