https://globalgpssystems.com/gnss/geoids-ellipsoids-and-undulation-height-understanding-the-basics/
Geoids, Ellipsoids, and Undulation Height: Understanding the Basics | Global GPS Systems
understanding the basicsgeoidsundulationheight
https://support.pix4d.com/hc/en-us/articles/360044174411
How to use vertical coordinate systems and geoids - PIX4Dmatic
This article explains how to select the correct vertical coordinate reference system, geoid, or geoid height in PIX4Dmatic.
how to usecoordinate systemsverticalgeoids