function e = radial( a, W, C, R, ox, oy, Tpy )
  f = a(1); Tpz = a(2); k = a(3);

  r2 = (C(1) - ox).^2 + (C(2) - oy).^2;
  e = (C(2)-oy).*(1+ k*r2 )+oy;
  e = e - ( W*(f*R(2,:) .+ oy*R(3,:))' + f*Tpy + oy*Tpz ) ./ ( W*R(3,:)' + Tpz );
  e = sum(e.^2);