Compare results of same matrix for different elements of matrix

Please firstly, see and plot the code,

xm=6; ym =3:0.01:10;
[x, y] = meshgrid(xm,ym);
angle = asind((-x.^2.*y.^2-y.^4+1296)./(2.*x.*y.^3));
angle = real(angle);
angle(angle < -50 | angle > 30) = NaN;
z = 6./(x.*sind(angle)+y);
figure
plot(z,angle);

I want to obtain matrix of absolute angle difference between z(i)=a and z(i)=a+0.2. Please help me in this regard.

[KSSV]

What is a?

[Cem Tuncer]

a is any point within the range of z values. Let say if z=1, I want to compare corresponding angles for z=1 and z'=1.2. And doing it for all range of z.

For example z=[0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2]

z'=[1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4] and we find the difference of corresponding angles. Please note that z is not basic as the example, it's elements are not ordered and have more fractions.

But it is difficult because the data is nonlinear and re-calculation of angles corresponding to z' is needed. Z is the result of a calculation of function including angle and I couldn't arrange reverse-calculation because of x-y meshgrid gives also huge mess for me.