  ALMA's high-resolution images allow to resolve the filamentary structure of
the ISM down to few thousand au at kpc distances. We aim to systematically
quantify the impact of the interferometric response and the effects of the
short-spacing information during the characterization of the ISM structure
using ALMA observations. We create a series of continuum ALMA synthetic
observations to test the recovery of the observational properties of dense
cores and filaments (i.e. intensity peak, radial profile, and width) at
different scales. We compare the results obtained with and without different
data combination techniques using different ALMA arrays and SD telescopes in
simulated data and real observations. Our analysis illustrates the severity of
interferometric filtering effects. ALMA-12m alone observations show significant
scale-dependent flux losses systematically corrupting (>30%error) all the
physical properties inferred in cores and filaments (i.e. column density, mass,
and size) before the maximum recoverable scale of the interferometer. These
effects are only partially mitigated by the addition of the ALMA ACA-7m array
although degrading the telescope PSF. Our results demonstrate only the addition
of the ALMA Total Power information allows to recover the true sky emission
down to few times the ALMA beamsize with satisfactory accuracy (<10% error).
Additional tests demonstrate the emission recovery at all scales is further
improved if the 7mTP data are replaced by maps obtained by a larger SD
telescope (e.g., IRAM-30m), even if the latter are noisier than expected. These
observational biases particularly affect partially resolved targets, becoming
critical especially for studies in nearby regions such as Taurus or Orion. Our
results demonstrate the need for the use of data combination techniques to
accurately characterize the complex physical structure of the ISM in the ALMA
era.
