Brief History of the Height Reference Systems Used along the Romanian Sector of the Danube River

Both geodetic engineers and specialists in the fields such as Hydrology, Hydrotechnics or Geography have faced countless times the problem of using multiple height reference systems on the territory of our country. Over the years, several "zero" points or fundamental points have been used in Romania: "zero" Black Sea Sulina, "zero" Adriatic Sea or "zero" Baltic Sea (most often used). Regarding the Romanian sector of the Danube River, all bathymetric navigation maps use Sulina as a fundamental “zero” point. Unfortunately, the (absolute) heights of the gauges “zero” installed along the Danube River differ from each other depending on the reference surface from which these heights are measured. For this reason, the water level determinations cannot be performed in the same height reference system. In practice, the transformations between different vertical datums used along the river are still a real challenge. This paper supports those who want to understand the history of using the main height reference systems on the Romanian sector of the Danube River and, moreover, represents a review of the relationships between them. Although they were determined long ago, some of them have been transposed into the GIS format to facilitate the height transformations.