New Datum Definitions for Modernization of the U.S. National Spatial Reference System
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The Need for a New Geometric Datum
To support improved GNSS positioning a new geometric datum is required. An excerpt from the aforementioned NGS 10-Year plan states "NGS [will redefine] the national horizontal datum to remove gross disagreements with the ITRF" (the ITRF is defined below). While the benefit from aligning NGD to ITRF is significant, the deployment of NVD requires a new geometric datum.
The geoid is one of an infinite series of progressively enveloped (essentially parallel) non-intersecting gravity geopotential surfaces, whose limit in the minimum is Earth’s mass center. To use a geometric datum that is not accurately referenced to the Earth’s mass center for ellipsoid heights, such as NAD 83, reduces the effectiveness of NVD (being geoid-based) and will continue to introduce error in the determination of orthometric heights with GNSS technology. To best implement and utilize the NVD, the NGD origin must be at the best estimate of the Earth’s mass center. Otherwise an ellipsoid height transformation will be required.
Note that the NGD datum will not be another realization of NAD 83, e.g. NAD 83(2022), but rather an entirely new datum.
In the early 1980’s, when the NAD 83 and WGS 84 datums were originally defined, our knowledge of the location of the Earth’s mass center was at an uncertainty of about one to two meters. Since then improved technology (e.g., more artificial satellites and more accurate tracking of those satellites) has allowed a refined estimate of the position of the Earth’s mass center. The offset between the location adopted for NAD 83 and the most accurate contemporary location is 2.2 meter (as defined by ITRF 08, or the International Terrestrial Reference Frame of 2008). The magnitude of the position shift between NAD 83 and the ITRF 08 at time 2022.00 (January 1, 2022) is shown in Figure 10. Note that this shift does not necessarily indicate the final horizontal position difference(s) between NAD 83 horizontal positions and NGD horizontal positions, but it does indicate the impact of changing the origin to the current best estimate of Earth’s mass center. Other factors, such as crustal motion or a change in the definition of ITRF, will affect the magnitude of the horizontal shift from NAD 83 to NGD. Additional information on ITRF in particular (and global reference systems in general) can be obtained from the International Earth Rotation and Reference System (IERS) at www.iers.org, and from the International GNSS Service (IGS) at igscb.jpl.nasa.gov/.
Since ITRF is a truly global reference system that is entirely compatible with GNSS, there is great benefit from minimizing the differences between the ITRF and a contemporary national geometric datum. Because of this, NGD will be defined to be as coincident as practicable with a modern realization of ITRF.
However, there is a complication. ITRF is fixed with respect to the overall Earth (using a no-net-rotation condition with regards to horizontal tectonic motions over the whole Earth), rather than to any specific tectonic plate. Because of this, tectonic movement of the North American plate causes ITRF coordinates to constantly change throughout the conterminous United States and Alaska, not just in locally tectonically active areas, such as western California. Similar issues exist for Hawaii and American Samoa (on the Pacific plate) and Guam and the Commonwealth of the Northern Mariana Islands (on the Mariana plate). Clearly there are significant advantages to a national datum whose coordinates do not constantly change. Because of this issue, there is currently a debate as to whether or not NGD will be fixed to the North American plate (also referred to as the Stable North American Reference Frame, or SNARF), so that most locations within the United States would not be subject to constantly changing coordinates (note that the horizontal rate of change of current ITRF coordinates within the stable part of the U.S., i.e. the part located on the North American plate rather than the Pacific plate, is about 2 cm/year). Another option to avoid continuously changing coordinates is to define NGD with respect to a specific date or "epoch" of ITRF.
To help visualize this coordinate change, Figure 11 (Craymer, et al., 2007) depicts the velocities, due to plate motion with respect to ITRF 2005, at stations in North America. The velocity vectors shown are a good indication of the rate of coordinate change one would expect if NGD were fixed to ITRF, rather than the SNARF or a specific ITRF epoch. Note that locations, such as portions of California located west of the San Andreas fault system, that are not on the North American plate will still see coordinate changes due to relative plate motion even if NGD is fixed to the SNARF.
Two benefits from aligning the origin of NGD to that of ITRF are improved GNSS positioning and a better definition of NGD. Because GNSS orbits are global, GNSS orbits are determined within ITRF, or a frame closely aligned and consistent with ITRF, such as that defined by the International GNSS Service (IGS) for precise orbits. Currently, high-accuracy GNSS surveying in the U.S. requires the vector processing and determination of differential station positions be done in ITRF/IGS. Once the vector processing is completed, a transformation and adjustment are performed to convert the ITRF/IGS station coordinates to NAD 83. ITRF is defined and monitored by use of GNSS as well as other independent space geodesy techniques such as Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). Therefore, if NGD is precisely aligned with ITRF, the relationship of NGD to all space-based systems is precisely monitored and maintained with minimal cost and error.
Figure 12 shows the difference in ellipsoid heights between NAD 83 and ITRF 08 (as ITRF 08 minus NAD 83). Note that while the new geometric system will change a station’s ellipsoid height as shown in Figure 12, this change will not cause a significant difference in its orthometric height (as shown in Part 3 of this series) since the vertical reference surface, i.e. the adopted reference geoid, will also "shift" along with the NGD ellipsoid.
