1. California Geodetic Advisor,
Vertical Datums
Marti Ikehara
California Geodetic Advisor,
Sacramento

2. GPS data is referenced to the ellipsoidal surface, which does not have a constant offset—they are not parallel to the equipotential surfaces.
It’s all about better determining N—geoid height, from better gravity data. What we have been doing for close to 20 years is collecting GPS data at leveled bench marks—the control for Ht Mod projects, and fitting the data in a least squares adjustment to the datum for leveling—NAVD88. We produce orthometric heights and input data to geoid models to try to account for the gravity term.

3. Datums at , North Spit

6. Datums at , North Spit