Interpolate y-values from x-values, outside the limit of the x-values

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Espen Mikkelsen
Espen Mikkelsen 2018-3-27
评论： Torsten 2018-3-27
I have one array with height values and another with density.
Height = [0 386.5 446.1 526.6 621.5 660.6 711.4 734.9 792.8 810.2 817.9 893.7 1136.8 1317 1420.2 1426.2]
Density = [1.225 1.216 1.203 1.194 1.182 1.178 1.171 1.169 1.161 1.160 1.160 1.151 1.122 1.101 1.091 1.091]
I want to interpolate, so I can find the density of any heights. When I try to interpolate for density values higher than the largest height value which is 1426.2, I only get a NaN value.
F.eks when I try to interpolate at height 2000m like this:
Out = interp1(Height,Density,2000);
What can I do?

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回答（1 个）

Torsten
Torsten 2018-3-27
https://de.mathworks.com/help/matlab/ref/interp1.html#btw2a2x
Best wishes
Torsten.
  2 个评论
Espen Mikkelsen
Espen Mikkelsen 2018-3-27
编辑：Espen Mikkelsen 2018-3-27

Thank you so much. I hope you could help me again. I found out that this code should work:

interp1(Height,Density,2000,'linear','extrap');

y-value(density) should decrease when x-value(height) is increasing. Instead y-value stays nearly flat and even slightly increasing when x-value is increasing. That doesnt make sense. The original curve is clearly decreasing linearly. I have no idea how I can solve this.

Torsten
Torsten 2018-3-27
I don't know MATLAB's extrapolation formula, but don't trust in extrapolation anyway.
If you know better, you should include your own correlation.
Best wishes
Torsten.

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