Illustration of how the direction of anisotropy is detected using the rotation-detection algorithm. Left: We first locate the region having elliptical contours and put the rotation center on the origin of the ellipses. Middle: Then we slowly rotate the contours so that we identify the major and minor axes, and the lengths of both axes are recorded for different contours. For example, the big dashed arrows show the major and minor axes of the dark blue elliptical structure. Right: The axes then provide necessary information (direction, anisotropic length) for magnetic field studies.