Notes on verification

Confirmed by multiple English and German sources, including Wikipedia articles on Normalhöhennull and Normalnull, official German state surveying agency documentation, and geographic reference sources. Strong historical and technical convergence. [cascade flags: wikipedia_dropped_other_sources_exist | tier=bronze indep_score=0.525 clusters=2 claim_tier=notable | framework convergence=single on Amsterdam Ordnance Datum (NAP) as the historical zero-point reference adopted by Prussia/Germany in 1879 and carried forward into modern German height systems (NN and NHN) (upstreams: Amsterdam Ordnance Datum (Normaal Amsterdams Peil/NAP), United European Levelling Network (UELN), Deutsches Haupthöhennetz) — publishers independent, evidence may not be | kind=empirical eligibility=high]