Resist the Reflex
Let's leave the classroom for a moment and walk into a situation that happens every day. A GIS technician arrives at a project site carrying a survey-grade GNSS receiver. Before collecting the first feature, they decide to occupy a nearby benchmark to verify that everything is working properly.
The receiver acquires satellites, connects to the RTK network, and reports a FIX solution. Horizontal precision is excellent. Vertical precision looks equally impressive. Confidence begins to build.
Then the elevation appears on the screen.
It doesn't match the published benchmark.
Immediately the investigation begins. Was the RTK correction lost? Is the antenna height incorrect? Was the benchmark disturbed? Is the receiver defective? Did something happen during the last firmware update?
Notice what has not happened yet. Nobody has asked whether the receiver and the benchmark are describing elevation relative to the same reference surface. That question often comes last, even though it should be the first.
The First Suspect Is Usually the Receiver
This reaction is understandable. The receiver is the newest piece of equipment on the job site, and it is producing the unexpected number. Human nature encourages us to blame the instrument that is speaking to us. Yet the receiver may simply be reporting exactly what it was asked to report.
An experienced GNSS professional develops a different habit. Before touching a single setting, they ask a series of questions that progressively narrow the possibilities. Was RTK fixed? Yes. Was the antenna height entered correctly? Yes. Does the horizontal position agree? Yes. Only then do they ask the question that changes the entire investigation:
What reference surface is each elevation using?
The fastest way to solve an apparent elevation problem is often to stop looking for measurement errors and start looking for reference differences.
Test Your Own Instinct
Before reading further, try this yourself. Your own receiver reports an RTK FIX solution. The horizontal position agrees with known control within a centimeter or two. The benchmark elevation, however, differs by nearly 30 meters.
Write down the first three explanations that come to mind.
Most readers list antenna height, poor RTK corrections, or a damaged benchmark. Very few list 'the receiver and the benchmark use different vertical references.'
The purpose of this exercise is not to see whether you guessed correctly. It is to expose the assumptions you naturally make under pressure. When we see an unexpected elevation, our minds immediately search for equipment failures because those are tangible. Reference systems are invisible. We cannot see an ellipsoid or a geoid standing in the field, so we rarely suspect them first.
A Better First Question
Imagine how differently the investigation begins if the first question changes from 'Why is my receiver wrong?' to 'Why do these two correct elevations disagree?' The first question assumes failure. The second assumes there is an explanation waiting to be discovered.
That single change in mindset is the real objective of this companion. Once you begin asking better questions, the correct technical answers become much easier to find.
Experienced GNSS users don't trust or distrust a number simply because it is large or small. They first determine whether the comparison itself is valid.
The next chapter starts at the actual beginning of the processing chain, before latitude and longitude ever enter the picture at all.