Towards The Unification Of The Vertical Datum Over The North American Continent

The United States adopted the North American Vertical Datum of 1988 (NAVD 88) for its official vertical datum in the 1990s. Canada has been using the Canadian Geodetic Vertical Datum (CGVD28) for its height applications since the 1930s. The use of the different datums causes inconsistent heights across the border between the two countries, and the topographic height data from the two countries are not compatible. Both datums rely on passive control and significant pre-modern survey data, yielding not only misalignment of the datums to the best known global geoid at approximately 1-2 m, but also local uplift and subsidence issues which may significantly exceed 1-2 m in extreme cases.Today, the GNSS provides the geometric (ellipsoidal) height to an accuracy of 1-2 cm globally. The use of current inaccurate vertical datums no longer serves the purpose it once did. Because of this, users have begun to demand a physical height system that is closely related to the Earth's gravity field to a comparable accuracy. To address this need, government agencies of both countries are preparing the next generation of vertical datums. Even if the new datums are based on the same concepts and parameters, it is possible to have inconsistent heights along the borders due to the differences in the realization of the datums. To avoid inconsistency, it is in the interest of both countries to have a united, seamless, highly accurate vertical datum. The proposed replacements for CGVD28 and NAVD88 shall be based on GNSS positioning and a high accuracy gravimetric geoid that covers the territories of the United States, Canada, Mexico and the surrounding waters (to include all of Alaska, Hawaii, the Caribbean and Central America). To account for the effect of the sea level change, postglacial rebound, earthquakes and subsidence, this datum will also provide information on these changes. Detailed description of the definition, realization and maintenance of the datum is proposed. The challenges in realization and maintaining the datum are also discussed.