@Preben Alsholm  Thnaks a lot

Just a small question: why the ~ ? 

@tomleslie Wow, thanks a lot!

Stef

@acer WOW! This looks so great, many, many thanks! I'm going to have a closer look at this, and adapt it a little bit, but... this is exactly what I'm looking for!

Thanks again (and so quickly!)

Stef

@acer I added the file (REStotaal.txt), and I load it via:

I get the following 3d-results, that I would like to see in 2d with contours (specifically the last one is the most interesting, of course):

@Carl Love : No, sorry, this is not the problem I'm trying to solve, "a" depends on other data and is somewhat "fixed".