Compute Derivatives of a scalar in a direction based on a Contour line

Hi all,
Starting from the vtk files of a 2-d case containing point data, I’ve been trying to compute the derivatives of a scalar (temperature). The Compute Derivatives or the Gradient filters, compute these in the X and Y directions.

What is important for me, is to compute these derivatives but in a direction which is normal and tangential to an isoline, which I generate using the Contour filter. With the Contour filter, as expected I get a curve, which has a variable direction.

When creating the Contour line(s), I see that there is also a “Compute Normals” check box, which according to the description it computes an extra array containing the normal values of the isoline. But I can’t locate this array. Of course I’m not sure if this is the proper way to get the derivatives in the normal direction of the isoline.

Any help is highly appreciated.