Matlab interp2() gives 'monotonically increasing' error - matlab

Question

I have many lines that represent a "z" value and want to interpolate at a specific value between them, at a specific x-axis location, to get the y-value. I'm trying interp2() but it throws a 'monotonically increasing' error.
The dataset below is a subset. I broke it out into xyz-1 and xyz-2 just for easy plotting in this question (i.e., making a repeatable example). How can I fix my interp2() or the inputs?
x1 = [0.02, 0.048, 0.108, 0.196, 0.279, 0.401];
y1 = [0.583, 0.43, 0.32, 0.279, 0.262, 0.259];
z1 = [50, 50, 50, 50, 50, 50];
x2 = [0.02, 0.048, 0.108, 0.196, 0.279, 0.401];
y2 = [0.747, 0.591, 0.435, 0.357, 0.326, 0.305];
z2 = [35, 35, 35, 35, 35, 35];
x_all = [x1, x2];
y_all = [y1, y2];
z_all = [z1, z2];
plot(x1, y1, 'blue', 'DisplayName', 'z1')
hold on
plot(x2, y2, 'magenta', 'DisplayName', 'z2')
xlabel('x')
ylabel('y')
legend
want_x = 0.2;
want_z = 40;
need_y = interp2(x_all, y_all, z_all, want_x, want_z, 'linear')
Error:
Error using griddedInterpolant
The grid vectors must be strictly monotonically increasing.
Error in interp2>makegriddedinterp (line 228)
F = griddedInterpolant(varargin{:});
Error in interp2 (line 128)
F = makegriddedinterp({X, Y}, V, method,extrap);

Answer 1

The function griddata is your friend for these tasks, It uses scatteredInterpolant under the hood, but in my opinion is more user friendly.
Keeping the same example code you gave, replacing the last line with:
>> need_y = griddata(x_all,z_all,y_all,want_x, want_z)
need_y =
0.329506024096386
The function can take vector inputs for want_x and want_z and return a vector output of need_y if you need to query more than one point.
You can also specify the interpolation method (linear, cubic, etc ...).
And just to make sure it worked as desired:
>> F = scatteredInterpolant(x_all.', z_all.', y_all.', 'linear'); %NOT y_all, z_all
need_y = F(want_x, want_z)
need_y =
0.329506024096386 % same result, yay!
For more details about using griddata, you can have a look at my answer to this question extremely similar to yours (just worded a bit differently): Interpolation between two curves (matlab)

Answer 2

You can interpolate the value with:
x1 = [0.02, 0.04, 0.09, 0.184, 0.309, 0.667];
y1 = [0.586, 0.447, 0.34, 0.279, 0.256, 0.256];
z1 = [50, 50, 50, 50, 50, 50];
x2 = [0.022, 0.044, 0.076, 0.125, 0.184, 0.293, 0.509, 0.667];
y2 = [0.747, 0.6, 0.49, 0.41, 0.363, 0.326, 0.303, 0.3];
z2 = [35, 35, 35, 35, 35, 35, 35, 35];
want_x = 0.2;
want_z = 40;
y1_ = interp1(x1, y1, want_x);
y2_ = interp1(x2, y2, want_x);
want_y = interp1([50 35], [y1_ y2_], want_z);

Answer 3

This question was answered here:
https://www.mathworks.com/matlabcentral/answers/637955-interp2-monotonically-increasing-error
quoting from that answer, in case the link breaks in the future:
interp2() is only for two-dimensional interpretation over a grid, not for interpolation of vectors. You need something like
F = scatteredInterpolatn(x_all, z_all, y_all, 'linear'); %NOT y_all, z_all
need_y = F(want_x, want_z);