A proposal to use geoid slope validation lines to validate models of geoid change

The United States National Geodetic Survey (NGS) has embarked on a ten year project called GRAV-D (Gravity for the Redefinition of the American Vertical Datum). The purpose of this project is to replace the current official vertical datum, NAVD 88 (the North American Vertical Datum of 1988) with a geopotential reference system based on a new survey of the gravity field and a gravimetric geoid. As part of GRAV-D, the National Geodetic Survey will develop a set of “geoid slope validation lines” at various locations of the country. These lines will be surveys designed to independently measure the slope of the geoid to provide a check against both the data and theory used to create the final gravimetric geoid which will be used in the geopotential reference system. The first of these lines is proposed to be established in the Autumn of 2011 in the west central region of Texas. The survey will be approximately 300 kilometers long, consisting of GPS, geodetic leveling, deflections of the vertical, surface absolute and relative gravity, including the use of relative meters for low-high surface gradient determination. This region was chosen for many factors including the availability of GRAV-D airborne gravity over the area, its relatively low elevation (220 meter orthometric height max), its geoid slope (from the latest high resolution models being a few decimeters over 300 km), lack of significant topographic relief, lack of large forestation, availability of good roads, clarity of weather and lack of large water crossings. Further lines are planned in the out-years, in more difficult areas, though their locations are not yet determined. Although the original intent of these lines was to serve as calibrations against geoid modeling data and theory, there may be additional uses relevant to geoid monitoring. A gap is being anticipated between the GRACE and GRACE-Follow On missions. GRACE has shown a quantifiable change (millimeters per year) in the geoid over parts of North America. As such, the GRAV-D project contains plans to monitor geoid change. However, without GRACE, some method of modeling geoid change and then testing that model must be developed. It is proposed, therefore, that as NGS develops more “geoid slope validation lines” that some consideration be made to placing one or more of them in areas of known, ongoing geoid change. Re-surveying of these lines would yield a direct, independent look at actual geoid change along the line. The sparseness and linear nature of such lines would not allow them to be used to directly create a continental model of geoid change, but they could stand as in-situ validations of models of geoid change coming from, say a model of mantle and glacial dynamics.