Zero Level Surface

The zero level surface is a defined equipotential surface of the Earth’s gravity field chosen as the reference for physical height systems. It represents the surface to which geopotential numbers are referred and approximates mean sea level on a global scale. In practice, it is associated with a conventional reference potential value (e.g., W₀) adopted for international height systems.

In geodesy, the zero level surface provides the physical reference for orthometric or normal heights and underpins the realization of global and regional vertical reference systems such as the International Height Reference System (IHRS). Its consistent definition is essential for unifying national height systems and ensuring comparability of height information worldwide.