Approximate dependence of the true impact parameter, y/b, in function of the estimated impact parameter, p, for bent cross sections derived from Démoulin & Dasso’s (2009) results (derived from r_Bx, see text in Sect. 4.5). The bending increases with the dimensionless parameter a. Linear force-free models (n = 1, Eq. (4)) are shown for two aspect ratio b. The black dashed line is the relation found by fitting the Lundquist field to the elliptic (a = 0) model with n = 1.