The local gravitational acceleration g at any arbitrary location inside the dedicated watt balance laboratory can be evaluated. A 3-dimensional (3D) spatial array of g values was measured by a Scintrex CG5 relative gravimeter inside the laboratory. Measurements were made at three different heights from the ground floor covering the volume of the entire room. From these measurements a 3D gravity map has been produced using a 3D fit. Predictions from this 3D gravity map were compared with off-grid experimental gravity measurements. They agreed to within 1 µGal. The absolute value of g was measured in the watt balance laboratory during the 2009 International Comparison of Absolute Gravimeters (ICAG-2009).

In conclusion, the present uncertainty on the determination of the gravitational acceleration g at the location of the test mass is not a limiting factor for a determination of the Planck constant at the 10^-8 level.