Pole Zero plot given a Transfer function
What you have are not the poles and zeros, but simply the filter coefficients, i.e., the coefficients of the numerator and denominator polynomials.

The poles are the roots of the denominator polynomial, and the zeros are the roots of the numerator polynomial. In Matlab they can be found by using the roots command:


p = roots(a);
z = roots(b);


Note that in general, poles and zeros are complex numbers, that's why they are plotted in the complex plane.

Just a remark: you used the zplane command with numerator and denominator interchanged, that's why the plot shows the zeros as crosses on the unit circle, and the poles as 'O's inside the circle. The correct call is zplane(b,a).