Need help with homework (simple backwards interpolation)

I want to calculate temperature of steam based on given enthalpy and pressure.

Example: https://imgur.com/a/u3rVOcD

The code should find temperature value (y axis) based on given enthalpy value (values in table) and pressure value (x axis). It is basically an backward interpolation, but with 2 variables.

Previously i have been using this code:

hnv_sek = 463.4
pk= 32
X_ZV = data_p_h(1,2:end)
Y_ZV = data_p_h(2:end,1)
Z_ZV = data_p_h(2:end,2:end)
x_ZV = pk
z_ZV = hnv_sek
fun_ZV = @(y_ZV)interp2(X_ZV,Y_ZV,Z_ZV,x_ZV,y_ZV)-z_ZV;
y_ZV = fzero(fun_ZV,mean(Y_ZV))
tnv_sek = y_ZV
interp2(X_ZV, Y_ZV, Z_ZV ,x_ZV , y_ZV)
end

And for values of data about and below 560, i have been getting this error (It was working fine with values that are high in table, until i get to low values. Any help would be greatly thanked for.):

Exiting fzero: aborting search for an interval containing a sign change
    because NaN or Inf function value encountered during search.
(Function value at 37.0123 is NaN.)
Check function or try again with a different starting value.

Answer 1

[Torsten]

Specify the extrapolation value in interp2.

[Stefan Bogdanovic]

Can you please define it more precisely? What do you mean by extrapolation value?

[Torsten]

If the requested data y_zv is not between min(Y_ZV) and max(Y_ZV), interp2 together with your interpolation method 'linear' returns NaN.

Therefore, you have to specify a value to be returned for the cases y_zv < min(Y_ZV) or y_zv > max(Y_ZV).

Look into the documentation for interp2 for more details.

https://de.mathworks.com/help/matlab/ref/interp2.html