meanSeaLevel geoid ellipsoid other
Mean sea level is the usual value here, since the elevation is usually tied directly or indirectly to control monuments. If elevation is based strictly on GPS, then geoid should be given. For some work, elevation may be measured from the ellipsoid, in which case that value is provided.
meanSeaLevel: This value should be used if the reference position for the elevation is derived from a control monument or benchmark.
geoid: This value should be used if the some other equipotential surface has been used as a reference or if the value has been determined using Global Positioning technology (GPS) referenced specifically to the geoid.
ellipsoid: This value is used if the reference surface is defined by the given theoretical biaxial ellipsoid (specified by EllipsoidChoice), resulting in geometrical heights.
other: Allows the specification of an arbitary verticalDatum.