difference between using interp1( X,Y, xi, 'linear', 'extrap') and doing the extrapollation after curve fitting

Hi

I know extrapollation are not ideal but I used xi= interp1( X,Y, xi, 'linear', 'extrap') and the results are totally onligical so I used fitobject = fit(X,Y, poly1' ); Use I had the equation of the line I can perform my extrapollation and found different values ? X and Y are 300 scattered measurement points, length(X)=length(Y)but of it happens that the same X give slightly different results for Y are there any special conditions to use interp1

I would expect both methods to give the same results?

thanks for any suggestions, regards,J

Answer 1

[José-Luis]

The methods are different and will not give the same results. The curve fit will fit a line (one and only one) to all your points (linear least squares if you don't specify anything else) and the extrapolation will be points on that line.

Interp1 will draw a line (provided you use the default option) between succesive points and use that for extrapolation. Therefore you end up with many segments and not a single line.

Cheers!

[Jürgen]

thanks for the clear explanation super

so when you do an extrapolation it is only based on the line trought the last two points?

[José-Luis]

Yes, for a linear fit.