  Systematic surveys of massive clumps have been carried out to study the
conditions leading to the formation of massive stars. These clumps are
typically at large distances and unresolved, so their physical properties
cannot be reliably derived from the observations alone. Numerical simulations
are needed to interpret the observations. To this end, we generate synthetic
Herschel observations using our large-scale star-formation simulation, where
massive stars explode as supernovae driving the interstellar-medium turbulence.
From the synthetic observations, we compile a catalog of compact sources
following the exact same procedure as for the Hi-GAL compact source catalog. We
show that the sources from the simulation have observational properties with
statistical distributions consistent with the observations. By relating the
compact sources from the synthetic observations to their three-dimensional
counterparts in the simulation, we find that the synthetic observations
overestimate the clump masses by about an order of magnitude on average due to
line-of-sight projection, and projection effects are likely to be even worse
for Hi-GAL Inner Galaxy sources. We also find that a large fraction of sources
classified as protostellar are likely to be starless, and propose a new method
to partially discriminate between true and false protostellar sources.
