The node analysis works with the potential of the nodes, and the potential is always relative to some point where we take it zero. Zero, like the potential of the ground. The "ground" means only U=0 for the chosen point. It need not be a physically real grounding.
Well, you can assign the notations U1, U2, U3, U4... to the nodes and write up all the equations, but you always get one less independent equations than the number of nodes. As an example, you have 4 nodes, and at the end you arrive to the solution U2=U1+6, U3=U1-2, U4=U1+10. You can choose any value for U1, but U1=0 is the simplest. If you fancy, it can be U1=1.4141 :)