MATLAB: Interpolate data: not monotonic

Question

Hey,

I am given a set of data with X and Y coordinates. The data is picturized below, with the circles corresponding to each data point. When I try to interpolate the data, to find new X coordinates for a set of Y coordinates (steps of 0.002 between minimum and maximum Y value), I get the error 'Error using griddedInterpolant. The grid vectors are not strictly monotonic increasing'. Is it because the X values constantly decrease and increase (and vice versa)?

What would be the best way to account for this so that I can still interpolate the data?

Answer 1

You’re correct. If you want to interpolate to get more values in your situation, a bit of deception is sometimes necessary. In similar situations, I simply flip the x and y arguments to the interp1 call, and make the appropriate changes to your interpolation vector to match your y data. I would use the 'linear' method with your data.