Untangling the Knot of CAD Preservation

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.

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.

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.

Here are some other CAD preservation resources, many of which refer to STEP:
• The United States National CAD Standard encompasses The American Institute of Architect’s CAD Layer Guidelines, the Construction Specification Institute’s Uniform Drawing System and the National Institute of Building Sciences Plotting Guidelines.
• MIT conducted a two-year project (which included digital preservation pioneer Stephen Abrams on their advisory board) called “Future-proofing Architectural Computer-Aided DEsign,” where they analyzed CAD data from three renown architects and their projects. The FACADE project’s final report (pdf) details recommendations and best practices.
• The National Archives’ “Revised Format Guidance for the Transfer of Permanent Electronic Records” lists “Preferred” and “Acceptable” formats for CAD.
• The Art Institute of Chicago Department of Architecture published “Collecting, Archiving and Exhibiting Digital Design Data” (pdf).
• In July, 2013, the Digital Preservation Coalition held a conference titled “Preserving Computer Aided Design.” In 2013, the DPC also released their Technology Watch report, authored by Alex Ball, “Preserving Computer-Aided Design.”
• ISO 13567 is a CAD layer standard.
• CAD standards on Wikipedia.
• ISO 16739 for BIM data.
• The “CAD: A Guide to Good Practice” is a collaborative effort from the UK’s Archaeology Data Service and the US’s Digital Antiquity.
• The Archaeology Data Service’s 3D Viewer project is an interactive 3D web-based working environment for the management and analysis of archaeological data
• Durable Architectural Knowledge: the DURAARK Project, funded by the EU
• The list of CAD file formats is stunning.

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.