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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 1 | Copyright
Atmos. Meas. Tech., 11, 593-609, 2018
https://doi.org/10.5194/amt-11-593-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 31 Jan 2018

Research article | 31 Jan 2018

All-sky photogrammetry techniques to georeference a cloud field

Pierre Crispel1,a and Gregory Roberts1,2 Pierre Crispel and Gregory Roberts
  • 1CNRM, Centre National de Recherches Météorologiques UMR 3589, Météo-France/CNRS, Toulouse, France
  • 2Scripps Institution of Oceanography, Center for Atmospheric Sciences and Physical Oceanography, La Jolla, California, USA
  • anow at: Météo-France DSM/AERO, Toulouse, France

Abstract. 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 3-D 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 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 3-D reconstruction is applied with cameras spaced 150m apart for two validation cases. The first validation case is carried out with cumulus clouds having a cloud base height at 1500ma.g.l. The second validation case is carried out with two cloud layers: a cumulus fractus layer with a base height at 1000ma.g.l. and an altocumulus stratiformis layer with a base height of 2300ma.g.l. Velocity fields at cloud base are computed by tracking image rectangular patterns through successive shots. The height uncertainty is estimated by comparison with a Vaisala CL31 ceilometer located on the site. The uncertainty on the horizontal coordinates and on the velocity field are theoretically 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.

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In this study, we use an all-sky stereo camera network to perform geolocation of individual elements of a cloud field in order to track individual clouds and estimate some of their morphological characteristics and their evolution in time. Furthermore, this allows use of cloud geolocation for cloud airborne measurements. For example, in the case of instrumented UAVs, the GPS coordinates of the target cloud may be communicated in real time to the autopilot.
In this study, we use an all-sky stereo camera network to perform geolocation of individual...
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