Datums, feet and GNSS vectors: The 2022 NGS upgrade

In what seems like just yesterday, GPS World published my article in the May 2017 issue of Survey Scene on the upcoming datum change by the National Geodetic Survey (NGS) in 2022.

With the calendar pages turning rapidly and as we get closer to the witching hour of geospatial voodoo, more items have surfaced to discuss and educate ourselves on in relation to “the change.”

Let’s delve into these topics and break each down into what the common surveying and geospatial practitioner will need to know with the advancements in coordinates, geodesy and our everyday uses.

NATRF2022: The continental U.S. replacement for NAD83 and NAVD88

It is no secret that with the advancing use of GNSS technology, flaws in both existing horizontal and vertical datums establishing our National Spatial Reference System (NSRS) have been identified and exposed.

NGS estimates that NAD83 is non-geocentric by over two meters, while the model establishing NAVD88 contains a tilt of approximately one meter across our continent.

For most geospatial practitioners, these flaws are minimal to the integrity of their data. It does, however, give us a glimpse of how assumptions of geodetic information can produce incorrect modeling of surveying and mapping data and could lead to more flawed earth models without significant changes to their structure.

With a great number of surveying and mapping practitioners using GNSS technology with little or no knowledge of the origins of our NSRS, it is a good time to provide the primers below to explain the history of our geodetic datums.

Besides my previous article, follow these links for much more thorough technical information:

• GPS World Contributing Author David Zilkoski

• NGS 2022 Informational Videos

• NGS Publications and Webinars

• NGS Video Library

• NGS / COMET Program YouTube Channel

With changes in both horizontal and vertical datums, slight variations in the data we are used to seeing will seem insignificant, but will require the user to pay close attention to potential data traps when converting between the old and new systems. The NGS graphics below depict the severity of datum change in the horizontal and vertical component across the U.S.

Image: NGS

Depending on where you are working, new state plane coordinates will vary from –2 meters to +4.5 meters from previously published values, with elevations fluctuating up to one meter from previous norms. All these changes are due to the increased knowledge of our world using various forms of emerging technology not thought possible several decades ago.

These new measuring methods and studies, including GNSS and gravity monitoring, have allowed scientists and geodesy experts to establish more accurate geographic location systems than past terrestrial ways and procedures.

We have geodetic monuments and marks everywhere; will they still be usable?

The short answer to this question is an unequivocal yes, but with some caveats. Use of GNSS monitoring has proven we reside on tectonic plates that move slowly over time; thus, the geographic values (latitude and longitude) used to calculate any number of coordinate value systems are changing as well.

Image: NGS

Relational data between established points are not likely to change, but studies have shown significant shifts in areas that result in movement of our previously considered “unmovable” monuments.

With additional parameters and characteristics being introduced with the 2022 datum, time and tectonic plate shift are main factors in establishment of a point.

The concept of a “permanent” point no longer exists in relation to a published and unchangeable coordinate value of horizontal and vertical data. The surveying and geospatial data collector must recognize that the user is establishing a particular X/Y/Z or N/E/Z value for that exact moment in time and it, theoretically, will change from the moment one steps away from the point.

This may be too “splitting of hairs” for most users, but the new system simply recognizes the reality of the moving data-collection stage, no matter how minute.

This datum re-establishment has been a monumental undertaking (no pun intended), and NGS deserves many kudos for coming up with a realistic solution for a complex problem.

However, most of its users still have a problem, and it lies within the standard unit of measurement: the U.S. survey foot. NGS (and its predecessor, U.S. Coastal and Geodetic Survey) have always used the meter for the basis of all units of measurement (as does the rest of the world.) The new 2022 datum is bringing us, the surveyors and mappers, to a new reality — nationwide adoption of the international foot. Let the grumbling and arguments begin!

The meter vs. international foot vs. US survey foot

The unit of measurement aptly named the “foot” has existed since early times, with most sources crediting King Henry I of England making a decree that his foot shall become the standard for measurement.

No matter where the definition of the foot came from, it has varied slightly throughout history. The origin of the meter (or metre, as it’s known worldwide) also has a variety of beginnings. The most established story starts from John Wilkins, an English philosopher, who published in 1668 what he described as a new standard of measurement based upon the length of a pendulum that swings approximately 38 inches across in one second. This length was eventually named the meter by an Italian scientist.

Another century later, King Louis XVI of France issued a integration law establishing the modern metric system with weights and measures having a base-ten system of units and sub-units. Within this system was the meter with a new length definition of being one ten millionth (1/10,000,000) of the distance from the North Pole to the Equator.

Upon completion of the calculations, a rectangular bar made of platinum and iridium was created to establish the “standard” meter from which all future measurements would be based.

The United States first recognized in 1866 the metric system and the meter (set forth as one meter equaling 39.37 inches). During this time, the International Commission of the Meter officially adopted the physical meter bar as the standard.

Over the next 100+ years, many studies were undertaken to re-establish the length of the meter. Using wavelengths of various elements, including cadmium, mercury, neon, zinc, helium, thallium and krypton, new definitions were created. In 1983, the current definition of the length of the meter was finalized.

The meter is now based upon the speed of light in a vacuum (299,792,458 m/s) with the meter being the length traveled in 1/299,793,458 of a second. While the length is very close to the original measurements set forth over the centuries, it is better defined for reproduction worldwide without having to possess a standard bar or other device.

