One Coordinate, Three Heights
A single physical point can be assigned several correct height values because each height is measured from a different reference surface. Ellipsoid height, orthometric height, and geoid height are not competing answers. Together, they describe the relationship between the antenna, the reference ellipsoid, and the gravity-based geoid.
Return to the Same Monument
Return to the survey monument that began this investigation. Picture both reference surfaces beneath the antenna at the same time: the smooth mathematical ellipsoid used by the GNSS positioning engine, and the irregular, gravity-based geoid. The antenna sits above both, and the measured height depends on which surface is chosen as the starting point.
The Three Measurements
The distance from the reference ellipsoid to the antenna is called ellipsoid height and is conventionally represented by a lowercase h. This is the geometric height that comes directly from the GNSS solution. It belongs to the same mathematical description as latitude and longitude and can be computed without introducing a geoid model or a conventional vertical datum.
The distance from the geoid to the antenna is called orthometric height and is represented by an uppercase H. This is the kind of height most people mean when they use the word elevation. It is intended to describe height relative to a gravity-based level surface, which is why it is useful for drainage, construction grades, flood studies, topographic mapping, and comparison with many published benchmarks.
The remaining measurement is the separation between the ellipsoid and the geoid. It is called geoid height, or geoid undulation, and is represented by N. Geoid height does not describe the antenna directly. Instead, it tells us how the two reference surfaces relate to one another at that location. Once that separation is known, the geometric height can be translated into the gravity-based height used by the project.
The three heights describe one physical point and two reference surfaces. Ellipsoid height measures from the ellipsoid to the antenna. Orthometric height measures from the geoid to the antenna. Geoid height measures between the ellipsoid and the geoid.
The Receiver Already Knows One of Them
The receiver does not begin with an orthometric elevation and work backward. Its native solution is geometric. After the satellite observations have been processed and the ECEF coordinates converted, the receiver already knows the ellipsoid height. At that stage, the position solution is complete even though the displayed number may not yet match the elevation expected by the user.
To report orthometric height, either the receiver or the software must add information that was not contained in the satellite geometry alone. It must determine the local separation between the ellipsoid and the geoid, usually by evaluating a geoid model at the horizontal position. The software then uses that modeled separation to express the same vertical coordinate relative to the gravity-based surface.
A receiver does not directly measure orthometric height from the satellites. It first solves ellipsoid height. Orthometric height is produced only after geoid information is introduced by the receiver, data collector, post-processing software, or another part of the workflow.
Three Numbers Do Not Mean Three Positions
This is where field comparisons often become confusing. One crew may record the ellipsoid height from a receiver status screen, while another crew records an orthometric elevation produced by a data collector. A third value may appear in the project files as the geoid separation used during processing. When those numbers are placed beside one another without labels, they can look like three conflicting answers for the same point.
They are not three positions. They are three parts of the same vertical relationship. The antenna remains at one location. The larger or smaller number does not reveal which observation is more accurate until the reference surface and processing path are identified. Before comparing any two elevations, the first question should therefore be the same question that has guided this investigation from the beginning: relative to what?
If the ellipsoid height and orthometric height differ by many feet, does that mean the GNSS solution contains a large vertical error? Not necessarily. The difference may simply be the local separation between the ellipsoid and the geoid.
Why the Labels Matter
The letters h, H, and N may seem like minor notation, but they prevent an important category of mistakes. A value labeled only as height or elevation leaves the reference surface unstated. Once the label is missing, it becomes easy to compare an ellipsoid height with an orthometric height, interpret the difference as receiver error, and begin changing settings that were correct from the start.
Clear labeling also reveals where a problem may have entered the processing chain. If ellipsoid heights agree but orthometric heights do not, the satellite solution may be fine while the geoid model, vertical datum, software configuration, or unit conversion differs. If the ellipsoid heights themselves disagree, the investigation belongs earlier in the chain. The three-height model therefore does more than explain terminology; it gives us a practical way to isolate the source of an elevation discrepancy.
One point can have three related height values: ellipsoid height h, orthometric height H, and geoid height N. They do not represent three different antenna positions. They describe the antenna and the separation between two different reference surfaces.
The three heights are connected by one compact relationship. In the next section, we will build that equation from the physical picture rather than memorize it, then use it to follow a real elevation through the final conversion from ellipsoid height to orthometric height.