Strange curve fitting with large polynomial degrees when using polyfit

I'm trying to fit a curve to my points by using polyfit, I'm getting normal results for small polynomial degrees, but when i want to use polyfit for bigger polynomial degrees, I get strange results.

This is what I get when for degree of 10:

It's okay and its normal, but when I use polyfit for the degree of 100, I get strange results for the starting points and the ending points:

What's the reason for this strange behavior? How can i fix this?

Answer 1

[Star Strider]

As a general rule, a polynomial fit with a degree grater than 7 is likely not appropriate, so a 100 degree polynomial is giving the expected result, with ‘ringing’ at the extremes.

It would appear that fitting a sine curve to the data would be appropriate. One approach to that is described in Curve fitting to a sinusoidal function. The one change I would make in that code is to replace the ‘zci’ fundtion with this one:

zci = @(x) find(diff(sign(x)));

It is more robust, and does not have the ‘end effect’ my original function does.