The kilogram is a measure of mass, not weight which is a measure of force. In order to really figure out mass, I'd think the localized gravity has to be accounted for. e.g. GRACE maps gravity variation around the earth by measuring the acceleration difference between a pair of satellites.
The measurements were taken at the same place at around the same time, so fluctuations in local gravity should have been negligible compared to actual mass fluctuations.
