How can I interpolate for " not strictly monotonic increasing vectors" ?

Hi All! I have tried to do some interpolation lately but unfortunately I have some issues.

I have a speed array called

Speed = [22 22 22.5 22.5] 
and I have a Angle array called

Angle = [130 180 130 180].

So I have run my script for those arrays and I have a output called RPM.

RPM = [91.1490 91.2120 93.2234 93.2860].

The way I have run my script is by "Crossing the inputs(Speed and Angle), so the first RPM (RPM(1)) is simply for Speed = 22 and Angle = 130, RPM(2) = speed = 22 Angle = 180 etc. I want to find the RPM for a given Speed and Angle in the range, lets say Speed = 22.3 and Angle = 150. When I use interp2 the error massage is "The grid vectors are not strictly monotonic increasing". How do I solve this problem? I hope the problem in understandable, otherwise feel free to ask!

Best regards Isa

Answer 1

[Guillaume]

Use griddedinterpolant, if your known points are properly gridded, or scatteredinterpolant if not.

If your known points are properly gridded, you first need to convert them into a grid:

Speed = [22 22 22.5 22.5]; 
Angle = [130 180 130 180]; 
RPM = [91.1490 91.2120 93.2234 93.2860];

Speedgrid = reshape(Speed, [2 2]);
Anglegrid = reshape(Angle, [2 2]);
RPMgrid = reshape(RPM, [2 2]);
g = griddedInterpolant(Anglegrid, Speedgrid, RPMgrid); %Angle and speed need to be in this order, or the whole lot transposed
%query for speed = 25, angle = 150:
rpm25_150 = g([150, 25])

Using scattered interpolant:

s = scatteredInterpolant(Speed', Angle', RPM')
rpm25_150 = s([25, 150])