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Documenting Vessels for HABS/HAER using CAD and Photogrammetry

Todd Croteau
HABS/HAER Maritime Program Coordinator
National Park Service
PO Box 37127
Washington, D.C. 20013-7127

The Historic American Buildings Survey/Historic American Engineering Record (HABS/HAER) of the National Park Service is a unique federal program whose responsibility is to document America's architectural and engineering heritage. Founded in 1933 to create work during the Great Depression, it is the oldest Federal program dealing with the built environment. Over the past 60 years HABS, along with its sister agency, the Historic American Engineering Record (HAER), established in 1969, has produced measured and interpretive drawings, photographs and historical reports on over 30,000 buildings and structures throughout the United States. Another WPA program developed was the Historic American Merchant Marine Survey (HAMMS). In eighteen months of operation, 426 vessels were recorded, and only two of those survive today. This collection is held at the Smithsonian Institution's Museum of American History, Transportation Division.

HABS/HAER Documentation is created using specific guidelines and formats, as defined in the Secretary of Interiors Standards and Guidelines for Architectural and Engineering Documentation, and affiliated guidelines for preparation of drawings, historical reports and photographs. The Guidelines for Recording Historic Ships is specific to the preparation of vessel documentation and discusses methods of recording. All materials are transmitted to the Library of Congress to be catalogued and preserved and held for perpetuity in the public domain.

Traditional HABS/HAER drawing projects typically require hand-measuring and hand-drafting onto mylar, however, a new approach to documentation is being developed; the introduction and use of computer-aided photogrammetry in conjunction with CAD. Architectural photogrammetry combines principles of photography and geometry in any method from which scaled drawings can be obtained from photographs; therefore photogrammetry, in its most basic sense is the science of using photographs to derive measurements. It is most important to treat photogrammetry as one of many tools in your box, not as the answer to measuring your structure. There are benefits, as well as disadvantages, to choosing CAD and photogrammetry. Multiple techniques and methods may be required to achieve the desired product.

The HABS/HAER CAD-Photogrammetry Studio uses a unique photogrammetric system that uses semi-metric cameras and a photogrammetry software which extract dimensions from photographic images by digitizing from photographic enlargements. As with all photogrammetric applications, basic necessary information must be provided to determine scale and orientation of the image.

The photogrammetric software which HABS/HAER uses is Desktop Photogrammetry's Photocad-Multi for obtaining three-dimensional measurements and Photocad-Single for two- dimensional (planer) measurements. Both programs operate from a pull down menu within AutoCAD and produce AutoCAD .DWG drawing files, which can be transferred as .DXF files also.

Photocad-Multi software uses multiple photographs, each taken from a slightly different angle and one known measurement. The software solves for the exact three-dimensional location of any point visible in at least two photographs. Using this method a three-dimensional model can be created of the vessel, from which lines drawings, profiles and sections can be obtained. Photocad-Single software uses a single photograph and four known points on flat facades or elevations and will correct for perspective distortion of the image. This method produces a two-dimensional drawing that can stand alone or be imported as a wallpaper in a three-dimensional program file. Often a combination of Photocad -Single, -Multi and traditional hand measuring are required to completely record a structure.

Photogrammetric documentation technology also provides a speedy method of documentation in cases of impending disasters or emergencies. In 1992, three years after the devastation of hurricane Hugo, a team traveled to Historic Charleston, South Carolina with one of the Metrica cameras to photograph important historic structures along the Battery. Although there have been no funding resources to digitize these photographs, they will provide an invaluable resource to aid in reconstruction should any of these buildings be damaged by a future hurricane. Additionally, for disaster preparedness a fleet of historic vessels could be hauled and photographed quickly, relative to the time it would take to make hand measurements. This would mitigate irreparable damage in a quick survey format, capturing indispensable information prior to their destruction. Similarly, photogrammetry can be used to record sites of a ruinous archeological remains that are difficult to access due to tidal intrusions or other hazardous environmental concerns.

Current projects of the HABS/HAER CAD-Photogrammetry Studio include proving Photocad-Multi to document the hull of historic vessels so as to lift the lines and create sheer and body plans. Successful results from these exercises are providing HABS/HAER the opportunity to further documentation capabilities, assisting in the preservation of historic vessels within the Maritime community, and adding to the permanent HABS/HAER collection in the Library of Congress.

Future applications of the 3-D computer models produced include developing "Virtual Reality" demonstrations that allow historic ships to "sail" again. Advanced Marine Enterprises, Inc., a ship design company, has developed software that converts CAD drawings into real-time movement that simulates actual physical properties, mechanical operations, sea-states and flood damage results. This is an excellent tool for interpreting the characteristics of vessels that no longer sail, or remain intact. Presently, AME is working under a grant from the National Preservation Training Center to develop a simulation for the MONITOR, which will provide researchers with insight into battle operations and maneuverability of the vessel.

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Inquiries regarding the software can be made to:
DESKTOP PHOTOGRAMMETRY, INC.
Attn: Bill Archibald
331 Lakeside Drive, Suite B
Foster City, California 94404
Voice: (415) 574-4225
Fax: (415) 345-4921
E-mail: 71151.2776@compusev.com

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Version 1.00, 30 July 1997