| Summary: | The effect of the construction of the 4th subway line of Budapest (Metro4) on the potential surfaces of the gravity field has been simulated, using the prism modelling technique. In the study mass loss due to the excavation of the two tunnels and of the stations has been considered. Mass variations deform the level surfaces; as so, the vertical reference surface of the levelling measurements is changing by time. In this study, the effect of the mass loss on levelling measurements was determined at a level 1 m above the ground, roughly simulating common instrument heights. Subsequently, the effect of the actual deformations of the surface on the levelling measurements has also been determined. According to the results, under certain arrangements of the levelling line with respect to the position of the excavations, the error due to the change of the vertical reference is in the 1μm order of magnitude, thus negligible.
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