Geoid

The geoid is a precise, imaginary surface that represents Earth's mean sea level across the entire planet, accounting for variations in gravity caused by differences in Earth's density and shape. It's essentially the shape the ocean surface would take under gravity and rotation, if undisturbed by tides, currents, or winds. Geodesists use the geoid as a reference point to measure elevations accurately, as it reflects true "sea level" globally. Understanding the geoid helps improve maps, GPS accuracy, and our knowledge of Earth's internal structure.