  We have carried absolute frequency measurements of the $(6s^{2})\,^{1}S_{0}$
$-$ $(6s6p)\,^{3}P_{1}$ transition in $^{171}$Yb (the intercombination line),
where the spin-1/2 isotope yields two hyperfine lines. The measurements rely on
sub-Doppler spectroscopy to yield a discriminator to which a 556 nm laser is
locked. The frequency reference for the optical frequency measurements is a
high-quality quartz oscillator steered to the GNSS timescale that is bridged
with a frequency comb. The reference is validated to $\sim3\times10^{-12}$ by
spectroscopy on the $^{1}S_{0}-\,^{3}P_{0}$ (clock) line in laser cooled and
trapped $^{171}$Yb atoms. From the hyperfine separation between the $F=1/2$ and
$F=3/2$ levels of $^{3}P_{1}$ we determine the hyperfine constant to be
$A(^3P_1)= 3\,957\,833\,(28)$ kHz.
