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The Historic American Buildings Survey (HABS–the Federal government’s oldest preservation program), and companion programs the Historic American Engineering Record (HAER) and the Historic American Landscapes Survey (HALS), have utilized 3D laser scanning technology since 2005 on over seventy-five projects, ranging from the Statue of Liberty to the Space Shuttle Discovery, from Heiaus in Hawaii to totem poles in Alaska, from the stairs of Carlsbad Caverns to the Civil War Ironclad Cairo at Vicksburg, and from the 12th century Victory Towers of Ghazni, Afghanistan to the World War I cemeteries in Belgium.
HABS/HAER/HALS uses 3D laser scanning technology primarily as a tool to derive one of our principle products–the measured and interpretive drawings that join large-format photographs and historical reports to become part of our permanent collection at the Library of Congress. These drawings must conform to the Secretary of the Interior’s Standards for Architectural, Engineering (and Landscape) Documentation: adequately convey the significance or value of the resource; be accurately prepared from reliable sources; be durable, standard, and reproducible; and be clear and concise.
3D laser scanning helps us capture some of the initial site data, and we create models and meshes to render the point cloud data into usable forms for presentation and interpretive purposes and for flattening into 2D architectural drawings.
Digital data capture, preparation, and presentation, when done with precision and an accurate end-product as the goal, have a steep learning curve; involve multiple, complex and at times incompatible software packages; are labor-intensive on both the front-end and the back; and vary from site to site. Data capture utilizing 3D laser scanning alone is rarely sufficient to provide enough accurate information to portray a site consistent with conventional architectural standards, and almost always benefits from utilizing complementary techniques such as traditional survey and hand measuring. The processing phase requires the ability to read and massage the point cloud in such a way that it is consistent with the resource and can be further worked into an accurate architectural rendering.
Data on historic sites, regardless of the manner of acquisition, serves three primary functions: the preservation and stewardship of the sites themselves; the creation of a permanent record of significance; and their presentation and interpretation to the public. Digital data capture and presentation offer some unique opportunities to accomplish these tasks in new ways that speak to those who will care for and visit these sites in the future.
Preservation and Stewardship
LIDAR is a valuable tool in data acquisition for the development of existing-condition drawings, with several significant caveats: that time is taken in the field to ensure that the site is accurately represented in the data gathered; that scanneracquired data is supplemented with data gathered by more traditional means, such as surveying and hand-measuring, to fill information gaps; and that time is taken post-capture to accurately translate and interpret scanner-acquired data to formats usable by historic site stewards and specialists.
Line drawings do not require up-to-date software and hardware platforms and can be easily duplicated and carried to a site, then referenced and marked up. "3D Laser Scanning for Heritage," published by English Heritage, notes that utilizing point cloud data to generate line drawings is "not a fully automatic process and requires skill and experience on the part of the users." This is something of an understatement. The labor-intensive process can entail manipulating large data sets by cropping and 3-D orbiting; building a solid model from scratch using the point cloud as an underlay or converting it to a mesh surface; and finally flattening the 3D model to obtain a set of lines. Each step involves a choice of software packages and generally mandates a powerful hardware configuration. In the end, the 3D and 2D graphics are, in the words of the Smithsonian’s Gunter Waibel, "lovingly handcrafted," a concept often overlooked in an era of immediate digital gratification.
HABS/HAER/HALS line drawings conform in design and format to the standards of the architecture profession and are easily usable by restoration, rehabilitation, or reconstruction project architects and engineers. To get from the point cloud to the 2D drawing requires knowledge of the way laser scans present layers of data and an understanding of the architectural conventions that determine how specific views and details are represented and of the line weights that add depth and clarity. This is where time spent getting to know the site, professional architect/ engineer training, and familiarity with the nuances of point cloud presentation combine to give accuracy, clarity and utility to the final product.
A Permanent Record
One of the primary reasons we document historic sites is to create a permanent record for future generations. Charles Peterson, an early organizer of the HABS program, argued in 1933 that "It is the responsibility of the American people that if the great number of our antique buildings must disappear through economic causes they should not pass into unrecorded oblivion." Similarly, Ben Kacyra refers to CyArk’s mission as ensuring that our "collective human memory" is not lost.
It is somewhat ironic that as digital technologies advance, the most durable records remain the older hard copies. Baseline documentation created by the Heritage Documentation Programs– the measured drawings, large-format photographs, and historical reports–is completed on materials tested for a 500 year shelf life and deposited at the Library of Congress, as they have been since the program’s founding in 1933. The Library makes available on-line digital versions in downloadable, publishable formats and stores the original hard-copy documentation in state-of-the-art facilities at Ft. Meade, complying in full with the Secretary’s Standard on durability mentioned above. The lack of industry standards for laser scanning and similar born-digital technologies, and the proprietary software that facilitates the transition from point cloud to 3D model and 2D line drawing, discourage the Library from preserving these intermediate formats. The National Archives currently pre-accessions only a limited number of file formats, and the National Park Service, which faces its own stewardship costs for the historic sites it maintains, cannot afford the millions of dollars it would cost annually to protect its own electronic records from file and media degradation. Thus, our formal, hard-copy documentation is better protected now than it was prior to the opening of the Library’s Ft. Meade facility, but we still face the challenge and expense of preserving our ever-growing bank of digital data.
Interpretation
Interpretation is where digital documentation really shines. On-line viewing tools permit manipulation of electronic images that give the viewer an almost on-site experience. Many of our historic sites are fragile and the resources endangered; fly-throughs and virtual tours permit us to present these to the public and still carry out our stewardship duties.
As our skill with the process has grown and better software has become available, digital presentations have become more sophisticated. The data from each site on which laser scanning was used–all seventy-five–was carefully and painstakingly worked through the modeling, meshing and flattening processes; checked, verified, and complemented by data obtained through traditional means; then carefully assembled so that the most significant aspects of the site were represented. The results are visually captivating but very precise, accurate renderings of complex historic sites that give the visitor something unavailable otherwise. (HABS/HAER/HALS YouTube channel: http://youtube.com/user/HDPNPS)
3D laser scanning is an excellent tool for presenting and interpreting historic sites. It helps preserve fragile resources by permitting visitors to experience them without the wear and tear of on-site visits.
When captured as part of a mitigation process, we can show the resource after it has been removed in as much or more detail than was available to the visitor before. We can also show expanded contexts that incorporate structures and landscapes. And, of course, there are the fly-throughs and virtual tours! Again, though, there is a skill level and time element to developing these tools once we have the data in hand, but as you can see, the results are very much worth the effort.
Conclusion
3D laser scan documentation, if it is to be useful for actual site preservation, requires sufficient time in the field to ensure accurate data capture, including the use of supplementary documentation techniques like hand measuring and survey, as well as post-capture time to translate the data into formats useful for site stewards, like measured drawings. The creation of a permanent digital record–maintenance and migration of files and sustainability of hardware platforms–is far more costly than similar requirements for hardcopy, and has yet to achieve consensus within and between public and private sectors. Digital interpretive products depend on quality data and both the skill and time to transform data from a variety of sources, via myriad software packages, to formats that can both educate the viewer and inculcate an appreciation of the significance of the resource. It may seem like we have come a long way from traditional hand recording and presentation techniques to 3D laser scanning and digital presentation, but we have yet to escape the reality that quality is neither fast nor cheap.
Richard O’Connor is Chief, Heritage Documentation Programs, and Acting Chief, Historic American Engineering Record (HAER), at the National Park Service. He received his PhD in History at the University of Pittsburgh and is the author of a number of articles and reports on historic industrial processes.
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