  In decomposing the HI rotation curves of disc galaxies, it is necessary to
break a degeneracy between the gravitational fields of the disc and the dark
halo by estimating the disc surface density. This is done by combining
measurements of the vertical velocity dispersion of the disc with the disc
scale height. The vertical velocity dispersion of the discs is measured from
absorption lines (near the V-band) of near-face-on spiral galaxies, with the
light coming from a mixed population of giants of all ages. However, the scale
heights for these galaxies are estimated statistically from near-IR surface
photometry of edge-on galaxies. The scale height estimate is therefore
dominated by a population of older (> 2 Gyr) red giants. In this paper, we
demonstrate the importance of measuring the velocity dispersion for the same
older population of stars that is used to estimate the vertical scale height.
We present an analysis of the vertical kinematics of K-giants in the solar
vicinity. We find the vertical velocity distribution best fit by two components
with dispersions of 9.6 +/- 0.5 km/s and 18.6 +/- 1.0 km/s, which we interpret
as the dispersions of the young and old disc populations respectively.
Combining the (single) measured velocity dispersion of the total young + old
disc population (13.0 +/- 0.1 km/s) with the scale height estimated for the
older population would underestimate the disc surface density by a factor of ~
2. Such a disc would have a peak rotational velocity that is only 70% of that
for the maximal disc, thus making it appear submaximal.
