vertikaldatum

Vertikaldatum, or vertical datum, is a reference surface used in geodesy and surveying to measure elevations and depths. It defines a zero level from which heights above or below the surface are referenced. A vertical datum can be global, regional, or local and is typically tied to a physical surface such as mean sea level or to a geopotential surface approximating gravity potential.

In practice, a vertical datum is realized through tide gauges, leveling networks, and geoid models. It serves as the anchor for height systems used in mapping and construction. National and regional datums include examples such as NAVD88 in North America and NGVD29; in Europe, EVRFs and national systems like Germany’s NHN or the Netherlands’ NAP.

Heights are often expressed as orthometric height H, the height above the vertical datum surface, in contrast with ellipsoidal height h. The relation H = h − N links orthometric height to ellipsoidal height through N, the geoid undulation (the separation between the ellipsoid and the geoid).

Vertical datums are essential for civil engineering, flood risk assessment, navigation, and land surveying. Converting between datums requires a geoid model and transformation parameters; differences between datums can cause significant height shifts across borders when not accounted for.