Altitude from GPS can be measured relative to the WGS84 ellipsoid or relative to the geoid, the height where gravity is constant. The difference between the two is normally called the undulation. The ellipsoidal altitude is normally used by surveyors since it is easier to translate from this altitude to a more suitable, local co-ordinate frame. The geoidal altitude is used more often at sea since the sea level follows the geoid.
The Inertial+ now includes an option so the type of output can be changed between ellipsoidal and geoidal measurements or a constant offset can be entered. This option should reduce the confusion over the altitude information that the Inertial+ outputs.
When the geoidal altitude is used, the undulation (difference between the geoid and the ellipsoid) is taken from the external GPS receiver. Normally a lookup table is used and different GPS receivers use different lookup tables. A common table to use is EGM96.
This option makes it easier for both the land surveying and the sea surveying communities.