Recording aboriginal rock art using cheap digital cameras and digital photogrammetry

Archaeologists, conservators and rock-site managers need simple and cost effective methods to record and document rock art, including both petroglyphs and pictographs. Combined laser scanning and photogrammetry can be effective but equipment remains expensive, is difficult to transport into the field and requires some expertise to use successfully during data capture. What is required is the development of a methodology that enables the inexpert, perhaps volunteer, field worker to acquire imagery suitable for photogrammetric measurement using cost effective digital sensors. This paper describes the desired alternative in which a cheap digital camera costing just US$300 is used to generate both accurate and dense DEMs and orthophotographs. These data are able to record detailed morphology, generate three dimensional visualizations and the ubiquitous fly through model. The methodology was developed and tested using a series of case studies, representing a diverse selection of aboriginal rock art. Imagery was acquired using a 3 Megapixel Nikon Coolpix 3100 costing US$300 and compared with imagery obtained using a Kodak DCS460, which originally cost US$ 30,000. Fieldwork was conducted at six field sites in Australia, including both petroglyphs and pictographs. Digital photogrammetry was carried out using the Leica Photogrammetry System and an external self-calibrating bundle adjustment; the combination generating medium accuracy (±3mm), high-resolution DEMs and orthophotos. The petroglyphs were small, typically 1-2m in length and located on horizontal sandstone outcrops. Simple stereopairs acquired using the Nikon Coolpix and simple scaled control in the form of a survey staff, generated dense DEMs (5mm), appropriate to record detailed morphology. An image processing technique implemented in the form of an Erdas “Spatial Model” tool allowed identification of the pecked and engraved grooves from the surrounding rock surface. The pictographs sites were located on vertical and curved rock faces within rock shelters, typically 2-4m high. 3D control was provided using a reflectorless Total Station and rotation of the control coordinates enabled the LPS software to function correctly. Lower resolution DEMs (50mm) proved sufficient to record the simplified morphology. Colour orthophotographs could be generated and multiple images mosaiced together to allow 3D dimensional visualization and fly through generation. The merits of the developed approach will be discussed and implications arising from adoption outlined.