In the Elevation Toolbox, the zero elevation value represents the relative elevation of the ground compared to your takeoff point. This means the elevation is relative to the launch location, not absolute.
The zero elevation can be randomly displaced a few feet or meters because of the drone's GPS error. To get the absolute elevation measurement, which is based on Mean Sea Level (MSL), you must apply Ground Control Points (GCPs) to your map.
V2.2