“GRAV-D has the potential to provide very significant benefits to the public”
Says, Dr Dru Smith, Chief Geodesist of NOAA’s National Geodetic Survey while explaining the GRAV-D project
What is the GRAV-D project?
GRAV-D stands for “Gravity for the Redefinition of the American Vertical Datum”. It is a multi-year project designed and led by the National Geodetic Survey (NGS). GRAV-D was initiated to measure and monitor the Earth’s gravity field with the goal of accurately defining a geopotential reference frame for the United States, to replace the current vertical datum NAVD 88 (the North American Vertical Datum of 1988).
What are the components of the GRAV-D project?
GRAV-D consists of three major data collection campaigns. The first is an airborne gravity survey over the United States and its surrounding territories. This is the largest campaign in both time and money, expected to be complete around 2022. It should yield a highly accurate “snapshot” of the gravity field over the USA. The second campaign is about monitoring the continental changes to the gravity field, through a combination of repeated terrestrial gravity measurements, satellite gravity missions and geophysical modelling. This is necessary for the long-term maintenance of the geopotential reference frame. The third campaign is mostly just an idea for now, consisting of partnership surveys in localized areas of significant geophysical change (such as southern Alaska, southern California or Hawaii). In that campaign, NGS would expect to cooperate with local groups with an interest in highly detailed change detection.
In addition to these data collection campaigns, the ultimate goal of GRAV-D is to use this gravity information to define an accurate geopotential reference frame (which includes a zero-height surface, known as “the geoid”). While data collection is ongoing, research is also being conducted to ensure that the equations used to model the geoid and other aspects of the gravity field are accurate enough to provide a geoid model to 1 centimeter where possible.
What are the current problems with ‘height’ that the GRAV-D project looks to address?
Firstly, there are many different definitions of “height”. It is likely that the most common use of “height” is the orthometric height, which is colloquially (but inaccurately) called a “height above sea level”. The orthometric height is the one most frequently plotted on topographic maps and is extremely good at telling the direction of water flow. Another different height is the ellipsoid height, which is a sort of mathematical abstraction which comes as an artefact of GPS positioning. Ellipsoid heights are determinable very accurately using GPS, but they are not good at determining the direction of water flow.
The improvements which GRAV-D will provide to “height” are twofold. The first improvement is solely in the determination of the “zero height surface” (or “geoid”) itself. The current official vertical datum of all civilian federal mapping authorities in the USA is NAVD 88. That datum was established using traditional levelling techniques over decades preceding the final release of NAVD 88 in 1991. Recent studies have shown that NAVD 88 has a zero height surface that is biased from the geoid by about 50 centimeters and tilted across the country by about 1 meter. Furthermore, NAVD 88 is accessed through the publication of heights on hundreds of thousands of passive geodetic marks (called “bench marks”) whose heights are rarely checked, and are therefore vulnerable to subsidence and bulldozing.
The second improvement which GRAV-D will empower is to no longer rely on unchecked passive control to provide access to heights. Rather, a GPS receiver, with a geoid model from GRAV-D, will provide immediate and accurate access to orthometric heights directly.
Who all are collaborating on the GRAV-D project with National Geodetic Survey (NGS)?
NGS has appreciated the collaborative efforts of a variety of other agencies in the few years since GRAV-D’s inception. Foremost amongst these are the Naval Research Laboratory (NRL), the U.S. Army Corps of Engineers (USACE), the National Geospatial-Intelligence Agency (NGA) and the Bureau of Land Management (BLM). However, the long-term operational funding for GRAV-D remains solely that of NGS at this time.
Besides helping NGS to fulfil its mandate to provide accurate positioning, there are several other benefits that will accrue from the completion of the GRAV-D project. Could you please elaborate on these benefits?
GRAV-D has the potential to provide very significant benefits to the public, beyond the NGS mission objectives. First, all of the data collected will be in the public domain, hopefully spurring academic research for years to come. Furthermore, the access to this data should improve all other models of the gravity field, not just those over the USA, due to the spatial correlation properties of gravity. This means that groups modelling the gravity field over Canada, or over the entire world, may see benefit from GRAV-D.
Other improvements may come from some collaborations, such as the flying of a magnetometer by USGS on GRAV-D flights (which has been done experimentally, but NGS hopes can become standard operating procedure).
Indirect benefits come from the performance of NGS’s mission, as the provider of geodetic control information to the rest of the federal government. That is, NGS expects that FEMA floodplain mapping, USGS topographic mapping and USACE levee monitoring surveys may all benefit from an accuracy improvement as GRAV-D finishes and the new geopotential reference frame is put in place.
Do you think a world-wide effort along the lines of GRAV-D will become essential in the years to come?
I do not feel that a world-wide effort would be essential, but I think some specific country-wide efforts will be so. Mapping the gravity field from space has rapidly improved our ability to use this information to improve heights, but even the best missions (GRACE and GOCE) cannot see the finest details of the gravity field which airborne (or terrestrial) gravity mapping can show. In many cases, budgets will prevent the accomplishment of GRAV-D style projects over many countries. Furthermore, mapping the ocean areas (aside from where they are nearest to occupied land) is not necessary. Ocean gravity mapping, in deep waters, is already a well-established science using altimetry data. With this in mind, I see large scale airborne mapping of the gravity field being mostly performed by large countries or groups of countries with the resources to do so, and the need for the highest accuracy heights.