  The optical lattice clock NICT-Sr1 regularly reports calibration measurements
of the international timescale TAI. By comparing measurement results to the
reports of eight Primary Frequency Standards, we find the absolute frequency of
the $^{87}\mathrm{Sr}$ clock transition to be
f(Sr)=$429\,228\,004\,229\,873.082(76)$, with a fractional uncertainty of less
than 1.8x10$^{-16}$ approaching the systematic limits of the best realization
of SI second. Our result is consistent with other recent measurements and
further supported by the loop closure over the absolute frequencies of
$^{87}\mathrm{Sr}$, $^{171}\mathrm{Yb}$ and direct optical measurements of
their ratio.
