GNSS with ground station assist is capable of measuring changes in elevation within millimeters. As an example, the US maintains the National Spacial Reference

GNSS with ground station assist is capable of measuring changes in elevation within millimeters. As an example, the US maintains the National Spacial Reference System (NSRS), which is a "consistent coordinate system that defines latitude, longitude, height, scale, gravity, and orientation throughout the United States" with very high accuracy. https://oceanservice.noaa.gov/facts/nsrs.html https://oceanservice.noaa.gov/facts/nsrs.html https://earthobservatory.nasa.gov/images/147436/taking-a-measure-of-sea-level-rise-land-motion https://earthobservatory.nasa.gov/images/147436/taking-a-mea... https://pubs.usgs.gov/tm/11d1/tm11-D1.pdf https://pubs.usgs.gov/tm/11d1/tm11-D1.pdf