How is land subsidence measured?
Land that has been heavily disturbed by humans in most developed countries, such as urban and rural regions will have been surveyed for topographical &amp; sub-division purposes.

Such data allows for a digital terrain model (DTM) to be created for such regions. When subsidence occurs, the subsided area can be resurveyed &amp; compared to the original DTM to ascertain the amount of subsidence that has occurred.

Undisturbed or poorly surveyed regions become problematic. The subsidence area &amp; the surrounding area can be surveyed, but then someone has to make a judgement call &amp; decide what the original surface may have looked like to infer an estimate for the degree of subsidence.

The other way is to have satellites periodically measure surface elevations &amp; then to do comparisons between each satellite measurement.

Radar could be used as the measuring system, but LIDAR (&amp; this reference also) can produce more accurate results.