What essential feature distinguishes the geoid model from geometric models?

The essential feature that distinguishes the geoid model from geometric models lies in its consideration of gravitational variations across the Earth's surface. The geoid represents the mean sea level of the Earth, influenced by the planet's gravity field, which varies due to changes in density and temperature of the Earth's materials, as well as topographical features. This means that the geoid accurately reflects the undulations of the Earth's surface based on how gravity varies from one location to another, allowing it to serve as a true reference surface for measuring elevations and depths.

In contrast, geometric models, such as the ellipsoid, are typically simplified representations of the Earth that do not incorporate the complexities of gravitational variations. These models are mathematically defined to serve specific applications, such as mapping or navigation, but they lack the ability to accurately represent the Earth's surface variations due to gravitational forces. By using the geoid model, geographers and GIS professionals can achieve a more precise understanding of phenomena that are influenced by gravity, such as sea-level rise, which is vital for coastal planning and management.