TERRESTRIAL 3D LASER SCANNING: A NEW METHOD FOR RECORDING ROCK ART

The mapping of archeological sites is an essential, but arduous task. It is further complicated by a need for repeat surveys to continuously manage and preserve the site integrity. Terrestrial laser scanners provide a non-destructive approach to record archeological sites. We present an example of laser scanning at a rock art site near Little Lake, California. The scanner is used to record four-levels of data within the site that include the site context, panels, panel elements, and motifs.

Mapping was conducted with a Cyrax 2500 laser scanner in conjunction with a Trimble 4400 RTK GPS surveying unit. The Cyrax 2500 has a 40° x 40° field-of-view (FOV) and can capture up to one million points in the FOV. The FOV limits the amount of space that can be scanned in a single scan. However, the scanhead can be turned or the scanner repositioned to capture the entire site. The placement of the scanner at various locations also permits scanning from multiple angles to produce a 3D view of the target object. Point clouds gathered from adjacent scans can be merged after they have been registered. Registration is conducted with 3D Cartesian coordinate system (real world coordinates developed with the aid of laser control targets or coordinates that have their origin at the scanner position) in the Cyclone software (v. 4.01). Registration of adjacent point cloud requires three common targets within each scan.

We produced an extremely accurate map of the spatial context of archeological features within a site (+1cm accuracy). Finer-scale scans recorded in great detail the spatial dimensions of motifs and panel elements. GIS analyses were conducted to assess the spatial patterns in the panel locations on the talus slopes. Our results indicated distinct local-scale clustering of panels near the toe of the talus slope.

The laser scanning approach provided valuable information for the preservation of the artwork at multiple spatial scales. Laser scanning also provides a way to monitor long-term changes to rock art from natural and visitor impacts through rapid repeat surveys. Finally, the repeat rapid reassessment of sites and archived imagery can also be used to determine the extent of vandalism or theft of objects in remote locations and provide information on the object that has been vandalized or stolen.