Setting the GROUND LEVEL in STAAD Pro

Hi all,

1) Is there a way to set the ground level in STAAD Pro or is the y=0 level the permanent ground level? This is for the purpose of computing the seismic forces because, from what I know, distribution of the computed base shear per level of a structure is related to the distance of a level from the ground surface.

For example, I have to design a 4-storey steel structure. The pedestal protrudes above the ground slab by a 0.5m and in modelling the the steel structure in STAAD, the support, which is the base plate connection, is above the ground slab by 0.5m.
Let's say that the distance from the 2nd floor to the ground level is 3.3m so the distance that will be used in modelling the steel from the 2nd floor to the base plate is 2.8m.

2) Do you neglect the 0.5m distance and just proceed in the modelling and assume the base plate connection as located in the ground?
3) If yes, will this not have any effect in the seismic forces?
4) If no, what do you do for this case?
5) Do I miss something?

Hoping to have some clarity regarding the matter.
Thanks in advance everyone!