Find X intercept with error estimates

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Arun
Arun 2025-4-8
编辑： Matt J 2025-4-8
Hi,
So i have a set of energy values x and corresponding spectral information y.
I fit a straight line to my data with the polyfit function (which uses least square fitting)
I end up with coefficients p and the struct S containing the error estimates.
I want to find the x intercept with the associated uncertainity. How do I do this?
My attempt :
If I can find the errors on the coefficients p, then I can easily find the uncertainity in the x intercept through propagation of errors. But somehow I cant find the bounds of p in S?
PS : I know finding the intercept is just -p(2)/p(1), its the uncertainity I need help with.

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Matt J
Matt J 2025-4-8
编辑：Matt J 2025-4-8
If I can find the errors on the coefficients p, then I can easily find the uncertainity in the x intercept through propagation of errors. But somehow I cant find the bounds of p in S?
The approximate covariance matrix is,
Rinv=inv(S.R);
cov=(Rinv*Rinv')*S.normr^2/S.df

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