  Nitrogen hydrides such as NH$_3$ and N$_2$H$^+$ are widely used by Galactic
observers to trace the cold dense regions of the interstellar medium. In
external galaxies, because of limited sensitivity, HCN has become the most
common tracer of dense gas over large parts of galaxies. We provide the first
systematic measurements of N$_2$H$^+$(1-0) across different environments of an
external spiral galaxy, NGC6946. We find a strong correlation ($r>0.98,p<0.01$)
between the HCN(1-0) and N$_2$H$^+$(1-0) intensities across the inner
$\sim8\mathrm{kpc}$ of the galaxy, at kiloparsec scales. This correlation is
equally strong between the ratios N$_2$H$^+$(1-0)/CO(1-0) and HCN(1-0)/CO(1-0),
tracers of dense gas fractions ($f_\mathrm{dense}$). We measure an average
intensity ratio of N$_2$H$^+$(1-0)/HCN(1-0)$=0.15\pm0.02$ over our set of five
IRAM-30m pointings. These trends are further supported by existing measurements
for Galactic and extragalactic sources. This narrow distribution in the average
ratio suggests that the observed systematic trends found in kiloparsec-scale
extragalactic studies of $f_\mathrm{dense}$ and the efficiency of dense gas
(SFE$_\mathrm{dense}$) would not change if we employed N$_2$H$^+$(1-0) as a
more direct tracer of dense gas. At kiloparsec scales our results indicate that
the HCN(1-0) emission can be used to predict the expected N$_2$H$^+$(1-0) over
those regions. Our results suggest that, even if HCN(1-0) and N$_2$H$^+$(1-0)
trace different density regimes within molecular clouds, subcloud differences
average out at kiloparsec scales, yielding the two tracers proportional to each
other.
