Dear Andrija Zorić

I think that this problem is due to the linear interpolation option.

run the following lines instead of surf(X,Y,Z)

mesh(X,Y,Z)

hold on

plot3(geom(:,1),geom(:,2),z,'o')

I think it could help you understanding what happened running 'griddata'

Matteo

Dear Matteo Sangiorgio

Thank You for Your answer.

I tried Your suggestion and I got points of surface exactly as I expected (mesh.jpg). But I don't know why are points on different sides connected.

It seems to me that problem isn't with linear interpolation option. I have also done it with cubic interpolation option and results are pretty much same (mesh cubic.jpg).

I would be grateful if someone explained to me what the problem was.

It is always useful to check where your data points are located. It looks that the points already form an almost orthogonal grid (some points are missing). This is the grid from a rectangular second order mesh (basically no middle point). I checked that with:

plot(geom(:,1),geom(:,2),'.k')

For me it was easier to generate another grid which will include that middle point:

Xm = 0:25:500;

Ym = 0:25:500;

[X, Y] = meshgrid(Xm,Ym);

Now I can interpolate my z value on the new grid

Z=griddata(geom(:,1),geom(:,2),W_cr,X,Y,'linear');

surf(X,Y,Z)

No strange lines obtained

Dear Mitrofan Curti

Thank You for Your answer.

The missing of middle nodes caused problem. I solved this using your recommendations. I still don't know why missing of middle nodes cause this numerical and graphical problem, but now I get correct surface. Thanks a lot.

Also thanks to other on the answers.