The NGS discusses the upcoming national spatial reference framework modernization.
Reference framework shift happens. The National Geodetic Survey (NGS)—the federal steward of the National Spatial Reference Framework (NSRS)—is planning a major (if not fundamental) update of the framework. This pending update is common knowledge among geodetic, surveying, and other geospatial communities.
Among the information circulating about the replacement of NAD 83 (North American Datum 1983), NAVD 88 (North American Vertical Datum of 1988), and IGLD 85 (International Great Lakes Datum of 1985) is plenty of high-level and foundational science. We have our ears to the ground at xyHt, and there is nevertheless a lot of apprehension among end-user constituencies.
We recently interviewed Dr. Dru Smith, NSRS modernization manager at NOAA-NGS, so we could pose some of the common, non-academic questions we hear from these end users. Our goal is to assure folks that the change will not turn into “Catch-2022.”
xyHt: After the new reference framework is adopted, models and mechanisms will be updated (OPUS, NADCON, GRAV-D, HTDP, et al.) so that end users will be “carrying the best control point with them” (as we’ve heard in NGS presentations and webinars). Will they be able to resolve the truest and most current geodetic value right then and there?
Dru Smith: Actually, that day is already here. Because the location of active control (Continuously Operating Reference Stations, or CORS) are regularly monitored and near-real-time GNSS positioning accuracy has improved steadily, a person using a Real Time Network (RTN) that is aligned with the NSRS can achieve centimeter-level positioning accuracy of NSRS coordinates at the epoch of their survey within seconds. Users requiring greater accuracy can use tools in the OPUS (Online Positioning User Service) suite with longer occupations to achieve cm or even sub-cm accuracies.
xyHt: What is the best and official name for the new reference framework? People hear different things (usually third-hand), like, “We’re moving from NAD83 to ITRF,” or “We’re going to be closer aligned to IGS,” or “We’ll be on WGS84 like the military.” What is the best shorthand version?
DS: NGS has been discussing this issue for years now without a final answer. We want to get this right. There are at least two issues that come up regularly:
- Using “North American” is no longer appropriate. NGS currently defines the “North American Datum of 1983” in American Samoa, Guam, and CNMI, places that are far away from the North American continent and not on the North American tectonic plate.
- Using “USA” or “American” are probably not good options either. NGS would like the new frames to be useful to, even adopted by, multiple nearby countries to improve cross-border consistency, and they should therefore not be named in a USA-centric fashion.
For now, NGS continues to call them the New Geometric Reference Frame and the New Geopotential Reference Frame, which I’ll shorten to NGeRF and NGmRF for the rest of the questions.
xyHt: The most common question we hear is, “What does the NGS mean by purely gravity-based datum?”
DS: It means that the geoid model in 2022 will not be a hybrid geoid. Currently, NGS takes a two-step procedure to provide a geoid model that can serve as a conversion between NAD 83 ellipsoid heights and NAVD 88 orthometric heights.
In step one, NGS computes a geoid model from purely gravity-based sources (a “gravimetric” geoid). In step two, NGS warps and twists that model, using GPS on benchmark data, to introduce the known systematic errors of NAD 83 and known systematic errors of NAVD 88 into that geoid. This second step yields a “hybrid geoid.” Users interested in converting height back and forth between NAD 83 and NAVD 88 use the hybrid geoid.
In 2022, the geoid, serving as the conversion between ellipsoid heights and orthometric heights, will rely solely on gravimetric data, and the concept of a “hybrid geoid” will go away.
xyHt: Will there be a new orthometric height value published for most benches, and will there simply be a tool like there was to convert between, say, NGVD29 and NAVD88?
DS: A new “VERTCON” will be provided that models the differences between the NAVD 88 orthometric heights and those of the NGpRF. This will function exactly like the NGVD 29 to NAVD 88 conversion.
In a similar fashion, the only place the new VERTCON will be applied is to marks that have not been surveyed (in this case, using GNSS) recently enough to get an updated NGpRF orthometric height. Users who care about the accuracy of the orthometric height of a mark are encouraged to survey that mark with GNSS and submit the data to NGS so that an updated NGpRF orthometric height can be computed and published for it.
xyHt: People ask, “What do I need to do work in the new framework?” Is it wrong to answer, “A lot of what we should have been doing all along”?
