This educational talk will explain why we need a vertical reference for our coordinates, how we define “up” and “height”. How elevations were measured in the past, and how we now use GNSS to do it, and the implications of that. What is “the geoid” (gravitational model of the earth) and its differences with respect to the ellipsoid. Different types of heights (orthometric, normal, dynamic) and how we use different geoid models. Finally I will talk about how PROJ.org (open-source library) is supporting vertical coordinate reference systems with the grid files available in PROJ-data (open-data).

WGS 84 (EPSG:4326) is the most commonly Coordinate Reference System used.
It is the default in QGIS, the one used by OpenStreetMap, and what many people have in mind talking about latitude-longitude (or even for projected coordinates!).
However in many cases users are not aware of the accuracy of coordinates in this system.

Nowadays with more affordable RTK devices (or PPK post-processing) people expect amazing accuracies (2 cm!), but forget that the reference system must keep that accuracy.

In this talk I will explain what is and what is not WGS84, when it is not a good idea to use it, and how we should be suspicious about data labelled as WGS84. Why people using it don't know or don't care about those problems (with or without a good reason).
Also I will talk about the pros and cons of such a CRS. For sure, not everything is bad.