code: def _sd_of_eigenvector(data, vec, measurement='poles', bidirectional=True):
      
      lon, lat = _convert_measurements(data, measurement)
      vals, vecs = cov_eig(lon, lat, bidirectional)
      x, y, z = vecs[:, vec]
      s, d = stereonet_math.geographic2pole(*stereonet_math.cart2sph(x, y, z))
      return s[0], d[0]
docs: Unifies ``fit_pole`` and ``fit_girdle``.