How to find curvature using points obtained by numerical integration?

Hi, I used ode45 to plot a curve and got the X,Z and phi value.

Where X and Z are points on the curve which change depending on phi. (Polar co-ordinates?Though I am not sure)

Equation governing the curve: 1/R1 + 1/R2 = 2 + constant*z

where R1 and R2 are two radii of curvature.

The equation governing the curve shows that if I find the curvature and plot it against 'z', I must get a straight line of intercept 2.

Kindly help me find the curvature.

Regards