I look for this: "Elevation in metres above 'mean high water spring' level."
Which means the geographic hight above MHWS: https://en.wikipedia.org/wiki/Mean_high_water_spring https://de.wikipedia.org/wiki/Springtide#Mittleres_Springhochwasser
I did find only this: https://www.wikidata.org/wiki/Q6452016 which is 'above MSL': https://en.wikipedia.org/wiki/Metres_above_sea_level https://de.wikipedia.org/wiki/H%C3%B6he_%C3%BCber_dem_Meeresspiegel
Bests, Markus