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Fitting Straight Lines

Suppose we have a set of N points and we wish to find the `best' straight line through them. To define `best' we need to give a quality of fit measure. Let us first choose an appropriate parameterization of the line;

where is the closest approach of the line to the origin and thus is normal to the vector p. We choose this form as it gives both parameters the same units. It is simple to show that the normal distance of such a line to a point is given by

If we assume a density distribution for this distance, , and that the points are independent samples from a distribution about the true line, we can construct a log likelihood function (see above)

which must be maximized. If is gaussian the problem can be solved by taking the line through the centroid of the N points parallel to the principle eigenvector of the covariance matrix of the data about the centroid. If, however, a non-gaussian distribution is used, we revert to using the optimization methods described above to choose the parameters which maximize . Typically we would use a local optimizer, obtaining a starting point by first solving with an assumed gaussian form for . If the function is twice differentiable then is twice differentiable, and the Levenberg-Marquardt method can be used.

For instance, robust line fitting techniques accommodate outliers by assuming a distribution for with longer tails than the gaussian [6]. For instance

or a gaussian with extended tails

Note that this latter is not strictly a probability density distribution since it does not have unit area, but this does not affect the parameter estimation. Note also that the latter is not differentiable at , so can only be optimized using a method which does not require derivatives, such as Simplex or Powell's.

In any parameter estimation problem it is important to estimate the confidence region about the parameter estimates obtained. If is gaussian then is a function. The found parameters can be considered to be drawn from a multivariate normal distribution about (which achieve the optimum) with a covariance matrix given by the inverse of the Hessian of f evaluated at . Confidence ellipsoids about can be drawn up at suitable levels using this covariance matrix [6]. If is not gaussian then more detailed analytic calculations or a Monte Carlo simulation are required to obtain the confidence limits.

The approach given above makes the assumption that measurement errors in the data points are equal in all directions. If this is not the case the merit function must be reformulated accordingly.

next up previous contents
Next: Fitting General Curves Up: Curve Fitting Previous: Curve Fitting

Bob Fisher
Fri Mar 28 14:12:50 GMT 1997