how do i find where 2 best fit lines cross?

[katy']

Hi

I was wondering if anyone knows a way to find where 2 best fit lines cross, I don't have the equation for the lines and i'm not sure if I can get it.

I currently have the program tracing the lines and comparing y values however it will only increment in steps of 0.1 on the x-axis and i need an accuraccy of 0.0001

[chrisger]

hi there

i suppose you have three 1D arrayswith the same number of elements: the x-values, y-values curve 1 and y-values curve 2. you can now use the Interpolate 1D Array - function to interpolate the y-values of the curves. see attachment (not tested, but it should work).

[DFGray]

If you have or can get a series of XY value pairs for both of the lines, getting the equation of the line is as simple as calling Linear Fit.vi.  Once you have the equations of both lines, the intersection is simple algebra, subject to the accuracy of your fit.  If the XY data pairs describe a curve, not a line, and you want more accuracy, you can try a polynomial fit using General Polynomial Fit.vi.  To find the intersection, set one polynomial equation equal to the other, compute the coefficients of the resulting polynomial equation, and find the root using Polynomial Roots.vi.  All this assumes you have a Pro or better version of LabVIEW.  If you don't, you can still do it, but you will need to write some code for the fits and roots.  Check out a copy of Numerical Recipes in C, by Press et. al. from your local library and it will give you the equations for all the above.  With LabVIEW's native matrix handling capabilities, the rest is straighforward.

Let us know if you need more help.