  Diffuse gamma-ray emission from interstellar clouds results largely from
cosmic ray (CR) proton collisions with ambient gas, regardless of the gas
state, temperature, or dust properties of the cloud. The interstellar medium is
predominantly transparent to both CRs and gamma-rays, so GeV emission is a
unique probe of the total gas column density. The gamma-ray emissivity of a
cloud of known column density is then a measure of the impinging CR population
and may be used to map the kpc-scale CR distribution in the Galaxy. To this
end, we test a number of commonly used column density tracers to evaluate their
effectiveness in modeling the GeV emission from the relatively quiescent,
nearby $\rho$ Ophiuchi molecular cloud. We confirm that both \hi\ and an
appropriate H$_2$ tracer are required to reproduce the total gas column
densities probed by diffuse gamma-ray emisison. We find that the optical depth
at 353 GHz $\tau_{353}$ from Planck reproduces the gamma-ray data best overall
based on the test statistic across the entire region of interest, but near
infrared stellar extinction also performs very well, with smaller spatial
residuals in the densest parts of the cloud.
