OGC Vertical Datum types
The following are definitions of vertical datum types from OGC 01-009 (CT Spec 1.0). They must be included as an argument to the VERT_DATUM keyword in OGC WKT according to the specification. It is unclear if there is a direct analog in the EPSG database.
12.3.9.8 CS_VD_AltitudeBarometric (Attribute) : 2003
The vertical datum of altitudes or heights in the atmosphere. These are approximations of orthometric heights obtained with the help of a barometer or a barometric altimeter. These values are usually expressed in one of the following units: meters, feet, millibars (used to measure pressure levels), or theta value (units used to measure geopotential height).
12.3.9.9 CS_VD_Depth (Attribute) : 2006
This attribute is used to support the set of datums generated for hydrographic engineering projects where depth measurements below sea level are needed. It is often called a hydrographic or a marine datum. Depths are measured in the direction perpendicular (approximately) to the actual equipotential surfaces of the earth's gravity field, using such procedures as echo-sounding.
12.3.9.10 CS_VD_Ellipsoidal (Attribute) : 2002
A vertical datum for ellipsoidal heights that are measured along the normal to the ellipsoid used in the definition of horizontal datum.
12.3.9.11 CS_VD_GeoidModelDerived (Attribute) : 2005
A vertical datum of geoid model derived heights, also called GPS-derived heights. These heights are approximations of orthometric heights (H), constructed from the ellipsoidal heights (h) by the use of the given geoid undulation model (N) through the equation: H=h-N.
12.3.9.12 CS_VD_Max (Attribute) : 2999
Highest possible value for vertical datum types.
12.3.9.13 CS_VD_Min (Attribute) : 2000
Lowest possible value for vertical datum types.
12.3.9.14 CS_VD_Normal (Attribute) : 2004
A normal height system.
12.3.9.15 CS_VD_Orthometric (Attribute) : 2001
A vertical datum for orthometric heights that are measured along the plumb line.
12.3.9.16 CS_VD_Other (Attribute) : 2000
Unspecified vertical datum type.
