Oblique aerial images: a review focusing on georeferencing procedures

The automatic processing of oblique aerial images is currently one of the most attractive topics in the fields of photogrammetry, remote sensing and computer vision. This kind of images has gained popularity throughout the last years, as it enables depiction of both vertical and horizontal structures of the urban environment, being ideal for various photogrammetric applications, apart from visualization and interpretation purposes. In most cases, the georeferencing of the images is indispensable for their metric exploitation, usually in terms of computation of their camera exterior orientation parameters. For other applications, georeferencing by means of image orthorectification is required, whereas a simple georeferencing in terms of mapping of the center point of the images or the outline of their footprint or image rectification via a 2D transformation also suffices in some cases. However, oblique airborne images pose several challenges in automated georeferencing procedures, due to complex image geometry and strongly varying appearance between images of the same scene taken from different viewing angles. In this paper, a thorough literature review is conducted in order to inform about the recent research in georeferencing of datasets including oblique aerial imagery. The review starts with a classification of oblique aerial imagery and a presentation of the most commonly used camera configurations for oblique imagery acquisition. A brief presentation of their history along with old applications is then made, followed by a categorization of the literature regarding state-of-the-art georeferencing procedures as well as other research trends and applications using oblique aerial images. Then, a more in-depth description of recent research on georeferencing of oblique image datasets is presented. Finally, the significant points, research results and conclusions derived from the literature are discussed, so that they can be utilized by future researchers in the development of georeferencing methods targeted to oblique airborne imagery.