What is Orthometric Height (H) measured from?

Orthometric height (H) is specifically defined as the vertical distance from the geoid surface to a point on the Earth's surface measured along a plumb line. The geoid is an equipotential surface that corresponds to mean sea level and represents the physical shape of the Earth's gravitational field. Thus, it serves as a reference surface for measuring height.

This measurement process is crucial because it reflects the true gravitational potential of the location being surveyed, taking into account variations in the Earth's gravitational field that may occur due to differing densities in the Earth's crust. Therefore, orthometric height provides a more accurate representation of elevation that is relevant for engineering, construction, and geological applications.

The other options do not correctly describe the concept of orthometric height. For instance, measuring from the highest elevation point in a region or an average landscape elevation does not adhere to the precise definition needed for accurate height determination relevant in surveying. Additionally, a fixed mark established for high points would not account for the gravitational variations that orthometric height considers.