  We measured the components of the 31-m-long vector between the two
Very-Long-Baseline Interferometry (VLBI) antennas at the Kokee Park Geophysical
Observatory (KPGO), Hawaii, with approximately 1 mm precision using phase-delay
observables from dedicated VLBI observations in 2016 and 2018. The two KPGO
antennas are the 20 m legacy VLBI antenna and the 12 m VLBI Global Observing
System (VGOS) antenna. Independent estimates of the vector between the two
antennas were obtained by the National Geodetic Survey (NGS) using standard
optical surveys in 2015 and 2018. The uncertainties of the latter survey were
0.3 and 0.7 mm in the horizontal and vertical components of the baseline,
respectively. We applied corrections to the measured positions for the varying
thermal deformation of the antennas on the different days of the VLBI and
survey measurements, which can amount to 1 mm, bringing all results to a common
reference temperature. The difference between the VLBI and survey results are
0.2 +/- 0.4 mm, -1.3 +/- 0.4 mm, and 0.8 +/- 0.8 mm in the East, North, and Up
topocentric components, respectively. We also estimate that the Up component of
the baseline may suffer from systematic errors due to gravitational deformation
and uncalibrated instrumental delay variations at the 20 m antenna that may
reach +/-10 mm and -2 mm, respectively, resulting in an accuracy uncertainty on
the order of 10 mm for the relative heights of the antennas. Furthermore,
possible tilting of the 12 m antenna increases the uncertainties in the
differences in the horizontal components to 1.0 mm. These results bring into
focus the importance of (1) correcting to a common reference temperature the
measurements of the reference points of all geodetic instruments within a site,
(2) obtaining measurements of the gravitational deformation of all antennas,
and (3) monitoring local motions of the geodetic instruments.
