important errors in estimating $\omega$ arises from imperfectly determining the PA of the major axis, resulting in an error $\theta$ in the PA. Since $\theta \neq 0$, we introduce the error $$\Delta \omega =\frac{1}{L^3} \int_{-\infty}^{\infty} x (x \tan \theta)
{\rm d}x \approx \frac{1}{3L^3} {L^3} \theta \approx \frac{\theta}{3}.$$ As $\theta$ is, in general, approximately 0.05$^\circ$ ($9\times10^{-4}$ radians), we estimate $\Delta \omega$ to be of the order of $3\times10^{-4}$ rad. A large warp can have a value of $10\times10^{-4}$ and a barely perceptible warp $5\times10^{-4}$ rad. In the remainder of this paper, we consider a galaxy as warped if its calculated $\omega$ is clearly greater than the associated error. (See Table 2 for our results.)

For those galaxies showing an appreciably warped disc, we fitted a number of additional parameters to the warp curves. We tried to use the function proposed by Jiménez-Vicente et al. (1997), but it had too many fitting parameters so the fits were resticted to the simple description:

$$y=\left\{
\begin{array}{ll}
0       & |x|<|A| \\
C (x-A) & |x|\geq|A|
\end{array}   .
\right.$$

This function reproduces the warp’s shape as a first approximation; i.e., it is flat up to a point and then deviates from the symmetry plane until it 