A Thought Experiment
Before we continue, I'd like you to forget everything you know about elevation for just a few minutes.
Imagine we build the world's most perfect GNSS receiver. It has perfect antennas, perfect clocks, perfect satellite orbits, perfect atmospheric models, and perfect RTK corrections. Every source of measurement error has somehow been eliminated.
Now we place this miraculous receiver on a survey monument. The receiver computes its position with absolute perfection. Not centimeters. Not millimeters. Perfectly.
Now here's the question.
What is its elevation?
Most people instinctively believe there must be only one correct answer. After all, we just imagined a perfect receiver making a perfect measurement. Surely perfection produces a single number.
It doesn't.
In fact, there are several completely correct answers. One application might report ellipsoid height. Another might report orthometric elevation. A GIS database may store the same point using a different vertical datum, while an engineering drawing may express the result in feet instead of meters. The physical location never changed. Only the language used to describe it changed.
Coordinates have no meaning by themselves. Every coordinate must be interpreted in the context of its reference frame, reference surface, vertical datum, and units. Two different numbers can legitimately describe the same physical point.
The Detective Mindset
When experienced GNSS users encounter an unexpected elevation, they rarely begin changing settings. They begin asking questions. Which reference surface is being used? Who performed the geoid conversion? Could the conversion have been applied twice? Is the benchmark referenced to the same vertical datum? The goal is not to guess which setting is wrong, but to reconstruct the complete journey of the coordinate from the satellites to the value displayed on the screen.
Stop asking, 'Why is my receiver wrong?' Start asking, 'Who produced the number I'm looking at?' That single change in perspective will solve more elevation problems than memorizing receiver menus.
The next chapter puts this instinct to the test directly: a real field scenario, and a chance to check your own first guess against what experienced GNSS professionals learn to ask first.