  The H2O Southern Galactic Plane Survey (HOPS) has mapped a 100 degree strip
of the Galactic plane (-70deg > l > 30deg, |b| < 0.5deg) using the 22-m Mopra
antenna at 12-mm wavelengths. Observations were conducted in on-the-fly mode
using the Mopra spectrometer (MOPS), targeting water masers, thermal molecular
emission and radio-recombination lines. Foremost among the thermal lines are
the 23 GHz transitions of NH3 J,K = (1,1) and (2,2), which trace the densest
parts of molecular clouds (n > 10^4 cm^{-3}). In this paper we present the NH3
(1,1) and (2,2) data, which have a resolution of 2 arcmin and cover a velocity
range of +/-200 km/s. The median sensitivity of the NH3 data-cubes is sigma_Tmb
= 0.20 +/1 0.06 K. For the (1,1) transition this sensitivity equates to a 3.2
kpc distance limit for detecting a 20 K, 400 Msun cloud at the 5-sigma level.
Similar clouds of mass 5,000 Msun would be detected as far as the Galactic
centre, while 30,000 Msun clouds would be seen across the Galaxy. We have
developed an automatic emission finding procedure based on the ATNF DUCHAMP
software and have used it to create a new catalogue of 669 dense molecular
clouds. The catalogue is 100 percent complete at the 5-sigma detection limit
(Tmb = 1.0 K). A preliminary analysis of the ensemble cloud properties suggest
that the near kinematic distances are favoured. The cloud positions are
consistent with current models of the Galaxy containing a long bar. Combined
with other Galactic plane surveys this new molecular-line dataset constitutes a
key tool for examining Galactic structure and evolution. Data-cubes, spectra
and catalogues are available to the community via the HOPS website.
