  Feedback driven massive outflows play a crucial role in galaxy evolution by
regulating star formation and influencing the dynamics of surrounding media.
Extracting outflow properties from spectral lines is a notoriously difficult
process for a number of reasons, including the possibility that a substantial
fraction of the outflow is carried by dense gas in a very narrow range in
velocity. This gas can hide in spectra with insufficient resolution.
Empirically motivated analysis based on the Apparent Optical Depth method,
commonly used in the literature, neglects the contribution of this gas, and may
therefore underestimate the true gas column density. More complex
semi-analytical line transfer (e.g., SALT) models, on the other hand, allow for
the presence of this gas by modeling the radial density and velocity of the
outflows as power laws. Here we compare the two approaches to quantify the
uncertainties in the inferences of outflow properties based on 1-D
"down-the-barrel" using the UV spectra of the CLASSY galaxy sample. We find
that empirical modeling may significantly underestimate the column densities
relative to SALT analysis, particularly in the optically thick regime. We use
simulations to show that the main reason for this discrepancy is the presence
of large amount of dense material at low velocities, which can be hidden by the
finite spectral resolution of the data. The SALT models in turn could
over-estimate the column densities if the assumed power laws of the density
profiles strong are not a property of actual outflows.
