3D SURFACE / CREATING BORDER / NAN VALUES

I have measured velocity data which I am analyzing using 3D surface (z-coordinate : velocity magnitude). And here the issue that I ran into:

I have missing values in measured data (marked as Nan). If I eliminate Nans, than values are interpolated throughout the domain (even at the places where data were not measured, I don’t like it)- figure A.

If I am keeping Nan values in data set, values are interpolated only where they were measured – and that’s what I want. However, Nan values show up as dots on the plot and I don’t know how to get rid of them –figure B.

My questions are:

-Is there any way I can input border for the surface, so that values will be interpolated only inside the given boundaries

-can I change settings somehow, so that Nan values just won’t show up in a plot at all (no dots)?

Thank you!