Articles | Volume 16, issue 22
Research article
16 Nov 2016
Research article |  | 16 Nov 2016

Cloud photogrammetry with dense stereo for fisheye cameras

Christoph Beekmans, Johannes Schneider, Thomas Läbe, Martin Lennefer, Cyrill Stachniss, and Clemens Simmer

Abstract. We present a novel approach for dense 3-D cloud reconstruction above an area of 10 × 10 km2 using two hemispheric sky imagers with fisheye lenses in a stereo setup. We examine an epipolar rectification model designed for fisheye cameras, which allows the use of efficient out-of-the-box dense matching algorithms designed for classical pinhole-type cameras to search for correspondence information at every pixel. The resulting dense point cloud allows to recover a detailed and more complete cloud morphology compared to previous approaches that employed sparse feature-based stereo or assumed geometric constraints on the cloud field. Our approach is very efficient and can be fully automated. From the obtained 3-D shapes, cloud dynamics, size, motion, type and spacing can be derived, and used for radiation closure under cloudy conditions, for example.

Fisheye lenses follow a different projection function than classical pinhole-type cameras and provide a large field of view with a single image. However, the computation of dense 3-D information is more complicated and standard implementations for dense 3-D stereo reconstruction cannot be easily applied.

Together with an appropriate camera calibration, which includes internal camera geometry, global position and orientation of the stereo camera pair, we use the correspondence information from the stereo matching for dense 3-D stereo reconstruction of clouds located around the cameras.

We implement and evaluate the proposed approach using real world data and present two case studies. In the first case, we validate the quality and accuracy of the method by comparing the stereo reconstruction of a stratocumulus layer with reflectivity observations measured by a cloud radar and the cloud-base height estimated from a Lidar-ceilometer. The second case analyzes a rapid cumulus evolution in the presence of strong wind shear.

Short summary
Sky imager cameras provide a full view of the sky with high spatial and temporal resolution and are used to derive cloud cover, cloud type or cloud-base height, if employed in a stereo configuration. The application of a dense fisheye stereo method provides dense, consistent and quite complete 3-D cloud boundaries and can be fully automated. We present validation of our approach and cloud examples with high geometric complexity. Applications are radiative closure studies and cloud dynamics.
Final-revised paper