  Footprint analysis, also known as the study of Influence areas, is a first
order method for solving inverse atmospheric dispersion problems. We revisit
the concept of footprints giving a rigorous definition of the concept (denoted
posterior footprints and posterior zero footprints) in terms of spatio-temporal
domains. The notion of footprints is then augmented the to the forward
dispersion problem by defining prior footprints and prior zero footprints. We
then study how posterior footprints and posterior zero footprints can be
combined to reveal more information about the source, and how prior footprints
and prior footprints can be combined to yield more information about the
measurements.
