MATLAB: Difference between using interp1( X,Y, xi, ‘linear’, ‘extrap’) and doing the extrapollation after curve fitting

curve fittinterpollation

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

Best Answer

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!
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