  The Lyman-alpha emission line is the strongest feature in the spectrum of
most high redshift galaxies, and is typically observed as being highly
asymmetric due to galactic outflows. Quantifying this asymmetry is challenging.
Here, we explore how measurements of one parameterisation, Lyman-alpha
skewness, are affected by instrumental resolution and detection signal-to-noise
and thus whether this can be extended throughout the archive. We model these
effects through simulated lines and apply our derived corrections to existing
archival data sets (including sources observed with FORS2 and DEIMOS) to
reconstruct the intrinsic line emission parameters. We find a large uncertainty
in parameter reconstruction at low resolutions (R<3000) and high skew values,
as well as substantial random errors resulting from the masking of sky lines.
We suggest that interpretations of spectral line asymmetry should be made with
caution, while a simpler parametrization, like B/R (blue-red flux asymmetry),
is likely to yield more robust results. We see a possible weak trend in
velocity width with mass, although there is no evidence in the data for a
reliable correlation of skew with galaxy mass, star formation rate or age at
z=4-5. Using our results, we investigate the possibilities of recovering
emission line asymmetry with current and future instruments.
