Re: Extrapolation

[Przemek Klosowski]

On 08/18/2013 02:37 PM, Sarah wrote:
> I want to do interpolation and extrapolation. I can interpolate by using
> interp2 command. But, this command didn't perform extrapolation. Is there
> any built-in function for extrapolation in octave? I have given an example
> in detail.
>
> a= [ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 1 2 3 4 5 6 7 8 9 10 11 12 13
> 14 15 16 17];

...
> cim= interp2(a, b, cc, ai, bi) % Interpolation matrix
>
>
> ce= interp2(a, b, cc, 18, 0.2)  % Extrapolation at some point
...
> How can I find 'ce' and 'cem' in this case?  Here, it gives "NA". How should
> I extrapolate to find 'ce' and 'cem' ?

'interp2' is indeed doing interpolation, and additionally it requires 
monotonic coordinates (your  a() array is not monotonic). I don't know 
of a standard extrapolation package--depending on what's appropriate in 
your case, you might try to add a guard-band around your data by 
duplicating the values for x=17.

I am sure you did read 'help interp2' already but take another look: it 
specifies the monotonicity requirement, and also talks about different 
algorithms that can be  used for interpolation.