  The origin and life-cycle of molecular clouds are still poorly constrained,
despite their importance for understanding the evolution of the interstellar
medium. We have carried out a systematic, homogeneous, spectroscopic survey of
the inner Galactic plane, in order to complement the many continuum Galactic
surveys available with crucial distance and gas-kinematic information. Our aim
is to combine this data set with recent infrared to sub-millimetre surveys at
similar angular resolutions. The SEDIGISM survey covers 78 deg^2 of the inner
Galaxy (-60 deg < l < +18 deg, |b| < 0.5 deg) in the J=2-1 rotational
transition of 13CO. This isotopologue of CO is less abundant than 12CO by
factors up to 100. Therefore, its emission has low to moderate optical depths,
and higher critical density, making it an ideal tracer of the cold, dense
interstellar medium. The data have been observed with the SHFI single-pixel
instrument at APEX. The observational setup covers the 13CO(2-1) and C18O(2-1)
lines, plus several transitions from other molecules. The observations have
been completed. Data reduction is in progress, and the final data products will
be made available in the near future. Here we give a detailed description of
the survey and the dedicated data reduction pipeline. Preliminary results based
on a science demonstration field covering -20 deg < l < -18.5 deg are
presented. Analysis of the 13CO(2-1) data in this field reveals compact clumps,
diffuse clouds, and filamentary structures at a range of heliocentric
distances. By combining our data with data in the (1-0) transition of CO
isotopologues from the ThrUMMS survey, we are able to compute a 3D realization
of the excitation temperature and optical depth in the interstellar medium.
Ultimately, this survey will provide a detailed, global view of the inner
Galactic interstellar medium at an unprecedented angular resolution of ~30".
