Cost-effective geocoding with exterior orientation for airborne and terrestrial archaeological photography: possibilities and limitations

Taking a photograph is often considered to be an indispensable procedural step in many archaeological fields (e.g. excavating), whereas some sub-disciplines (e.g. aerial archaeology) often consider photographs to be the prime data source. Whether they were acquired on the ground or from the air, digital cameras save with each photograph the exact date and time of acquisition and additionally enable to store the camera's geographical location in specific metadata fields. This location is typically obtained from GNSS (Global Navigation Satellite System) receivers, either operating in continuous mode to record the path of the camera platform, or the position is observed for each exposure individually. Although such positional information has huge advantages in archiving the imagery, this approach has several limits as it does not record the complete exterior orientation of the camera. More specifically, the essential roll, pitch and yaw camera angles are missing, thus the viewing direction and the camera rotation around it. Besides enabling to define the exact portion of the scene that was photographed (essential for proper archiving), these parameters can also aid the subsequent orthophoto production workflows and even guide photo acquisition. This paper proposes a cost-effective hard- and software solution (camera position: 2.5 m and orientation in static conditions: maximally 2°, both at 1σ) to record all indispensable exterior orientation parameters during image acquisition. After the introduction of the utilized hardware components, the software that allows recording and estimating these parameters as well as embedding them into the image metadata is introduced. Afterwards, the obtainable accuracy in both static (i.e. terrestrial) and dynamic (i.e. airborne) conditions are calculated and assessed. Finally, the good use of this solution for different archaeological purposes will be detailed and commented where needed, while an outlook on future developments finalizes this article.

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