Why Terrestrial Reference Frame and not Datum?
NATREF2022 stands for “North American Terrestrial Reference Frame of 2022”. It is going to be the new national reference, replacing NAD83. So why “Terrestrial Reference Frame”, and not “Datum”? On the NGS web site, the page that has all the information about the new systems is titled “New Datums”, so one might infer that they mean pretty much the same thing; they do. The difference is at an academic level. Geodesy is an interesting field because there are subtle nuances to word definitions, and slight differences to how those words are used in other mathematical sciences such as geometry. “Datum” in a mathematical sense, is simply a singular form of “data”. In geodesy, this indicates a single point from which to begin measurement in a relative measure. Classically, our geodetic datums are formed from the location of a single place of reference such as an astronomical observatory. In modern systems, they are formed by a network of points that are geometrically related into a single collective, a sum of many parts, rather than relying on the single point as an anchor definition. So rather than defining it by a single point out of many, it is recognized as a geometric network, and the reference that network provides is a Geometric Reference Frame.
I’m going to say it: Conceptually, a geometric reference frame is just a new datum.
To the GIS practitioner, map maker, or surveyor, they provide the starting point and context for our relative descriptions of location. Geometric Reference Frame is currently the popular term in geodesy. It is academically appropriate and conveniently serves as a way to make the new name different from the old, which in this case I can get behind. Can you imagine reading someone’s sloppy handwritten field notes of NAD27 vs NAD22? It would invite disaster. Sometimes, change for the sake of change is not a bad thing. So aside from a mouthful, what are we getting?
From “Fixed” to Time-Based Reference Frames
There are actually going to be four new reference frames: One each for the Continental US/Canada/Mexico; the Mariana [tectonic] plate; the Pacific plate; and the Caribbean, each with similarly abbreviated names. We’ve never had that kind of unified coverage before, so that’s pretty cool. Each of these frames will be plate-fixed, but also, at the time of realization, geocentric. This gets right to the heart of why this is happening now. As it turns out, NAD83 wasn’t as geocentric as intended when it was created. That is to say, the middle of the datum should theoretically have been at the geocenter but it wasn’t; it was off by about two meters.
Over time, with tectonic motion, the effect of this offset grew and its effect on surface positions could no longer be ignored. What does that mean? Well, most of our positioning work in modern times is done based on GNSS devices (Global Navigation Satellite System), GNSS by nature is geocentric since the positions are calculated from satellites which orbit the center of mass of the planet. If our national reference frame is not geocentrically related, then it is not directly compatible with GNSS. As motion continues into the future, the new models will acknowledge this and will dynamically change over time following the rotations and motions of the plates. This is necessary because if we are working on the surface of a plate that is moving relative to the geocenter, we need to track that motion if our survey devices stay with the geocenter. So once again, the new models are fundamentally different from the old and a significantly different name will really help to acknowledge that. This is going to require a new mindset for a lot of GIS users. Right now, many still deal with coordinates in “fixed” reference frames where we may acknowledge a reference epoch (date), but that date isn’t actually used for anything other than metadata. Time-based coordinates are inevitable in the future, so it’s time to start getting comfortable with them.
One question I heard directed to the NGS at the Summit was along the lines of, “If we’re just going to have to update again in a few years, why don’t you fix the problem at 2022 so we don’t have to deal with it again?” The problem here is not with the system that needs to be updated (with the implication being that it is flawed now), but in our understanding of the system we’re moving to. We are currently using a system in which we don’t acknowledge that things move and a lot of people have come up through their careers comfortable with there being a fixed relationship between any two given coordinate systems. We are moving to a system where time is not only a factor, but is fully acknowledged as necessary in a moving system. Data epoch is no longer optional. We need to know where our data was and when it was there in order to know where it is a few years later.
Under the hood of this new name NATRF2022, we are adding an entire dimension of measurement, and that’s far more exciting than adding a few new words in the name of the datum.
Preparing for the New Reference Frames of 2022
Over the next few years, we will need to make a few fundamental changes to GIS in order to be ready. First and foremost, we need to make sure our colleagues are comfortable with the new terminology and the concepts of time itself as being an important part of position. After the new systems are in place, we will likely also have new projected coordinate reference systems to deal with. It is very likely that we will have new versions of the US State Plane coordinate system zones. Furthermore, many states are undergoing a push to support new Low Distortion Projections such as the efforts in Minnesota, Wisconsin, Oregon, Iowa, and others. With 4 new plate models, we’re also going to have new Coordinate Transformations to relate them to each other and the older systems, the new reference frames will require it.
As a key player in the geospatial software industry, Blue Marble is already working on changes to our software in preparation for the upcoming new reference frames. Much of this will be invisible in our tools for the time being, while other components are already there, such as epoch settings, transformations that are not stuck to WGS84, and the ability to dynamically bring in new parameters to the database. We have been paying attention and are ready for the coming changes and will strive to help our users be ready, too, as we all learn exactly what these new reference frames will look like over the next five years. As an industry, we have grown very comfortable and perhaps complacent with our systems and transformations in the US for some time. Change is coming, and the time to prepare is now.
Sam Knight is the Director of Product Management for Blue Marble Geographics. With Blue Marble for over 13 years, Sam has lead hundreds of GIS and Geodetics courses and is a frequent speaker at industry conferences, trying to make tricky geodetics concepts accessible at a practical level.