MATLAB: Extrapolating dataset and find intersection

datasetextrapolationhelpintersection

Hello there,

I'm quite new to Matlab and stuck at a problem for a while and thus this is my first question.

What I have: A (x/y) dataset which is depicted by the blue line in the figure below

What I want:

Two extrapolating lines (depicted by dotted lines) through the maximum slope of the peak and the "ground" line respectively
Intersection Point of these lines to retrieve its x-coordinate

What I have tried: I tried using the following line for a first try, just to see what happens, however I am unable to plot the extrapolated line (Error Msg. Vectors must be the same length)

y1=interp1(t,hf,[2500:2600],'linear','extrap')
plot(t,y1)

As said I have not that much experience yet so I would highly appreciate any helpful input for solving this particular problem.

Best regards

Best Answer

You should be having coordinates of the lines: use this:

https://in.mathworks.com/matlabcentral/fileexchange/35606-line-line-intersection-2d

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MATLAB: Line intersection without polyfitting

From the plot?! Why would polyfit be useful for finding the intersection of lines? Do you mean the polyxpoly command?

Did you ask an internet search engine for "Matlab intersection lines" already? You will find:

MATLAB: How to use pchip to interpolate between data points in cartesian coordinate format

The all-increasing or all-decreasing issue you're encountering isn't a restriction of pchip, but rather of the : operator. If you try to define a vector with a positive step size but decreasing values, you'll get an empty vector. For example:

 >> 5:1:2
 ans =
   Empty matrix: 1-by-0

I'm guessing that what you're really after in this case is a parametric interpolation, where you "fill in" points in both the x- and y-direction.

 % Some fake data
 theta = linspace(0, 2*pi, 10);
 x = cos(theta);
 y = sin(theta);
 % Interpolate
 told = 1:10;
 tnew = linspace(1,10,50);
 xnew = pchip(told, x, t);
 ynew = pchip(told, y, t);
 % Plot
 plot(x,y,'-b.', xnew, ynew, '-ro');

You can get more sophisticated to evenly space your final points, but this gives you there general idea.

Best Answer

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MATLAB: Line intersection without polyfitting

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