IHRS – International Height Reference System

The IHRS is the system definition for a global reference of heights, providing a unified framework for measuring elevations consistently worldwide. It establishes the conventions, models, and procedures needed to define height surfaces based on gravity and sea level. In geodesy, the IHRS serves as the standard system that is realized by height reference frames, which provide concrete coordinates and connections for global and regional applications, supporting precise leveling, GNSS-derived heights, and studies of sea level and crustal motion.

More Info: https://geodesy.science/item/height-reference-frame/