Dr. Dru Smith of the National Geodetic Survey describes a modernized National Spatial Reference System: what’s changing, why it’s changing and what surveyors should expect.
If you do geospatial work in the United States using the National Spatial Reference System (NSRS), you may be keeping tabs on framework modernization, now undergoing a delay that could push completion out by several years.
Despite the delay, the “2022” terms will remain the same, as outlined in this new datums FAQ.
Dr. Dru Smith, NSRS Modernization Manager at the National Geodetic Survey, provided guidance on the NSRS modernization plans earlier this year as a guest speaker for a popular webinar by the Trimble Business Center team and Jeff Ryan, TBC Product Manager.
Smith, who has been with NGS for 25 years, kicked off his talk by describing why the NSRS needs to be modernized and what will replace the current national datums: the North American Vertical Datum of 1988 or NAVD 88 and the North American Datum of 1983 or NAD 83.
Those datums were perfect for their time, Smith said, but “space-aged geodesy has led us to a much better understanding of how to put out a vertical datum, so this is no longer the right tool for the future.”
Perpetuating the myth of single coordinate through time
Why does the official vertical datum need to change?
Smith explained that the NAVD 88 suffers from the use of infrequently surveyed geodetic control marks that are almost never re-checked for movement, disappear by the thousands every year, are not funded for replacement, are not necessarily in convenient places, don’t exist in most of Alaska, weren’t adopted in Canada, and were part of a level network constrained to a single point, allowing cross-country error buildup.
That buildup has manifested in two ways:
- From the vertical standpoint, NAVD 88 suffers from a known bias of about 50 centimeters and a northwest-southeast trending tilt of about 1 meter across CONUS (Conterminous United States). “These are systematic errors in all of the heights in CONUS alone. It’s actually slightly worse in Alaska and then each of the individual territories of the United States, specifically islands: American Samoa, Puerto Rico, U.S. Virgin Islands, etc. They have their own vertical datums with similar issues.”
- From the geometric standpoint using NAD 83, the geocenter of the world is more than two meters off from the current best estimates of the International Terrestrial Reference System or ITRS, a geocentric system of coordinates. This causes systematic errors around the -2 to +2-meter range in latitude,longitude and ellipsoid heights.
In addition, the current NSRS relies on geodetic marks set in the Earth’s crust, but the crust is dynamic. “The way NGS has put out the NSRS has perpetuated a longstanding myth in the surveying community that you can put a coordinate on a marker, and that coordinate sits in perpetuity,” Smith said. “But that’s not true, especially with the accuracy of surveying today, we can see this rotational signal in CONUS, plus a deformational signal on the West Coast.”
When surveying at centimeters of accuracy, these changes at the multi-centimeter, per-year level are significant, Smith said, and they manifest in coordinates, day in and day out.
Decision to change
In 2008, the NGS announced it would modernize and put out a 10-year plan for the NSRS, the geodetic control to the nation.
“We took out a white sheet of paper,” Smith said. “If we could do things all over again from the beginning, what would we do, and how would we do it?’”
The resulting plan focused on replacing NAVD 88, as well as NAD 83 (the geometric— historically called “horizontal”—control datum for the U.S., Canada, Mexico and Central America) at the same time. “They interplay with one another,” Smith said, “and frankly, we did not want to try to upend the system twice to our users, so we’re going to do it all at once in a single go.”
Broadly, the modernization involves transitioning to a geoid-based vertical datum and leveraging multiple constellations and real-time networks.
Role of time dependency
As part of modernization efforts, points in the NSRS with defined coordinates will have epochs associated with them, based on the time actual data were collected at those points.
“We wanted to fully embrace it,” Smith said of time dependency, “rather than just perpetuating the myth of a single coordinate through time.”
From the “Blueprint for 2022, Part 3: Working in the Modernized NSRS” technical report, here are additional details:
- Such coordinates will be known as “survey epoch” coordinates, SECs (if associated with finite time spans of data collection) or “active” coordinates (if associated with continuous data collection).
- Consequently, passive control will have less reliability than active control, and NGS will treat the NOAA CORS Network as having the definitive, up-to-date coordinates within the NSRS. A change of business will result: users of the NGS tool “OPUS” will need to collect Global Navigation Satellite System (GNSS) data, even when performing leveling or classical traverse surveys to ensure coordinates computed in those surveys are up-to-date and are “tied to” the NSRS through the NOAA CORS Network (NCN). In order to bridge users into a time-dependent NSRS, NGS will also be estimating, and providing to the public, “reference epoch” coordinates (RECs) on points at five-year intervals, beginning with epoch 2020.00. Such estimates will mimic the current status quo [the 2010.00 epoch of NAD 83(2011), for example], but will be estimates of coordinates at times different from the data collection and therefore expected to have less accuracy than survey epoch coordinates (SECs)
- Whereas users will have the option, via an updated OPUS, to take any campaign survey at any date and adjust their surveys to any epoch(s) of their choosing, we at NGS will not do this. Rather, if your survey data is submitted to NGS, we will compute survey epoch coordinates (SECs) at the epoch of your survey. Then, in the future, those same data will be used to estimate reference epoch coordinates (RECs).
- We will be providing tools to users, under the catch-all name “OPUS,” for uploading, processing, analyzing, and submitting survey data of all types, such as: GNSS, RTK (Real Time Kinematic), RTN (Real Time Network), leveling, gravity, or classical. Additionally, OPUS will have tools for ingesting and analyzing continuous data (e.g. GNSS, gravity). The tool will be browser-based and will fully integrate all data types, so that, for example,a single project, containing both GNSS and leveling could be uploaded and processed under the same project name.
Surveyors should expect the modernized NSRS to impact them on the geometric side and vertical side, with coordinates experiencing shift and drift, Smith said. Also, if surveyors are sitting on data, they will have to get it into the new system.
“Right now, we’re putting out some new tools that will continue to support the modernized NSRS,” Smith said. These will include providing NGS’ default NADCON and VERTCON transformations, both of which were recently updated and incorporated into NGS’s new flagship tool NCAT (NGS Coordinate Conversion and Transformation Tool). Also, the familiar OPUS tool will change slightly and have a slightly different look and feel.
In addition, Smith encouraged users to try the new mobile-friendly mark recovery webpage.
“If you have ever been out in the field and thought, ‘man, I wish I could find the survey marks that are near me,’ now you can,” he said.
Surveyors will want to monitor updates on NSRS modernization by checking the website for new developments or subscribing to NGS email updates here. One way you can directly impact the modernization is the collection, and submission to NGS, of high-quality GNSS data on vertical bench marks as part of the GPS on BM campaign. Such data will be used to improve the VERTCON transformation tool when the modernized NSRS is released.
Trimble is taking a variety of steps to prepare for the changes underway for NSRS modernization, including:
- Improving Trimble geodetic libraries to ensure optimum accuracy when transforming coordinates between any static, plate-fixed or dynamic Datums
- Updating Coordinate System Manager with new Datums
and State Plane Coordinate System Zones 2022 provided by NGS.
- Focusing on providing a seamless user experience, while updating UI and integrating new libraries across Trimble Firmware (GNSS receivers)
and Trimble Software (Trimble Business Center and Trimble Access)
- Engaging with NGS to understand the modernization program.
- Acting as industry leader providing industry & customer feedback
- Educating survey professionals, such as through webinars.