EPSG:1121

This transformation involves the application of a height difference interpolated from a hydroid surface correction model. The model provides the offset of the tidal surface from the ellipsoid, positive when the tidal surface is above the ellipsoid, of a specified geographic 3D CRS (the source CRS) at nodes on a regular grid of latitude and longitude intersection points. The geodetic latitude and longitude used to interpolate within the grid are not affected by this transformation.

The grid is referenced to a specific geographic CRS (the horizontal component of the source CRS) and interpolation must be made in the latitude and longitude of this system.

Calculation of the height difference is achieved through a bi-linear interpolation of the grid, using the latitude and longitude of the point. This step provides the height correction (ζ) of the target datum above the ellipsoid of the source Geographic 3D CRS. Because it is applied in the target CRS in the depth domain, the height correction model relationship to ellipsoidal height has its sign reversed. Then:
           D = ζ – h

In a marine context depth of the seabed below the tidal surface is derived from observed water depth and observed three-dimensional position including ellipsoidal height, both measurements reduced to a common vessel reference point, and the ellipsoid height of the tidal surface interpreted from the hydroid model. Then:
	D = (Dobs – hobs) + ζ

where 	
D	        is the depth below the tidal surface (for example, depth below LAT)
Dobs 	is an observed depth below the vessel reference point
hobs 	is an observed ellipsoidal height of the vessel reference point
ζ 		is the height of the tidal surface above the reference ellipsoid interpolated from the hydroid model

The method is not reversible because the depth is 1-dimensional and not associated with a horizontal CRS.