Introduction As part of the English Heritage Rock Art Pilot Project (1999), laser scanning as a method of documenting rock art in the field, was examined. The petroglyphs studied are located on Rombald’s Moor in West Yorkshire. The areas scanned were approximately 1.2 by 0.5 m in size. Laser scanning was found to be a good technique for the 3D non-contact recording of rock art. The equipment is suitable for use in the field – even in remote locations. The data obtained documented the petroglyphs to a high level of detail. Importantly, the results were not subjective to lighting conditions at the time of data capture. Indeed, once the data had been post-processed, and was examined under varying lighting conditions, a distinct wear pattern was located on the surface of one of the rocks. This pattern had not been discernable from photography, nor with the naked eye. The results of laser scanning can be exploited in a wide variety of imaging formats, providing a flexible digital archive. Images of contour maps of the surfaces and a scale replica in polyurethane were also produced from the data.

 Instruments and software A ModelMaker H laser scanner (3D Scanners UK) mounted on a six-axes Faro silver arm was used to record the petroglyphs. A sensor with a 40 mm stripe width was used, and the scanner recorded at a rate of twelve stripes per second. Stripe-stripe separation is dependent on the speed at which the sensor is moved across the surface by the operator, and ranges from 4 mm – 0.2 mm. The petroglyphs were recorded with high accuracy, hence with a slow scanner speed, producing a dense point cloud. Data was captured using ModelMaker V1 software. Meshing and post-processing was also carried out in ModelMaker software. All data was checked on site. Areas which may have been missed during data capture were easily highlighted and ensured as complete a data set as possible was recorded. The raw point cloud was also crudely meshed on site to check the quality of the scan data. ModelMaker V1 was used to create a contour map of the individual rocks. Each rock art panel required 3-4 hours to scan, including the time it took to move the equipment and prepare it for data capture.

 Why was scanning selected? Commonly used 2D techniques such as sketching, rubbing, and photography employed to record rock art are subject to lighting conditions. Moreover, they suffer inaccuracies due to the difficulty of rendering a 3D surface in 2D. Close range photogrammetry, whilst providing a 3D archive is still subject to the lighting conditions at the time of recording. Moulding and casting techniques, while both 3D and non-light subjective, can damage the weathered surface of a petroglpyh. Non-contact recording using laser scanners provided a solution to all of these problems. In addition, by fixing datum points close to the rock panel it is possible to rescan the rock at a later date. The two data sets can then be compared to measure and monitor any changes or decay on the surface of the petroglyph.

 What problems were encountered? The ambient light levels on the moor were too bright for laser scanning (scanning was undertaken during summer). As this was anticipated, a small tent was erected over the site. The tent suitably reduced light levels to enable data capture. Calibration of the equipment on site was difficult. There was a lack of solid surfaces to mount the geometric cube used for calibration. It was necessary to calibrate the arm off-site, move the equipment with care, and check the calibration on completion of scanning, off-site. In this way it was possible to ensure that calibration had remained within specification throughout data capture.

What were the final deliverables? The raw data was provided in CTA (3D Scanners file) and ASCII format. The postprocessed data was provided as STL files, and the 3D contour maps were delivered in IGES format. In addition, the data obtained by laser scanning was used to produce a scale replica of one of the rocks in lightweight polyurethane using 3-axes cnc (computer numerically controlled) machining.