  We examine the diagnostic power of rest-frame ultraviolet (UV) nebular
emission lines, and compare them to more commonly used rest-frame optical
emission lines, using the test case of a single star-forming knot of the bright
lensed galaxy RCSGA 032727-132609 at redshift z~1.7. This galaxy has complete
coverage of all the major rest-frame UV and optical emission lines from
Magellan/MagE and Keck/NIRSPEC. Using the full suite of diagnostic lines, we
infer the physical properties: nebular electron temperature (T_e), electron
density (n_e), oxygen abundance (log(O/H)), ionisation parameter (log(q)) and
interstellar medium (ISM) pressure (log(P/k)). We examine the effectiveness of
the different UV, optical and joint UV-optical spectra in constraining the
physical conditions. Using UV lines alone we can reliably estimate log(q), but
the same is difficult for log(O/H). UV lines yield a higher (~1.5 dex) log(P/k)
than the optical lines, as the former probes a further inner nebular region
than the latter. For this comparison, we extend the existing Bayesian inference
code IZI, adding to it the capability to infer ISM pressure simultaneously with
metallicity and ionisation parameter. This work anticipates future rest-frame
UV spectral datasets from the James Webb Space Telescope (JWST) at high
redshift and from the Extremely Large Telescope (ELT) at moderate redshift.
