Abstract
Abstract. In multimedia photogrammetry, multi-camera systems often provide scale by a calibrated relative orientation. Camera calibration via bundle adjustment is a well-established standard procedure in single-medium photogrammetry. When using standard software and applying the collinearity equations in multimedia photogrammetry, the refractive interfaces are modelled in an implicit form. This contribution analyses different calibration strategies for bundle-invariant interfaces. To evaluate the effects of implicitly modelling the refractive effects within a bundle adjustment, synthetic datasets are simulated. Contrary to many publications, systematic effects of the exterior orientations can be verified with simulated data. The behaviour of interior, exterior and relative orientation parameters is analysed using error-free synthetic datasets. The relative orientation of a stereo camera shows systematic effects, when the angle of convergence varies and when the synthetic interface is set up at different distances to the camera. It becomes clear, that in most cases the implicit modelling is not suitable for multimedia photogrammetry. An explicit modelling of the refractive interfaces is implemented into a bundle adjustment. This strict model is analysed and compared with the implicit form regarding systematic effects in orientation parameters as well as errors in object space. In a real experiment, the discrepancies between the implicit form using standard software and the explicit modelling using our own implementation are quantified. It is highly advisable to model the interfaces strictly, since the implicit modelling might lead to relevant errors in object space.
Highlights
Photogrammetry dealing with more than one optical medium is known as multimedia photogrammetry
We hypothesize that, when scale is provided only by the relative orientation of a multi-camera system, systematic errors occur in forward intersection due to the inevitable correlations between the interior and exterior orientation of a multi-camera system resulting in incorrect scaling
Using planar interfaces and the implicit estimation of interior and relative orientation, the bundle adjustment leads to significant errors when scale is provided by the relative orientation only
Summary
Photogrammetry dealing with more than one optical medium is known as multimedia photogrammetry. It becomes increasingly important in underwater applications such as fish farming, industrial inspection tasks or surveying of archaeological and cultural heritage sites. Camera calibration via bundle adjustment is a well-established standard procedure in single-medium photogrammetry and might be a reasonable tool for multimedia photogrammetry. In case of underwater photogrammetry, the waterproof camera housings and the water itself bend the optical path of light. These refractive effects need to be compensated implicitly by the standard calibration parameters or modelled explicitly by an extended camera model within the calibration process. We hypothesize that, when scale is provided only by the relative orientation of a multi-camera system, systematic errors occur in forward intersection due to the inevitable correlations between the interior and exterior orientation of a multi-camera system resulting in incorrect scaling
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