All-sky photogrammetry techniques to georeference a cloud field

Pierre Crispel (1) , Gregory Roberts (2)

1 IMFT - Institut de mécanique des fluides de Toulouse
2 SIO - UC San Diego - Scripps Institution of Oceanography

In this study, we present a novel method of identifying and geolocalizing cloud field elements from a portable all-sky camera stereo network based on the ground and oriented towards zenith. The methodology is mainly based on stereophotogrammetry which is a 3D reconstruction technique based on triangulation from corresponding stereo pixels in rectified images. In cases where clouds are horizontally separated, identifying individual positions is performed with segmentation techniques based on 5 hue filtering and contour detection algorithms. Macroscopic cloud field characteristics such as cloud layer base heights and velocity fields are also deduced. In addition, the methodology is fitted to the context of measurement campaigns which impose simplicity of implementation, auto-calibration, and portability. Camera internal geometry models are achieved a priori in the laboratory and validated to ensure a certain accuracy in the peripheral parts of the all-sky image. Then, stereophotogrammetry with dense 3D reconstruction is applied with cameras spaced 10 150 m apart for two validation cases. The first validation case is carried out with cumulus clouds having a cloud base height at 1500 m.agl. The second validation case is carried out with two cloud layers: a cumulus fractus layer with a base height at 1000 m.agl and an altocumulus stratiformis layer with a base height of 2300 m.agl. Velocity fields at cloud base are computed by tracking image rectangular patterns through successive shots. The height uncertainty is estimated by comparison with a Vaïsala CL31 ceilometer located on the site. The uncertainty on the horizontal coordinates and on the velocity field are theoretically 15 quantified by using the experimental uncertainties of the cloud base height and camera orientation. In the first cumulus case, segmentation of the image is performed to identify individuals clouds in the cloud field and determine the horizontal positions of the cloud centers.

Mots clés: photogrammetry techniques; georeference

Domaines: Sciences de l'environnement

Pierre Crispel, Gregory Roberts. All-sky photogrammetry techniques to georeference a cloud field. Atmospheric Measurement Techniques, 2017, ⟨10.5194/amt-2017-203⟩. ⟨hal-02415958⟩