To further muddy the standardization of units, in 1959 an international agreement was made by Australia, Canada, New Zealand, South Africa and the United Kingdom so one yard would equal 0.9144 meters. Meanwhile, the U.S. National Bureau of Standards published a notice that all survey-related measurements will remain based so one one yard equals 3600/3937 meters or 0.91441083 meters.

Image: NGS

We have two different measurements for the foot. What’s the big deal?

The difference between the two standards is two parts in one million; while that doesn’t affect everyday physical measurement, it does cause havoc on coordinate systems with values beyond the millions. (See NGS video “Two Right Feet?” for details).

What makes it even more confusing is that states across our country vary on which “foot” is standard within their legislation and daily practice. Currently (at the date of publication), six states recognize the International Foot as their standard unit of measurement, with four states not defining it. The remaining states have officially adopted the U.S. survey foot as their standard unit of measurement.

NGS has suggested that starting with the 2022 datum change, the U.S. survey foot will not be supported in applications and software produced by them for geodetic computations. It will be limited to meters and the international foot, so they are recommending that states update their existing definitions to change to the international foot along with recognizing the 2022 datum as the official coordinate-system base.

How to train our profession, the construction industry and John Q. Public on the new datum

I would be lying to you if I said I’m not concerned with the rollout of the new datum and with converting all surveying and mapping work to the international foot. My biggest concern is not with those direct relationships I have with my staff and fellow professionals within my company.

My main concern starts with these two areas: the tens of thousands of surveying practitioners working within projects containing state-plane coordinate systems in addition to contractors and other mapmaking providers using survey-grade equipment for construction and other mapping applications.

Both groups have little to no technical knowledge of the intricacies of state-plane coordinate systems and the geodesy network “behind the curtain.” To paraphrase a well-known mortgage company with an app-based home loan system, “push button, get data” is the limit of most users’ knowledge when it comes to state-plane coordinates.

Add to this the double-edged sword of real-time networks, where the user does not have to be concerned with setting up a base station, and the potential problems could get worse.

While there will be a few early and timely embracers of the new datum, the majority will dig their heels in and refuse to switch. When the conversion to the 2022 datum is upon us, many users will drag their feet on learning about the new system as existing projects continue under the old datums.

Until there is a mandate by government agencies and others, many newer projects beginning around the adopting time will remain on NAD83 and NAVD88 until directed otherwise.

Most practitioners I have spoken with on this issue agree that it will be a tricky period for surveying and mapping. Rather than get bogged down with negativity and fight change, the surveying, mapping and geospatial community should do the following:

• Rally our professions around these significant changes to educate our technicians and future professionals.
• Coach contractors and other trades who rely on the technology to understand the new system.
• Work with governmental agencies at all levels to educate them about what these changes entail and why to make the appropriate revisions to codes and statutes now.
• Capitalize on this opportunity to teach the public about who we are and how spatial data is part of everyone’s life.

All these points are paramount to the success of the datum upgrade and need to be followed through to the end. Ultimately, the faster we adopt and adapt, the better our geospatial world will be. There is lots of work ahead of us, but as the staff at NGS has shown us, the hard work necessary to make significant change is well worth the effort.

CALLING ALL SURVEYORS AND GEOSPATIAL PROVIDERS!

NGS announces GVX data format for GNSS vector processing

The National Geodetic Survey (NGS) is requesting input and feedback on a new data format for sharing real-time kinematic (RTK) GNSS vector information.

The new format will be like the static GNSS standard, Receiver Independent Exchange (RINEX), and is utilized by most software packages and the Online Positioning User System (OPUS).

The new GNSS Vector Exchange format (GVX), will introduce a new industry standard for sharing of RTK vectors across differing platforms and software packages.

Earlier users of GPS-based data collection remember the number of proprietary files created by each manufacturer, and having their own unique format for data and attribute interpretation. In response, the NGS created RINEX to help standardize data collection as a universal file format that would easily be adopted by receiver and software producers.

That same goal is being set with the introduction of the GVX format as the next step in data-collection standardization for GNSS RTK vectors. GVX elements include (but are not limited to) the following:

• Mark-to-mark Earth-Centered, Earth-Fixed (ECEF) vector components
• Variances and covariances of vector components’
• Reference frame information
• Start and stop time of the observation
• A-priori coordinates for the end points of each vector
• Receiver and antenna types
• RTK and real-time network (RTN) settings, if applicable
• Quality control metadata (e.g., PDOP, number of satellites used, orbit type, etc.)

The introduction to the new format along with technical specifications and examples are on the NGS website.

The National Society of Professional Surveyors (NSPS) works directly with NGS to provide input on maintaining and updating the National Spatial Reference System and will include significant assistance with educating geospatial data providers with the upcoming 2022 datum change and implementation of the North American Terrestrial Reference Frame of 2022 (NATRF2022).

Image: NGS

You can send your feedback here.

For more information, visit the NGS website.

Takeaways from this geospatial refresher…

The surveying, mapping and geospatial professions have exciting times ahead with these cool upgrades from NGS, so we need to take advantage of the calm before the storm to educate ourselves to make the most of the opportunity.

Geospatial data surrounds all of us, and we are the profession specifically educated for correctly and efficiently keeping a handle on it all. It all starts with growing your knowledge a little bit each day. Please join me in growing the profession as well.