Hi @mauleon.fb you would want to look for areas of no change at all, like a mountain, for example. It’s best to choose an area that historically holds a trend of no change. You would know this by having a good historical understanding of your project area. Take a look at your interferogram and identify a few places that show no change. Then take a look at an aerial to see what’s there. Make the best decision based on your findings. Once you have picked a spot, you may want to have a historical look at the area to make sure it in fact has looked the same for a while to indicate that its a solid known zero. You can use Google Earth to view historic images, and also use any other data in the area that you may have, like survey data, ect. Once you are sure about your area, take an averaged sample of the area to get a good number to use for the subtraction to get absolute displacement.