  (Abridged) We constrain simulated all-sky maps in total intensity, linear
polarization, and rotation measure (RM) by observations. We test a number of
large-scale magnetic field configurations and take the properties of the warm
interstellar medium into account. From a comparison of simulated and observed
maps we are able to constrain the regular large-scale Galactic magnetic field
in the disk and the halo of the Galaxy. The local regular field is 2 microG and
the average random field is about 3 microG. The known local excess of
synchrotron emission originating either from enhanced CR electrons or random
magnetic fields is able to explain the observed high-latitude synchrotron
emission. The thermal electron model (NE2001) in conjunction with a proper
filling factor accounts for the observed optically thin thermal emission and
low frequency absorption by optically thick emission. A coupling factor between
thermal electrons and the random magnetic field component is proposed, which in
addition to the small filling factor of thermal electrons increases small-scale
RM fluctuations and thus accounts for the observed depolarization at 1.4 GHz.
We conclude that an axisymmetric magnetic disk field configuration with
reversals inside the solar circle fits available observations best. Out of the
plane a strong toroidal magnetic field with different signs above and below the
plane is needed to account for the observed high-latitude RMs. Our preferred
3D-model fits the observations better than other models over a wide frequency
range.