In its 10-year plan, NGS states it will provide a transformation tool between the contemporary realization of NAD 83 and NGD (contemporary at the time of adoption of NGD). Note that there is not currently an NGSsanctioned transformation between the NAD 83 High Accuracy Reference Network (HARN) realizations and the NAD 83(NSRS2007) realization (although such a transformation is currently under development by NGS). Therefore, at present, to reliably transform high-accuracy geospatial data based on realizations prior to NAD 83(NSRS2007) to NGD, the user must first migrate those data to NAD 83(NSRS2007) through processes that will, most likely, require additional survey work (or readjustment of existing GNSS vectors). For those facing this problem, the NGS utility OPUS (Online Positioning User Service, www.ngs.noaa.gov/OPUS/) could be an extremely cost-effective part of their solution until NGS releases the new transformation tool in mid-2012.
Much work on improving NAD 83 itself remains to be done long before the NGD replaces it in 2022. Improved CORS coordinates in NAD 83 have been computed and issued in 2011, and a new set of passive c
ontrol coordinates (based on GNSS technology and tied to CORS) will be published in June 2012. That is, there is a new realization of NAD 83 (identified as NAD 83(2011) epoch 2010.00) based in the improved CORS coordinates, and it is the basis of a new nationwide adjustment of passive control referred to as the National Adjustment of 2011 (NA2011). For more information on the NA2011 Project, see www.ngs.noaa.gov/web/surveys/NA2011/.
The NA2011 Project will keep the passive control consistent with a new realization of the CORS network, referenced to an epoch date of 2010.00 (this is the new Multi-Year CORS Solution, which is based on new orbits and absolute GNSS antenna models, among other improvements). Shortly after the new realization of NAD 83 is released, a tool will be developed to perform coordinate transformations between NAD 83(NSRS2007) and NAD 83(2011). This is in addition to the previously mentioned transformation under development by NGS between NAD 83(NSRS2007) and the various HARN realizations.
To gauge the user community’s ability to adapt to and adopt the new NVD and NGD, in May 2010, NGS hosted their first Federal Geospatial Summit on Improving the NSRS (Smith, 2010; NGS, 2011b); the next summit is in July 2012 (see www.ngs.noaa.gov/2012GeospatialSummit/ for more information). One option presented at the 2010 Summit was to define and adopt NGD before NVD. Definition of a new geometric datum and transformation of NSRS coordinates to that datum are relatively easy to achieve, whereas the definition of NVD requires completion of the GRAV-D program. This would allow NGS, along with its customers and partners, to enjoy the benefits of a fully modern NGD geometric reference system without waiting until the completion of GRAV-D in 2022 (or later). Obviously, adoption of the NGD geometric datum prior to NVD would require generation of new hybrid geoid models to be used with NGD consistent with NAVD 88, since the NGD ellipsoid heights will, as shown in Figure 12, differ significantly from NAD 83 ellipsoid heights. For the purposes of minimizing impacts to the user community by going through two changes at different times, NGS has decided to reject this option and stick to the plan of a 2022 joint release.
The State Plane Coordinate System
The NAD 27 State Plane Coordinate System (SPCS) was first defined, at the request of and in collaboration with the user community, by NGS in the 1930s. Since the introduction, also by NGS, of the NAD 83 SPCS, advances in technology (e.g., computers with geospatial software that allow "re-projection-onthe-fly") have diminished the value of NGS support of the SPCS or any other "general coverage" projected coordinate system. This seems especially relevant since few (if any) practitioners use SPCS for the reason it was originally intended (to allow surveyors to perform "geodetic" surveys using plane trigonometry). SPCS is still used as a projection for Geographic Information Systems (GIS), other mapping products (such as USGS quadrangles), and by some government and private organizations for surveying and engineering projects. However, many users of SPCS for precise positioning (especially those in high elevation areas) use a "modified" version that is "scaled" to reduce map projection distortion (a practice which compromises accurate georeferencing of spatial data, among other problems). As such, it can be argued that the utility of SPCS is decreasing in many geographic areas, and therefore a new SPCS based on NGD may not be warranted, or at least as projected coordinate systems covering large geographic areas (with the accompanying relatively large linear distortion). The nominal distortion of SPCS is 1:10,000 (100 parts per million), but this is with respect to the reference ellipsoid (more-or-less at "sea level"); much greater distortion occurs at high elevations (e.g., up to about 400 parts per million at an elevation of 6000 feet for SPCS). Given the evolution in user requirements and capabilities for projected coordinates (such as minimizing distortion), NGS has not yet determined how or whether an NGD version of SPCS will be developed.
As with the implementation of an SPCS based on NAD 83, definition of a new SPCS as part of NGD, if done, would likely be based on new projection parameters. Doing so helps differentiate between SPCS based on the NGD, NAD 83, and NAD 27 datums by creating large differences in projected coordinates. Such an update of the SPCS will likely require changes in statute in many jurisdictions of the U.S.
Invitation for Comment or Inquiry
Readers who wish to comment on the definition and implementation of NGD are invited to send their comments, or questions, to: Joe.Evjen@noaa.gov. You are also invited to contact your nearest NGS Geodetic Advisor (www.ngs.noaa.gov/ADVISORS/AdvisorsIndex.shtml) for more information or to comment on the new datum.
References for this four-part series are available in the Exclusive Online-only Content area of amerisurv.com
David Minkel is a geodesist in the Geodetic Services Division of the National Geodetic Survey. Formerly the NGS Geodetic Advisor to Arizona for 12 years, he has worked in NOAA’s charting and geodetic services for the last 36 years.
Michael Dennis is a geodesist at NGS and is currently manager of the National Adjustment of 2011 (NA2011) Project. He is also a licensed engineer and surveyor, including ownership of a consulting and surveying firm.
A 1.785Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE