exactly. These changes are of relative nature between the first and the second image. But you don’t know your "level value"above which there is uplift and below which is subsidence. Therefore you need to define a point at an area where you don’t exoect changes. Take the value of the unwrapped phase at this location and substract this value from the whole image.
Where you select this point depends on your knowledge of the study area, landuse distribution, topography and the patterns in your unwrapped interferogram.
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