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I am meshing a 3D surface with a surface structured mesh with 44x16 divisions. The algorithm creates an unrealistic jump, or better a step, in the bottom left corner of the surface (see figure). The surface was simply created as 4-sides surface, given the four lines of the boundary. The lines are not straight, especially that to the left.
Thanks for help.

attach a zip file with the .gid folder of the model, to see how are your curves and the created surface

The 'unexpected' (but valid) shape of the elements depends on the parametrization of the surface
The surface could be defined in different ways,
1) as a plane trimmed with the curves, and then could be of two subtypes a PlSurface or a NURBSurface
2) as a untrimmed NURBSurface, with exactly 4 boundary curves.

Creating a NURBS surface, in the contextual menu is possible to force some of them,
e.g. NoTryPlanar to force a untrimmed 4-sided shape, also if the lines are coplanar.

Thank you for the answer. The lines do not lay on a single plane. Furthermore, the shorter lines are not straight but come from real topography. The surfaces are built by their contour.

I have attached the model containing 2 adjacent surfaces, in order to show that for the surface to the south, surface & mesh are accurately built, while the one to the east show that unexpected feature.

Thank you for your help.
Elisa

This surface has been created as a 'planar rectangular shape' trimmed with the curves. (it as been detected as possible planar within a tolerance)

Do you have exactly 4 lines, it is possible to force to create a non-planar and untrimmed surface.
You can convert the current inner shape of your surface (without lost other information like meshing data, groups, layer, conditions,...) with

Geometry->Edit->Rebuild surface from boundary
then click the mouse <right>-button to show the contextual menu, and select "Don't try planar"
then select the surface 2 and press Escape to finish.

Then the same and the mesh will be as you expected

Note: to see the difference in the definition of the NURBS surface can use
Geometry->Edit->NURBS surface...
This will draw the control polygon
the initial surface (a planar shape trimmed) vs the final surface (force to be untrimmed, with 4 sides, topologically a rectangle)