DS: That isn’t a bad answer at all. But one philosophic change should come with it: NGS wants users to trust to active control as much as possible. That is, step one of performing a geodetic survey should not be “go find passive control around my area.” This is because NGS considers such control to be “tied to” the NSRS, but not truly “part of” that NSRS.
Once a surveyor takes their instrument off of a passive control point, knowledge of that point’s location begins to grow stale. This does not happen with active control. This is one reason that NGS is so eager to work with RTN administrators: it would be beneficial to the entire geospatial community if every RTN could be validated as “aligned with” the NSRS to some quantifiable level of agreement.
xyHt: The NGS is providing a lot of essential tools for working in the new framework, but what are the best things that states and localities can do to prepare? If, say, a state were to allocate budget to prepare for (and continue to work in) the new environment, what would be the best bang for the buck (e.g. GPS on benches, height mod, more CORS, establishing new reference marks …)?
DS: The best preparation we can recommend now is two-way communications. NGS is ramping up the amount of information we’re providing to the public, but it must go both ways. If there are significant concerns that NSRS users have about the change, NGS needs to hear about them early and often. That will give us the opportunity to consider them and provide an appropriate response.
However, the biggest bang for the buck will be to get involved in GPS on benchmark campaigns, especially as NGS ramps up the national campaigns in the late 2010s. Having accurate GNSS data on a published vertical benchmark, as near to 2022 as possible, will give us the data necessary to update VERTCON as part of the 2022 change.
xyHt: Except for places like the West Coast and Alaska, many folks have never really had to deal with substantial velocity. Will the plate-fixed era of NAD 83 come to an end, and will many more people have to deal with velocity? Will the “4th D: temporal” become more important than ever?
DS: Yes and no. We have heard some very positive things from the user community about their ability to move to a more dynamic NSRS, but not everyone has the tools to deal with moving coordinates. As such, NGS is considering an approach that will split the difference.
For instance, coordinates at their most fundamental level will remain in the IGS frame with IGS coordinates; that may look like (or be identical to) the IGS coordinates and velocities. From these, NGS should be able to easily compute the secular plate-wide motion (the “plate rotation”). Such motion can be removed from a moving coordinate so that it has the appearance of being plate-fixed (or epoch-fixed, depending upon how one looks at it), at least in latitude and longitude.
But that leaves two very important dynamic signals in the NSRS still: height changes and episodic movements (such as earthquakes).
Height changes cannot be easily modeled the way a plate rotation can be; they are notoriously localized features. NGS got users used to the ability to remove earthquake movements by introducing the Horizontal Time Dependent Positioning (HTDP) program, which allows users to determine their coordinates at different epochs, through various models of earthquakes, tectonic rotation, etc.
NGS, however, has never been in the business of actually modeling that movement ourselves. We receive such models from other scientists and have introduced them into HTDP. It is not clear that this will continue in the future. NGS considers the re-surveying of points as “best practice” when movement is suspected rather than reliance upon a model of movement. [Ed: re-surveying is not necessarily limited to re-observation by conventional traverse or level runs, but through tools such as OPUS, OPUDS projects etc.]
xyHt: The NGS is providing a lot of essential tools for working in the new framework, but what are the best things that states and localities can do to prepare? If say a state was to allocate budget to prepare for (and continue to work in the new environment); what would be the best bang for the buck? (E.g. GPS on benches, height mod, more CORS, establishing new reference marks…?)
DS: The best preparation we can recommend now is two way communications. NGS is ramping up the amount of information we are providing to the public, but it must go both ways. If there are significant concerns which NSRS users have about the change, NGS needs to hear about them early and often. That will give us the opportunity to consider them and provide an appropriate response.
However, the biggest bang for the buck will be to get involved in GPS on benchmark campaigns, especially as NGS ramps up the national campaigns in the late 2010’s. Having accurate GNSS data on a published vertical benchmark, as near to 2022 as possible, will give us the data necessary to update VERTCON as part of the 2022 change.
xyHt: Except for places like the west coast and Alaska, many folks in the U.S. have never really had to deal with substantial velocity. Will the plate fixed era of NAD 83 come to an end and will many more people have to deal with velocity? Will the “4th D – temporal” will become more important than ever?
