  Our position inside the Galaxy requires all-sky surveys to reveal its
large-scale properties. The zero-level calibration of all-sky surveys differs
from standard 'relative' measurements, where a source is measured in respect to
its surroundings. All-sky surveys aim to include emission structures of all
angular scales exceeding their angular resolution including isotropic emission
components. Synchrotron radiation is the dominating emission process in the
Galaxy up to frequencies of a few GHz, where numerous ground based surveys of
the total intensity up to 1.4 GHz exist. Its polarization properties were just
recently mapped for the entire sky at 1.4 GHz. All-sky total intensity and
linear polarization maps from WMAP for frequencies of 23 GHz and higher became
available and complement existing sky maps. Galactic plane surveys have higher
angular resolution using large single-dish or synthesis telescopes. Polarized
diffuse emission shows structures with no relation to total intensity emission
resulting from Faraday rotation effects in the interstellar medium. The
interpretation of these polarization structures critically depends on a correct
setting of the absolute zero-level in Stokes U and Q.
