Among the general public, there is a misunderstanding about the crucial role gravity data plays in geospatial, AEC and their daily lives — and the unseen benefits thereof. When you ask someone about “elevation”, they envision a height above sea level. Indeed, common references to elevation are expressed as such. But what truly is “sea level”? Sea level, expressed as a theoretical surface of “equipotential gravity”, seems too esoteric a concept on which to frame discussions of the value of gravity observations and research. But how to relate the subject in an accessible and broad way? Perhaps by starting from the ground up.
People can relate to the elevation on a hiking map they have, or an elevation sign at a mountain pass they are driving over, but not the reference frameworks (datums so to speak), and models that derive such elevations. The magic happens unseen to the end user. In this age of high precision GNSS, the casual user sees a resultant elevation, yet Space–based navigation and positioning systems only provide an ellipsoid height: height above (or below) the reference ellipsoid. Such ellipsoids roughly fit the oblate shape of the Earth but have a direct geodetic relation to the satellite tracking and orbits. Gravity data is used to define geoid models. These are further refined by applying terrestrial data to create geoid difference models, which are applied as a ghost-in-the-machine to yield the elevation the GNSS equipment displays.
This process of ellipsoid+geoid derivation (more correctly described as producing “orthometric elevations”), is well known among surveyors, geodesists, and geospatial professionals. To these downstream users, the value of improvements in gravity data is obvious. The densification and improvements to more than a century of gravity measurements is not an inexpensive proposition. Terrestrial, airborne, and satellite-based gravity modernization programs can represent substantial investments. There are economic studies that have put cost-benefit figures to such programs, but they are not generally considered outside of the geospatial, scientific, and defense communities. The value has to be expressed in terms that funding agencies, legislators, and the public can relate to, which is a challenge.
But recent events have seen the term “gravity”, in the geodetic sense, reaching broader audiences — even through mainstream media. News items about the 2019 Everest measurement expedition of the Survey Department of the Government of Nepal, and the 2020 Everest measurement expedition of the Department of Land Surveying and Mapping of the Ministry of Natural Resources, China noted that both projects included gravity measurements. The two countries plan to jointly announce results later this year.
Not only were wide networks of gravity measurements featured in both initiatives, but also companion conventional terrestrial geodetic observations — with GNSS and conventional instruments — to tie the gravity data to existing geodetic reference frameworks. Susheel Dangol, the coordinator and Chief Survey Officer of the Sagarmatha (Everest) Height Measurement Secretariat (Nepal), said that a broader geodetic program was part of the Everest height measurements. This included a network of 298 survey marks with gravity and GNSS measurements on each. The GNSS measurements were referenced to an array of temporary GNSS base stations, further tied to the Nepal national reference framework and continuously operating reference station (GNSS CORS) network. Subsets of these survey marks were also observed with digital and differential levels. Providing the companion gravity-based data is a vital part of establishing updated elevations for the mountain and surrounding region.
In the United States, the National Geodetic Survey (NGS) is the federal agency charged with the establishment and maintenance of the nation’s spatial reference framework. Modernized elevation data is seen as the key to improving, for instance, the national flood mapping system. The 2019 NGS report: Scaling the Heights: Socio-economic Study of the NGS Gravity Program notes that in the 30-year–period (1988-2017), an average of 86 fatalities per year from floods was experienced. In 2017, flash floods killed 103 people, injured eight, and caused $59 billion in property and crop damage. In addition to floods, in 2017, tropical storms and hurricanes killed 43 people, injured 62, and caused $23 billion in property and crop damage — many more were killed by winter storms, river floods and rip currents.
Natural disasters are chief in the public eye, but there are additional benefits of refined elevation data related to agricultural, waterborne commerce, and public water resources. Legacy irrigation and water distribution systems were often designed with inconstant elevation data, and such head loss has been targeted for improvement by local and regional water conservation initiatives.
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With the advent of advances in airborne and satellite-based gravity measurement, the ability to refine global gravity models took a tremendous leap forward. Among these new resources were the 2002-2017 Gravity Recovery and Climate Experiment (GRACE); a joint mission of NASA and the German Aerospace Center (DLR). And the 2009 ESA Gravity field and Ocean Circulation Explorer (GOCE), Earth Explorer missions. Using this data, various government, scientific, and commercial entities around the globe have been able to deliver “centimeter-grade” geoid models. This means that geoid difference models derived from such data, when applied to GNSS ellipsoid heights to derive orthometric elevations, add little more than a centimeter to the combined precision.
