  An accurate estimate of the interstellar gas density distribution is crucial
to understanding the interstellar medium (ISM) and Galactic cosmic rays (CRs).
To comprehend the ISM and CRs in a local environment, a study of the diffuse
$\gamma$-ray emission in a mid-latitude region of the third quadrant was
performed. The $\gamma$-ray data in the 0.1--25.6~GeV energy range of the Fermi
Large Area Telescope (LAT) and other interstellar gas tracers such as the HI4PI
survey data and the Planck dust thermal emission model were used, and the
northern and southern regions were analyzed separately. The variation of the
dust emission Dem with the total neutral gas column density NH was studied in
high dust-temperature areas, and the NH/Dem ratio was calibrated using
$\gamma$-ray data under the assumption of a uniform CR intensity in the studied
regions. The measured integrated $\gamma$-ray emissivities above 100~MeV are
$(1.58\pm0.04)\times10^{-26}~\mathrm{photons~s^{-1}~sr^{-1}~H\mbox{-}atom^{-1}}$
and
$(1.59\pm0.02)\times10^{-26}~\mathrm{photons~s^{-1}~sr^{-1}~H\mbox{-}atom^{-1}}$
in the northern and southern regions, respectively, supporting the existence of
a uniform CR intensity in the vicinity of the solar system. While most of the
gas can be interpreted to be HI with a spin temperature of $T_\mathrm{S} =
125~\mathrm{K}$ or higher, an area dominated by optically thick HI with
$T_\mathrm{S} \sim 40~\mathrm{K}$ was identified.
