  Aims. We study the impact of the parallax on the search for very long
timescale microlensing events towards the Magellanic Clouds due to dark massive
compact objects within the past MACHO and EROS, the ongoing MOA and OGLE, and
the future LSST surveys. We quantify the impact of neglecting this effect on
the classical event selection process and also quantify the parallax
detectability without the help of follow-up observations.Methods. We define a
distance between true events affected by parallax and the closest events
without parallax. This distance is used to estimate the probability of missing
the preselection of events because of parallax, for any survey characterised by
its time sampling and photometric performance. We also define another distance
to quantify the detectability of the parallax effect, in order to trigger
complementary observations.Results. We find that the preselection of years long
time scale events is marginally affected by parallax for all surveys, provided
the criteria are reasonably tight. We also show that the parallax should be
detectable in the majority of the events found by the LSST survey without
follow-up observations.
