Calculating radius of circle

Looking for a simple function. I would like calculate radius of a circle using three points. anyone done this before or know a function that will do this?

[Jacob Ward]

What do you mean by three points? Will these points be along the circumference of the circle? Will one be the center of the circle?

Answer 1

[Jacob Ward]

Using the following equation:

You have three unknowns: h, k, and r.

So you'll need to solve a system of three equations using your three different x-y pairs:

(x1 - h)^2 + (y1 - k)^2 = r^2

(x2 - h)^2 + (y2 - k)^2 = r^2

(x3 - h)^2 + (y3 - k)^2 = r^2

Solving this system of equations will give you r and MATLAB's solve(eqns,vars) command should do the trick.

Something like this:

 syms h k r
 eqns = [(x1-h)^2+(y1-k)^2==r^2,(x2-h)^2+(y2-k)^2==r^2,(x3-h)^2+(y3-k)^2==r^2];
 S = solve(eqns, [h k r]);

 radius = S.r(2);

The S.r(2) takes the second r solution because the first one is the negative of what you want, the second is the positive and desired solution.

Answer 2

[Image Analyst]

See the FAQ: http://matlab.wikia.com/wiki/FAQ#How_can_I_fit_a_circle_to_a_set_of_XY_data.3F