  Theoretical description of off-specular grazing-incidence nuclear resonant
scattering of synchrotron radiation (Synchrotron M\"{o}ssbauer Reflectometry,
SMR) is presented. The recently developed SMR, similarly to polarized neutron
reflectometry (PNR), is an analytical tool for the determination of isotopic
and magnetic structure of thin films and multilayers. It combines the
sensitivity of M\"{o}ssbauer spectroscopy to hyperfine interactions and the
depth selectivity of x-ray reflectometry. Specular reflection provides
information on the depth profile, while off-specular scattering on the lateral
structure of scattering layers. Off-specular SMR and PNR intensity formulae of
a rather general multilayer with different domains, based on a Distorted
Incident-Wave Approximation (DIWA) are presented. The Distorted-Wave Born
Approximation (DWBA) results are given in an Appendix. Physical and numerical
implications, why using DIWA, are explained. The temporal character of SMR
imposes specific differences between SMR and PNR. In order to reveal the limits
of DIWA and to compare the two analytical methods, two-dimensional diffuse SMR
and PNR maps of an antiferromagnetic multilayer are calculated and critically
compared. Experimental '$\omega-2\theta$' SMR map of a periodic $[
\mathrm{Fe}/\mathrm{Cr}]_{20}$ multilayer is presented and compared with
simulations by the present theory.
