Geo-referencing and stereo calibration of ground-based Whole Sky Imagers using the sun trajectory

Abstract

Ground-based Whole Sky Imagers (WSIs) are now commonly used for cloud observations. Upon deployment, they may not be exactly level or precisely face north. This significantly affects subsequent processing of the images, especially for applications where two or more imagers are required, e.g. 3D volumetric cloud reconstruction. We present a method to remove this mis-alignment using the sun position in images captured over a whole day. Coupled with precise coordinates of the device locations, this method also improves the geo-referencing accuracy of the captured images. We detect the sun in the images and compute the corresponding 3D vectors using the lens calibration function. These vectors are compared to the actual sun direction. The mismatch between the two sets of vectors is then corrected using a 3D rotation matrix. The method can also be applied to other celestial bodies, such as stars or the moon.