Todd A. Croteau - Maritime Program Coordinator for the Historic American Engineering Record, U.S. National Park Service PREFACE Bodie Island Lighthouse represents one of few remaining towers that continue to shine its guiding light through its original First Order Fresnel Lens. Located along Cape Hatteras in North Carolina, Bodie Light is in need of restoration to allow visitors access to the tower and lense. As the recipient of a Save America's Treasures grant for the rehabilitation, its stewards, the National Park Service, began development of a restoration plan that included HABS/HAER documentation of the site. This paper will discuss the basics of HABS/HAER projects and detail the use of digital surveying techniques to record the measurements of such a massive structure using efficient and highly accurate digital equipment. HABS/HAER/HALS The Historic American Buildings Survey (HABS) was created as a Depression-era program to employ architects, historians and photographers in 1933, following a need to mitigate the rapid loss of Colonial architecture in the United States. HABS continues today as the oldest federal preservation program with the addition of its sister programs; Historic American Engineering Record (HAER) in 1969 and Historic American Landscapes Survey (HALS) in 2000. The mission of HABS/HAER/HALS is to develop an archivally-produced collection of drawings, photographs and historical reports that record America's built environment for future generations. The collection is created using consistent formats, and located in the Library of Congress Prints and Photographs Department. The collection is made available to the public in the Library and via the "Built in America" website; (http://memory.loc.gov/ammem/collections/habs_haer/) A PROJECT AT BODIE ISLAND LIGHHOUSE Prior to the rehabilitation of the Bodie Lighthouse project, managers will need accurate and detailed drawings of the structure to prepare a restoration plan. While many original plans for the tower were located in various archives, few drawings detailed the lense and its base. Additionally, it was not known if those original drawings reflected the existing conditions of the tower. The HABS project served to locate original drawings, field check measurements and develop new drawings of components with little existing documentation. Interpretive drawings were also developed to illustrate how cast-iron components were connected to aid in the removal and reassembly of sections to be repaired. Historic drawings were scanned, inserted into AutoCad, and scaled to represent a 1:1 ratio. Dimensions for any part in the drawings can be taken directly from the digital image and new drawings can be traced over the scanned images as needed. Traditional hand measuring techniques were employed to check measurements of smaller details in the field, however, most of the exterior was inaccessible for easy measuring due to the great heights and fragility of the structure. In addition to developing a cost-effective solution for measuring this large structure, the NPS was interested in proving new laser scanning technologies that had been commercially developed for use in reverse engineering and industrial plant retrofits. LASER SCANNING To cross-check exterior measurements, it was determined that Lidar or Laser Scanning technologies could provide a vast quantity of measurements in limited time, while overcoming the need for elaborate scaffolding, cranes, or climbing gear. The NPS contracted with the surveying firm of SIS, based in Rockville, MD to undertake the laser scanning and provide two forms of product. The first product is the rough measurements, which is known as a Point Cloud. The second product was to create section cuts through the point cloud of the tower at ten-foot intervals to determine if the tower was warping out of true circular shape and to see if the tower was plumb or leaning in any direction. Chris and Christine Korkalo served as the scanning technicians for SIS. They setup their Cyrax brand laser scanner and began work at 9am. Nine hours later, the team had taken over 30 million measurements, each being represented by a dot on the screen that when viewed together in a point cloud, had a nearly photographic appearance of the lighthouse. What would have taken months by hand, was accomplished in one day...and much more accurately. Each brick, base stone, and grout line was measured by the equipment. The system also records color intensity, which served to accurately measure the paint scheme of the tower's day marks. The three-dimensional computer model that results from the scanning can be rotated to various views such as Plan, Elevation, or even Sections at a specific plane. An aerial "flyover" can be mapped and animated to give an overall understanding of the site. After some post-processing time, we received the data, which showed that the tower maintained true circular shape throughout its rise and did not deviate from the centerline. The tower was apparently built on a better foundation than the first tower that carried its name. FARRO ARM SCANNING The next phase of digital surveying would require the use of a different piece of equipment. Laser scanners send light beams out to an object, which then reflects it back to the machine for calculations. Laser beams would penetrate the glass prisms of the Fresnel Lense and not return to the device. There is also limited space between the lens and the lantern perimeter. Another device, known as a Farro Arm, uses an articulated series of rods with a metal probe at its tip to record x,y,z coordinate information from an object. For this work the NPS contracted with Direct Dimensions from Baltimore, MD. Again working over the course of one day, the team was able to develop thousands of point measurements that could be converted to a three-dimensional computer model and then to line drawings. The drawings produced by this technology are probably the most accurate ever produced of an existing Fresnel Lense. The computer model can also be surfaced, rotated, and cut to provide various views. Additionally, the model can be converted to a stereo lithography file (.stl) and "printed" three-dimensionally on a rapid prototyping machine. Miniature or full-size replicas of lenses or individual prisms can be produced for gifts or as molds for casting new prisms to replace the old fractured ones. CONCLUSIONS Based on the results of this project, and a growing number of other historic structures scanned with Lidar technology, it is apparent that laser scanning will become a more common practice when measuring buildings, especially those of high complexity, large scale, or fine detail. The HABS drawing set produced for Bodie Island Lighthouse utilized a range of tools available; from the artistic ink drawings of the original designers to the digital models developed by laser scans. The techniques employed resulted in a savings of time and cost to the project, while providing highly accurate documentation that will transcend the drawing sheet. Like all new technologies, there is a learning curve and a high initial cost factor. While the cost of scanning may appear high to some, the results should outweigh the costs, especially on larger projects where time and accuracy are critical. For more information about this or other maritime documentation projects contact:
Todd A. Croteau
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