How to find intersection of data sets

Hi I'm very new to Matlab and programming in general. I have two curves each represented by data sets of x,y coordinates. I want to find the point at which the two lines intersect. Obviously I could plot both sets and then find the intersection just by sight, but I want to do it mathematically.

The curves need to be evaluated by spline to be a piecewise polynomial, but after that I don't know how to solve them to find the intersection.

Answer 1

[Image Analyst]

Do you want to find the index where they are closest? What is no index matches and the crossing point is in between indexes? Do you want to find out where the cross in between indexes? So you'd have to set one cubic equal to the other in between the two indexes where it crosses.

[Andrew]

The crossing point is in between the indexes. The problem I am having is setting the results from the spline to be equal to each other in order to solve. I am having trouble manipulating the results from spline in order to do this.

[Andrew]

I used a combination of your suggestion of equating the cubic functions at the relevant indices and Azzi's method to find the relevant indices. It was much easier once I figured out how to dig into the spline structure. Thanks very much!

[Image Analyst]

Glad you got it working.

Answer 2

[Azzi Abdelmalek]

Look at this example

x=0:0.01:10;
y1=sin(x);
y2=cos(x);
plot(x,y1,x,y2)
idx=sign(y2-y1)
i1=strfind(idx,[1 -1]);
i2=strfind(idx,[-1 1]);
i3=find(idx==0);
idx=unique([i1 i2 i3])

[Andrew]

Thanks very much! I used a combination of your method to find the relevant indices and Image Analyst's suggestion of equation the cubic functions at those indices.