  Selection effects can bedevil the inference of the properties of a population
of astronomical catalogues, unavoidably biasing the observed catalogue. This is
particularly true when mapping interstellar extinction in three dimensions:
more extinguished stars are fainter and so generally less likely to appear in
any magnitude limited catalogue of observations. This paper demonstrates how to
account for this selection effect when mapping extinction, so that accurate and
unbiased estimates of the true extinction are obtained. We advocate couching
the description of the problem explicitly as a Poisson point process, which
allows the likelihoods employed to be easily and correctly normalised in such a
way that accounts for the selection functions applied to construct the
catalogue of observations.
