# Unstructured grid to structured grid

21 views (last 30 days)
Lorenzo on 20 Dec 2014
Commented: Emanuel on 31 Jul 2019
Dear all, what would be the best way, in your opinion, to get a structured grid out of an unstructured grid?
What I have is something like this:
x y f
1 12 7
3 10 4
1 11 2
2.4 15 0
So basically x and y are totally random and in no specific order. What I need is to resample the values of f on a structured grid with a constant step.
I had a quick look at interp2 and it looks like it will only work if my vectors are strictly monotonic which is not the case as I have plenty of repeated values for x and y.
Thanks a lot

David Young on 20 Dec 2014
Edited: David Young on 20 Dec 2014
scatteredInterpolant may do what you need. Like this:
x = [1 3 1 2.4].';
y = [12 10 11 15].';
f = [7 4 2 0].';
si = scatteredInterpolant(x, y, f); % using default linear interpolation
xint = (0.5:0.5:3.5).'; % regular grid
yint = (9.5:0.5:15.5).';
fint = si({xint yint})
Emanuel on 31 Jul 2019
How does this work if you have a lookup table with x and y values only. So, transfer non-momotonic x values into a fixed (strictly momotonic) x step size with the corresponding (interpolated) y values ?

### More Answers (1)

Lorenzo on 20 Dec 2014
Things are getting even more difficult… I actually don't have a grid but a set of cylindrical coordinates in a plane, namely a set of values for a radius and a set of values for an angle…
Is there any way in matlab to perform the interpolation in cylindrical coordinates? The issue with treating them as cartesian is that I end up with errors at the boundaries, in my case I lose the continuity information of my surface at theta=0° or theta=360°
Any idea?
Thanks again!
##### 1 CommentShowHide None
David Young on 21 Dec 2014
I'd have to think that through - but scatteredInterpolant doesn't depend on having a grid at any stage - you can use the returned function, called si in my answer, to sample at any required points. To avoid the discontinuity in theta I would expect that converting everything to Cartesian coordinates might be the right thing to do.