  We propose an adaptation of the semilinear algorithm for the prediction of
the impact corridor on ground of an Earth-impacting asteroid. The proposed
algorithm provides an efficient tool, able to reliably predict the impact
regions at fixed altitudes above ground with 5 orders of magnitudes less
computations than Monte Carlo approaches. Efficiency is crucial when dealing
with imminent impactors, which are characterized by high impact probabilities
and impact times very close to the times of discovery. The case of 2008 TC3 is
a remarkable example, but there are also recent cases of imminent impactors,
2018 LA and 2019 MO, for which the method has been successfully used. Moreover,
its good performances make the tool suitable also for the analysis of the
impact regions on ground of objects with a more distant impact time, even of
the order of many years, as confirmed by the test performed with the first
batch of observations of Apophis, giving the possibility of an impact 25 years
after its discovery.
