The Question We Should Have Asked
A surveyor, GIS technician, engineer, or utility worker walks onto a site, occupies a known benchmark, and watches the displayed elevation. It doesn't match the published value.
Why is my elevation wrong?
That's almost always the first question that comes to mind, and it feels like a reasonable one. After all, there is a known benchmark and there is a number on the screen. If the two numbers differ, something must be wrong.
Or does it?
Throughout this companion we are going to challenge that assumption. Not because receivers never make mistakes, they certainly can, but because the first conclusion is often the wrong conclusion. Before questioning the receiver, we should first question whether we are comparing two values that describe the same reference surface.
This companion is not about proving your receiver is wrong. It is about understanding why a perfectly correct GNSS elevation can appear to be wrong when compared against another perfectly correct elevation.
Changing the Question Changes the Investigation
Imagine calling a mechanic because your speedometer reads 62 mph while a roadside radar sign flashes 100 km/h. Without realizing it, you've compared two different units describing the same speed. Neither instrument is broken. The apparent disagreement exists entirely because the measurements are being expressed differently.
Elevation problems often follow the same pattern. One value may be referenced to the ellipsoid. Another may be referenced to a geoid model. A third may use a different vertical datum altogether. Until those references are identified, comparing the numbers tells us very little.
An elevation should never be evaluated by its numeric value alone. Every elevation must be accompanied by the question, 'Referenced to what?'
What This Companion Will Teach You
By the end of this companion you should be able to look at two different elevations and determine why they differ. More importantly, you will know how to identify which one is appropriate for your project. The objective is not to memorize software menus. It is to develop the habit of following the coordinate through every transformation until the apparent disagreement has been completely explained.
The next chapter pushes this even further. What happens if we imagine a perfect receiver, with every possible measurement error eliminated? Even then, there isn't just one correct elevation.