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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.