A vertical coordinate system defines the origin for height or depth values.

Like a horizontal coordinate system, most of the information in a vertical coordinate system is not needed unless you want to combine a dataset with other data that uses a different coordinate system. The most important part of a vertical coordinate system is its unit of measure. The unit of measure is always linear. Another important part is whether the z values are heights (elevations), or depths. The axis directions are, positive respectively, up or down. Depths are always displayed as positive values.

A vertical coordinate system (vcs) can be referenced to two different types of surfaces:

• spheroidal (ellipsoidal) OR
• gravity-related (geoidal)

Most vertical coordinate systems are gravity-related.

A gravity-related vertical coordinate system is often only loosely connected to a particular geographic coordinate system. Any particular vertical coordinate system may be used with different horizontal coordinate systems. A gravity-related vertical coordinate system may set its zero point through a local mean sea level or a benchmark. Mean sea level will vary at different places due to topography, atmospheric effects, etc. A gravity-related vertical coordinate system will include a vertical datum as part of its definition. An example is shown below.

VERTCS["National_Geodetic_Vertical_Datum_1929",VDATUM["NGVD_1929"],PARAMETER["Vertical_Shift",0.0],PARAMETER["Direction",1.0],UNIT["Meter",1.0]]

A spheroidal vertical coordinate system defines heights that are referenced to spheroid of a geographic coordinate system. GPS natively reports heights relative to the WGS84 ellipsoid. An on-board geoid model (discussed below) converts the ellipsoidal heights to gravity-related elevations. A spheroidal height is a geometry quantity and does not have a physical sense as a geographic coordinate system's spheroid may fall above or below the actual earth surface. Spheroidal heights for an area may not reflect movement due to gravity, that is, the flow of water. Water can run uphill when working with spheroidal heights.

A vertical coordinate system with heights or depths that are referenced to the spheroid will include a datum , rather than a vertical datum definition. An example is shown below.

VERTCS["WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PARAMETER["Vertical_Shift",0.0],PARAMETER["Direction",1.0],UNIT["Meter",1.0]]

Geoid

The geoid is an equipotential, or level, surface of the Earth's gravity field (Torge). Imagine the oceans allowed to settle under the influence of gravity only and not subject to tidal or atmospheric forces. Tunnels are also used to connect the oceans so that the water can move freely. The resulting surface is a representation of the geoid. The geoid is approximately equal to mean sea level (MSL) and generally differs from local mean sea level by a meter or so. It is a complex shape. The geoid is influenced by the composition of the earth so it may have discontinuities in its slope. This means that the surface is an analytic surface as opposed to a mathematical surface like an ellipsoid. The geoid generally differs from an earth-centered horizontal geodetic datum by less than 100 meters. In the UK, the geoid and the horizontal datum (OSGB36) differ by less than five meters.