DS: Yes and No. We have heard some very positive things from the user community about their ability to move to a more dynamic NSRS, but not everyone has the tools to deal with moving coordinates. As such, NGS is considering an approach which will split the difference. For instance, coordinates at their most fundamental level will remain in the IGS frame with IGS coordinates; that may look like (or be identical to) the IGS coordinates and velocities. From these, NGS should be able to easily compute the secular plate-wide motion (the “plate rotation”). Such motion can be removed from a moving coordinate so that it has the appearance of being plate fixed (or epoch fixed, depending upon how one looks at it), at least in latitude and longitude.
But that leaves two very important dynamic signals in the NSRS still: height changes and episodic movements (such as earthquakes). Height changes cannot be easily modelled the way a plate rotation can be – they are notoriously localized features. NGS got users used to the ability to remove earthquake movements by introducing the Horizontal Time Dependent Positioning (HTDP) program, which allows users to determine their coordinates at different epochs, through various models of Earthquakes, tectonic rotation, etc. NGS, however, has never been in the business of actually modelling that movement ourselves. We receive such models from other scientists and have introduced them into HTDP. It is not clear that this will continue in the future. NGS considers the re-surveying of points as “best practice” when movement is suspected, rather than reliance upon a model of movement. [Ed: re-surveying is not necessarily limited re-observation by conventional traverse or level runs, but through tools such as OPUS, OPUDS projects etc]
xyHt: You have said in past seminars that the “geoid cannot be bulldozed”. If the Earth’s center and geoid model are the ultimate single monument, but those are modelled, who are the parties that agree upon a center, and how do they determine it and does it never change?
DS: NGS has relied upon the International GNSS Service (IGS) to define the center of the reference frame. Currently NAD 83(2011) has a mathematical relation to the latest IGS frame (IGS08) that contains over 2 meters of translational offset. That is, NAD 83 is not geocentric. When the new geometric reference frame (name still to be determined) is released in 2022, NGS anticipates that it will share the frame origin with whichever IGS frame is current at that time, at some specific epoch. Thankfully knowledge of the location of the IGS frame center has converged over the years and any changes are expected to be very small.
xyHt: The NGS performed some rigorous tests on GRAV-D’s ability to yield an accurate geoid model. This included post processed GNSS, RTN, etc. There is a segment of the end user constituencies that will not believe that (a) one could ever get 2 cm vertical with GPS and (b) that a hybrid geoid model could yield a 2 cm orthometric height. What say you to those folks? And aren’t things only going to get better with modernized constellations?
DS: What I would say is seeing is believing. As part of the Geoid Slope Validation Survey of 2011 (GSVS11) NGS proved the following, without any doubt: In the coastal region of Texas the addition of GRAV-D’s airborne gravity to current gravimetric geoid modelling yielded differential geoid undulations with an accuracy of 1 cm over all distances from 0 to 300 km. NGS is continuing to quantify the impact of GRAV-D by analysing GSVS14 in Iowa (a higher region than Texas, but still topographically flat) and the planned GSVS16 in Colorado (a high, rugged terrain). [Ed: see the article on the NGS study in Iowa in the January issue of xyHt – http://goo.gl/bZ8E7V] NGS considers the accuracy of differential heights to be the critical target, as absolute heights have (by necessity) some academic, but generally unimportant, issues when being defined. NGS believes that most users care about the relationship of heights, not their absolute values.
Differential GNSS combined with differential geoid undulations will yield differential orthometric heights. However, the accuracy achieved with geodetic levelling is expected to remain better than GNSS/geoid levelling, at least over distances less than a few dozen kilometers.
Those with lingering concerns about GRAV-D’s accuracy are invited to read the details of GSVS11 at: https://goo.gl/8LHuSx
xyHt: Many people tend to consider a very old published value as being kind of “god points”. But those old realizations were quite noisy (not to mention changes since due to velocity, subsidence, etc). How can we get people to get past those notions and to accept geodesy resolved via observation?
DS: Historic points carry significant interesting information and can certainly help inform us about how historic maps, surveys or charts might differ from their true positions today. But with GNSS accuracy capable of showing even the slightest changes to positions over time, NGS views the reliance upon coordinates at passive control to be of less reliability than active control. NGS will continue to perform accuracy studies, and will plan to provide guidelines to users before 2022, but we can only show people the door. They will have to walk through on their own.