Practical and Technical Considerations for the Replacement of NAVD 88 and IGLD 85

English Abstract:

The North American Datum of 1988 (NAVD 88) was co-defined with the International Great Lakes Datum of 1985 (IGLD 85) through the determination of geopotential numbers at some 600,000 passive geodetic control bench marks. Those geopotential numbers were computed from geodetic spirit leveling and surface gravity measurements. Most of the field work was performed in the 1970’s and 1980’s, though some of the leveling dates from NGVD 29 and CGVD 28.

The primary method by which surveyors and other geospatial professionals access NAVD 88 and IGLD 85 is through the publication of Helmert orthometric heights (for NAVD 88) or Dynamic Heights (for IGLD 85), both derived from geopotential numbers, at passive control. NAVD 88 and IGLD 85 were significant achievements and improvements over previous height systems on the North American continent. However, they have a significant number of disadvantages, including: the general lack of vertical change monitoring; the removal of passive control during construction projects; the inconsistency between marks which move with the crust and those which are deep-driven rods and move only with underlying bedrock; the inconvenience of marks not existing in an area of interest to a surveyor; the propagation of leveling errors over thousands of kilometers and the failure to incorporate 2^nd and 3^rdorder effects into the original determination of geopotential numbers.

In 2008, the National Geodetic Survey (NGS) formally announced its policy to replace NAVD 88 with a geopotential reference frame implicitly linked to a new geometric reference frame (which will replace NAD 83). The geopotential reference frame will have the data and tools needed to provide to users their orthometric height, dynamic height, deflections of the vertical or acceleration of gravity at any point in the USA or its territories. The orthometric heights will be accessed from CORS stations, a time dependent gravimetric geoid model and GNSS positioning techniques. The dynamic heights will be similarly accessed, but will also rely on a time-dependent surface gravity field.

This talk will focus on the technical and practical considerations surrounding the definition of such a frame, its upkeep and its method of access by geospatial professionals. Topics will include data collection, models, tools, accuracy and sustainability.