# Interpolation error for one column increasing and one column decreasing

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Brian N. on 12 Apr 2021 at 20:58
Edited: John D'Errico on 12 Apr 2021 at 21:21
I am attempting to find an interpolation point from a matrix with one column having data in descending order and one column having data in ascending order. When I run the following line
h = interp1(Atmos2(:,1),Atmos2(:,2),1.1);
I get the error: Sample points must be unique and sorted in ascending order.
how do I alleviate this problem? Here is a sample of the data

John D'Errico on 12 Apr 2021 at 21:15
Edited: John D'Errico on 12 Apr 2021 at 21:21
This is impossible to answer, since what you show is clearly not what you have.
The error that is given indicates the vector (x) is NOT a strictly increasing vector. But since you are interpolating density as a function of height, all I need do is look at the vector in column 2. It IS strictly increasoing. Therefore, you have not shown us a set of data that would cause that error.
Regardless, you need to insure that x (height) is an INCREASING vector.
Brian N. on 12 Apr 2021 at 21:21
How would I make x an increasing vector? Because density decreases with increasing altitude.