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 \pm 0.5 km s ^ { -1 } and 18.6 \pm 1.0 km s ^ { -1 } , 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 \pm 0.1 km s ^ { -1 } ) with the scale height estimated for the older population would underestimate the disc surface density by a factor of \sim 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 .