This report presents azimuthal dependent and polarization dependent x-ray resonant magnetic scattering at the Ir L-3 edge for the bilayered iridate compound Sr3Ir2O7. The two magnetic wave vectors, k(1) =(1/2, 1/2, 0) and k(2) = (1/2, -1/2, 0), result in domains of two symmetry-related G-type antiferromagnetic structures, denoted A and B, respectively. These domains are approximately 0.02 mm(2) and are independent of the thermal history. An understanding of this key aspect of the magnetism is necessary for an overall picture of the magnetic behaviour in this compound. The azimuthal and polarization dependence of the magnetic reflections, relating to both magnetic wavevectors, show that the Ir magnetic moments in the bilayer compound are oriented along the c axis. This contrasts with single layer Sr2IrO4 where the moments are confined to the ab plane.