Masking low coherence areas in deformation displacement map

Hello everyone!!
Once more here to know if the community can solve my doubts :slight_smile:
I have seen in a tutorial that I have found, that, after computing the displacement map of a region, the points with low coherence can be masked (deleted from the map) and for creating this mask a reference point is selected. I don’t know why a reference point is chosen and its value added to the rest of values in the map. Someone could help me?

Thanks!!!

That’s because all InSAR phase-measurements are relative.