Automated archiving of oblique and vertical aerial photographs

Abstract

The main purpose of any aerial photo archive is to allow quick access to images based on content and location. Therefore, next to a description of technical parameters and depicted content, geo-referencing of every image is of vital importance. This can be done either by identifying the main photographed object (geo-referencing of the image content) or by mapping the centre point and/or the outline of the image footprint. 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 (e.g. Exif). 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 angles of the camera 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, these parameters can also aid the subsequent orthophoto production workflows and even guide photo acquisition. The presentation proposes a new image archiving workflow. The new pipeline is based on the parameters that are logged by a commercial but cost-effective GNSS/IMU solution and processed with in-house developed software. Together, and with support of freely available terrain elevation data, these components allow to automatically geo-locate and rectify the (oblique) aerial images (by a simple planar rectification using the exterior orientation parameters) and retrieve their footprints with reasonable accuracy. The latter is automatically stored as a vector file, thus enabling a fast and almost automatic GIS-based archiving of all the imagery.