This poster was presented at the 3D Digital Documentation Summit held July 10-12, 2012 at the Presidio, San Fransisco, CA.
Saving the Lines: Documenting Ships and Small Craft Using Laser Scanning
Recording the shape of ships is especially challenging as they are typically constructed with compound curves and complex surfaces. Traditional hand-measuring techniques are time-consuming and only record a small fraction of the overall surface. EDM machines or Total Stations speed up this process, but still only record a relatively small number of points along the hull surface. Within a relatively short amount of time, Laser Scanners can record the majority of the hull surface, including plank seams and other features attached to the hull, in addition to recording the deteriorated nature of the material.
The point clouds generated from the scans can be processed through a variety of software solutions to create what is known as a “Lines Plan and Table of Offsets”. A Lines Plan is a two-dimensional drawing that describes the curves generated by slicing the surface with multiple planes in the x, y, and z directions. The drawing is similar to a contour map for landscapes. These drawings and the resulting table of offsets would allow a builder to construct the boat. Surface models created from the data points can be used to perform hydrostatic and hydrodynamic testing of the vessel to determine seaworthiness and other characteristics of its design. Additionally, the photo data created during the scanning process can be used to show views of the ship that might not be obtainable using traditional methods. This includes the capability of creating panophotos that allow a person to view an area under, or a space within, the vessel that might be closed to visitor access for safety, security, or accessibility reasons.
Although laser scanning can greatly reduce the time in the field for surveying a vessel, it is only one of several techniques used to create the complete record for a ship. Laser scanners are very adept at capturing the exterior hull surface, but there are many interior spaces on a ship or up in the rigging that are difficult to access or are too cluttered with machinery, structural components or other obstructions. It is usually necessary to supplement the scanning with hand-measurements and photography. In many cases, it may be more efficient to hand-
measure features. Another unique dilemma to documenting ships is the fact that often times they are documented in the water and move with the currents and tides while moored. The underwater portion of the hull cannot be scanned. Ideally, a vessel would be blocked on land, hauled out on a marine railway, or lifted on a drydock if the budget allows.
Overall, laser scanning is a very useful tool in the survey of historic vessels. Scanning can rapidly capture data from curved surfaces with high accuracy and record the deformation of shape inherent in old ships. However, it would be difficult to say that laser scanning is the only tool for the job. A combination of techniques is required to fully document a ship and to prepare a set of measured drawings that would meet the Secretary of the Interior’s Standards for Architectural and Engineering Documentation.