Whether caused by thermal instability of permafrost, pumping of subterranean acquifers, or some other geomorphic process, subsidence is a big deal. Even a few centimeters of change can be detected with fodar.
Here are some thermokarst slumps in the Noatak River valley. Mouse-over to see the change over 1 year. Ice rich ground beneath the tundra melt, causing the ground above to subside and flow downhill towards the stream.
Here is a shaded relief of the same slumps. Mouse-over to see the terrain over 1 year. The headwalls here are about 2-3 meters high and they migrated about 10 meters. The difference in acquisition resolutions causes differences in feature appearance, but does not adversely affect the precision of the results.