  The spatial distribution of dust in galaxies affects the global attenuation,
and hence inferred properties, of galaxies. We trace the spatial distribution
of dust in five fields (at 0.6-0.9 kpc scale) of M31 by comparing optical
attenuation with the total dust mass distribution. We measure the attenuation
from the Balmer decrement using Integral Field Spectroscopy and the dust mass
from Herschel far-IR observations. Our results show that M31's dust attenuation
closely follows a foreground screen model, contrary to what was previously
found in other nearby galaxies. By smoothing the M31 data we find that spatial
resolution is not the cause for this difference. Based on the emission line
ratios and two simple models, we conclude that previous models of dust/gas
geometry need to include a weakly or non-attenuated diffuse ionized gas (DIG)
component. Due to the variation of dust and DIG scale heights with galactic
radius, we conclude that different locations in galaxies will have different
vertical distributions of gas and dust and therefore different measured
attenuation. The difference between our result in M31 with that found in other
nearby galaxies can be explained by our fields in M31 lying at larger galactic
radii than the previous studies that focused on the centres of galaxies.