With such data, for instance, a survey crew measuring elevations for an irrigation system with high-precision GNSS (capable of solving ellipsoid heights of 2-3cm) can yield orthometric elevations under 5cm. And relative to adjacent elevations, even more precisely.
The GRAV-D (Gravity for the Redefinition of the American Vertical Datum) project of the NGS, which has been ongoing since 2007, is an ambitious nationwide initiative that has combined terrestrial and satellite gravity data, together with airborne gravity over coastal areas. It has modernized not only the essential geoid models but is a key component of a new national spatial reference framework set for a 2022-2025 completion. There are similar initiatives underway, or already completed, across the globe.
Several research projects have been undertaken by the NGS to test the quality of the geoid products. The Slope Validation Survey of 2011 (Texas), and another in 2014 (Iowa), compared geoid derived elevations from GNSS observations (both static and real-time kinematic) with elevations on survey marks established with terrestrial differential (digital level) methods. The results demonstrated that GNSS-geoid derived elevations could be carried up to 100km with the same precisions as high-order legacy methods. To further validate the achievability of the GRAV-D goals, the NGS issued GPS Kinematic Challenges in 2010 and 2013, seeking GPS and GPS-IMU data from public and private sources to check the quality.
The 2019 NGS study of the socio-economic benefits of elevation improvements from modernized gravity data concluded:
“The costs for infrastructure work without the final geoid could be higher by 30%-50%, which translates into a cost reduction from the levels before the reduction of 23%-33%. For other applications the costs without the Gravity Program would be higher by 10%-20%, implying a reduction of 9%-17%. Combining these into an assumption for geospatial activities as a whole results in an estimate of cost savings of 11%-19%.”
“Spending on geospatial activities was estimated at $22.1-$30.4 billion in 2018. Spending is updated to 2019 by the estimated 4.8% change in nominal GDP from 2018 which places it at $23.16-$31.86 billion. It is assumed that 50% of the spending or $11.58-$15.93 billion is in the purview of the Gravity Program. This allows for work done using other technologies, work done at distances at which geoids do not provide an advantage, and work in the included occupations that is unrelated to orthometric height measurement.”
“Applying 22%-38% to $11.58-$15.93 billion results in an estimate of potential direct economic benefits of the NGS Gravity Program for geospatial activities of $2.55-$6.05 billion per year at 100% adoption based on 2019 economic activity. In comparison, U.S. spending on structures was $1.4 trillion in 2018. For nonresidential structures alone it was $637 billion.”
The benefits are further explored in the Geobuiz, Geospatial Industry Outlook & Readiness Index, 2018 edition (Geospatial Media and Communications).
While such estimates may not see a direct projection forward, considering the unforeseen global crisis in 2020, we should consider that having improved resources and tools could help with recovery.
The invisible force for change
That gravity has and will continue to play a vital role in development of the built world, and the management and preservation of the natural world, has notable historical precedent. The 18th century Geodetic Mission to the Equator (Mission Géodésique à L’Équateur) was undertaken primarily to measure degrees of latitude to help refine matters of the shape of the Earth, and to settle the debate of whether the Earth was elongated at the poles, or bugling a the equator. An elaborate triangulation network — that took three grueling years of field work with physical measuring rods, heavy surveying and celestial observation instruments — included several other groundbreaking, albeit less publicized, scientific works. Among these was the use of gravity measurements (e.g. the swing of a pendulum) to support the transfer of elevation from the coast to the surveyed area far inland. But they were also able to measure the effects of deflection from vertical due to the gravity pull of the mass of large mountains — a milestone in the study of gravity and physical geodesy.
In the 20th century, at the cusp of the Space Age, pioneering scientists and mathematicians like Dr. Glady B. West — a hidden hero of the story of GPS — doing work for the U.S. Navy, began to put the final pieces of global geodetic models together. These would be crucial to the success of the first truly global navigation satellite systems, the Navy Transit system, and later the U.S. Air Force Navstar (GPS) system. Among the work that West and others tackled was putting numbers to the relationships and dependencies of gravity with orbits, sea level, and in refining the vital geoid models. Satellite based Earth Observation systems, radar altimetry, and even those satellite maps you see on your phone have all benefited from such key work involving gravity.
The best tools will be needed to get global growth back on track, play catch-up, and brace for growing infrastructure demands that were already well underway. Gravity again, will be a hidden hero.
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