Error in interp1 (line 186) F = griddedInt​erpolant(X​,V,method)​;

Hello everyone,

I am using interp1 function, in MATLAB R2015a, to interpolate lift and drag coefficients. These coefficients are given in function of the angle of attack.

The angle of attack in the original excel file are sorted in ascending order. However, i am getting the following error:

Error using griddedInterpolant

The grid vectors are not strictly monotonic increasing.

Error in interp1 (line 186)

F = griddedInterpolant(X,V,method);

Please could anyone explain to me where the problem although the angles of attack are sorted and how to fix it?

Best Regards

Answer 1

[Steven Lord]

although the angles of attack are sorted

Have you confirmed that this is true or are you just assuming that this is true? Call issorted on the first input to interp1 and confirm that this is true.

My guess is that at some point the angle data crosses from say 359 degrees to 1 degree (or the equivalent in radians.) In that case the unwrap function may be of use to you.

[nado]

Thank you for your reply !!!

you were right !!! In fact, the original data file was containing some errors. In general, the lift and drag coefficients are given for angles of attack varying between -180 and 180 degree.

However, in the Excel file, they forgot the minus sign. That's why MATLAB was lost and gave errors.

Best Regards