  The polarimeter vector field for multibody decays of a spin-half baryon is
introduced as a generalisation of the baryon asymmetry parameters. Using a
recent amplitude analysis of the $\Lambda^+_\mathrm{c} \to p K^- \pi^+$ decay
performed at the LHCb experiment, we compute the distribution of the
kinematic-dependent polarimeter vector for this process in the space of
Mandelstam variables to express the polarised decay rate in a model-agnostic
form. The obtained representation can facilitate polarisation measurements of
the $\Lambda^+_\mathrm{c}$ baryon and eases inclusion of the
$\Lambda^+_\mathrm{c} \to p K^- \pi^+$ decay mode in hadronic amplitude
analyses.
