I would bet they had a standard construction form (2 meter tall Sonotube, etc) and they simply dug down until they reached the rock and poured the concrete. Lik

I would bet they had a standard construction form (2 meter tall Sonotube, etc) and they simply dug down until they reached the rock and poured the concrete. Like you say, they height doesn't matter since it will be referenced from some other datum.