The aim of this investigation is to study how to use a gravimetric (quasi) geoid for levelling by GPS data in an optimal way. The advent of preci~ gex-xJetic GPS has made the use of a technique possible, which might be called GPS- gravimetric geoid detemfination. In this approach, GPS heights above the reference ellipsoid are determined for points whose levelled ( orthometric ) height H is above ~ level people have already surveyed; for these points, we thus have the values of the geoid undulation N. These values are then used to constrain the geoid undulations N".