  Absolute gravimeters (AG) are used in geodesy, geophysics, and physics for a
wide spectrum of applications. Stable gravimetric measurements over timescales
from several days to decades are required to provide relevant insight into
geophysical processes. Users of AGs participate in comparisons with a
metrological reference in order to monitor the temporal stability of the
instruments and determine the bias to that reference. However, since no
measurement standard of higher-order accuracy currently exists, users of AGs
participate in key comparisons led by the CIPM. These comparisons provide the
reference values of highest accuracy compared to the calibration against a
single AG. The construction of stationary, large scale atom interferometers
paves the way towards a new measurement standard in absolute gravimetry used as
a reference with a potential stability up to 1 nm/s$^2$ at 1 s integration
time. At the Leibniz University Hannover, we are currently building such a very
long baseline atom interferometer with a 10 m long interaction zone. The
knowledge of local gravity and its gradient along and around the baseline is
required to establish the instrument's uncertainty budget and enable transfers
of gravimetric measurements to nearby devices for comparison and calibration
purposes. We therefore established a control network for relative gravimeters
and repeatedly measured its connections during the construction of the atom
interferometer. We additionally developed a 3D model of the host building to
investigate the self-attraction effect and studied the impact of mass changes
due to groundwater hydrology on the gravity field around the reference
instrument. The gravitational effect from the building 3D model is in excellent
agreement with the latest gravimetric measurement campaign which opens the
possibility to transfer gravity values with an uncertainty below the 10
nm/s$^2$ level.
