QGIS average/mean Z-value of polygon

Question

I am trying to derive an average/mean Z-value of the GPS measured polygons. The standard z($geometry) I use for point layers doesn't work obviously. I could use z(start_point($geometry)), but that is only the first point.
Is this possible to derive directly from the polygons, thus without converting the polygones to points? And how would this be possible?

Kazuhito · Answer

I recommend Extract Z values tool in the Processing Toolbox > Vector geometry.
(As a sample I created a brown polygon geom_from_wkt('POLYGON((4 5 2, 4 6 2.5, 7 6 1, 7 5 1))')

Execute Extract Z values tool.
Click on a small ellipsis button [...] of the Summaries to calculate list.
Select Mean and run the tool.

The tool will produce an Extracted layer with which attribute table contains the mean of Z value. (In this example mean of {2.5, 1.0, 2.0, 1.0} is 1.625).

QGIS average/mean Z-value of polygon

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