Laser scanning can be used to create an image of an object in space, but the construction of this image comes by a very different method than is used in conventional photography. When a photograph is taken, the image is captured through a lens. The specifications of the lens determine the observed qualities of perspective in the final image. The lens also introduces distortions that make it difficult to extract accurate drawings or measurements. The orthographic drawings commonly used by architects and engineers are drawn without perspective so that relationships between any given points on the drawing can be measured at a constant scale.
Laser scanning measures the distance from the scanner, itself, to a number of points in space. After making these measurements, the relationship between the gathered points can be expressed in the constructed 3D space of the computer. By taking thousands or millions of measurements, a “cloud of points” emerges, which accurately describes the subject being scanned. Finally, when rendered from the point of view of a synthetic, orthographic camera in the 3D world space of the computer, accurate views are generated.
In selecting viewpoints, the goal is to describe the scan subject completely, while maintaining common objects between views to register the resulting data. In areas where detail is important, sub-viewpoints are designated and scanned at higher resolution. Note that each viewpoint contains only a fraction of the total information for the subject. Just as a light projects a region of shadow on an object, here each view contains areas where data is "shadowed." In order to register the data from multiple viewpoints, spheres of known diameter are to be ncluded in the scanned scene.
As the spheres appear the same from any view and are uniform in diameter, they can be quickly isolated by the computer, named, and treated as distinct objects. Because the views are designed with a view to the same spheres, it is then possible to register the coordinate space of each separate viewpoint into an integrated model. Components of the object and its context can be used as "surrogate spheres." After locking together each of the views, the resulting cloud of points can be visualized in a variety of ways, including final rendered files. The scanning operation of each fragment shall involve multiple viewpoints, to be later correlated. It is crucial that all spheres can be left in place during scans, and are precisely named in the computer for registration purposes.
While it is still a challenge to deal with the large amount of data generated from 3D scans, the scanning process remains the best method to quickly gain comprehensive documentation of a site. This is especially crucial for imperiled sites, where details risk being lost before they can be documented. In these crisis cases, 3D scan data could potentially be the only resource available to future researchers.
To go further, however, we need to question the technique and an integrated approach is helpful, balancing laser scanning with traditional techniques. While 3D scanning can be accurate and rapid when compared to traditional techniques, it is important to test the data provided via traditional techniques, where and when possible. There are also clear advantages to traditional techniques in terms of cost and time, depending of the project.
3D scanning enables a site to be accurately measured in a relatively short amount of time. 3D scanning remains up till now the only viable way of documenting the precise measurements of a complex subject.
Importantly, scan data models can be transferred into popular formats (e.g. AutoCAD) for use by architects and engineers. These same files can be used as the basis for reconstructions, physical models, or object movies. Provided that the files are continually migrated, 3D scanning is a permanent, durable record of the site.
During fieldwork, our team has found that 3D scanning hardware is inevitably somewhat delicate. While scanners differ in the robustness of their performance, all require special handling and careful operation. Also, as mentioned above, the amount of time required to complete large-scale scans and the heaviness of the resulting files continues to be a significant problem.
It remains very difficult to accurately capture epigraphy with 3D laser scanning. Large-scale scanners are not designed to read detailed inscriptions while small-scale scanners are not equipped to deal with the scale of a building. Since epigraphy must be scanned at high resolution to capture vital detail, heavy files are again an issue.
Ultimately, 3D scanning continues to be a fertile area for experimentation and research; within only a few years this technology will be well established and widely available as a powerful tool for all but the smallest research teams.