  Since its discovery in the aurorae of Jupiter ~30 years ago, the H$_{3}^{+}$
ion has served as an invaluable probe of giant planet upper atmospheres.
However, the vast majority of monitoring of planetary H$_{3}^{+}$ radiation has
followed from observations that rely on deriving parameters from
column-integrated paths through the emitting layer. Here, we investigate the
effects of density and temperature gradients along such paths on the measured
H$_{3}^{+}$ spectrum and its resulting interpretation. In a non-isothermal
atmosphere, H$_{3}^{+}$ column densities retrieved from such observations are
found to represent a lower limit, reduced by 20% or more from the true
atmospheric value. Global simulations of Uranus' ionosphere reveal that
measured H$_{3}^{+}$ temperature variations are often attributable to
well-understood solar zenith angle effects rather than indications of real
atmospheric variability. Finally, based on these insights, a preliminary method
of deriving vertical temperature structure is demonstrated at Jupiter using
model reproductions of electron density and H$_{3}^{+}$ measurements. The sheer
diversity and uncertainty of conditions in planetary atmospheres prohibits this
work from providing blanket quantitative correction factors; nonetheless, we
illustrate a few simple ways in which the already formidable utility of
H$_{3}^{+}$ observations in understanding planetary atmospheres can be
enhanced.
