National Adjustment of 2011 (NA2011) Project
Frequently Asked Questions (FAQs)
Last updated April 30, 2012
List of abbreviations used in these Frequently Asked Questions:
| CNMI | Commonwealth of the Northern Mariana Islands |
| CONUS | Conterminous United States |
| CORS | Continuously Operating Reference Stations |
| FGDC | Federal Geographic Data Committee |
| GNSS | Global Navigation Satellite System |
| GPS | Global Positioning System |
| HTDP | Horizontal Time-Dependent Positioning (NGS software) |
| IGS | International GNSS Service |
| MYCS | Multi-Year CORS Solution |
| NA2011 | National Adjustment of 2011 |
| NAD 27 | North American Datum of 1927 |
| NAD 83 | North American Datum of 1983 |
| NAVD 88 | North American Vertical Datum of 1988 |
| NADCON | North American Datum CONversion (NGS software) |
| NGS | National Geodetic Survey |
| NGSIDB | National Geodetic Survey Integrated Data Base |
| OPUS | Online Positioning User Service |
| VERTCON | VERTical CONversion (NGS software) |
Questions:
1. What is the intent of the National Adjustment of 2011 (NA2011) Project, and what is it called?
2. Why is NGS performing a national adjustment so soon after the last adjustment?
3. Is NA2011 an adjustment or a readjustment?
5. What are "passive" and "active" control stations?
6. When will the NA2011 Project be completed?
7. Does this mean NGS will have created a new datum once NA2011 is complete?
9. What will the new, resultant realization of NAD 83 be called?
11. What does the term "epoch 2010.00" mean/signify?
13. Are the CORS and CORS96 datum tags for NAD 83 the same?
14. What passive control stations will be included in (affected by) this adjustment?
17. Are all CORS included in the MYCS? If not, what CORS are excluded?
18. Will all CORS with MYCS coordinates be constrained (i.e., used as control) for NA2011?
19. What is the expected magnitude of the position/height shifts, when compared to NAD 83(NSRS2007)?
22. Will all stations with NAD 83(2011) coordinates have very small (near zero) velocities?
25. What stations will NOT be included in NA2011?
Questions added on November 8, 2011:
31. When will projects submitted after the August 31st NA2011 cutoff date be published?
Answers:
1. The National Adjustment of 2011 Project is abbreviated "NA2011 Project" or "NA2011." It is a nationwide, geometric adjustment of passive stations, with positions determined using Global Navigation Satellite System (GNSS) technology, which includes the Global Positioning System (GPS). NA2011 is called "geometric," because it will yield new values for latitude, longitude, and ellipsoid height; it will NOT affect the orthometric heights of stations. However, a future nationwide vertical adjustment is being considered to determine GNSS-derived orthometric heights, based on the results of NA2011. (Additional information on this is provided in the questions below).
2. NGS has performed a reanalysis of Continuously Operating Reference Station (CORS) data, in conjunction with an international effort coordinated by the International GNSS Service (IGS). This reanalysis, known as the Multi-Year CORS Solution (MYCS), has yielded new geometric coordinates and velocities on CORS using data from 1994 through the present. For NGS products and services to be mutually aligned, it is necessary to perform an adjustment (constrained to the new MYCS coordinates) of as many of the GNSS vectors held in the NGS Integrated Data Base (NGSIDB) as possible. These vectors represent GNSS observations between passive stations, and between passive stations and CORS (i.e., active stations). The vectors that are tied to CORS will make it possible to determine new coordinates on passive control consistent with the MYCS. More information on the MYCS is available on the NGS CORS Coordinates and MYCS FAQ web pages.
3. The terms "adjustment" and "readjustment" are, for all practical purposes, synonymous. Other terms, such as "improvement" and "update" are also applicable. NGS has traditionally leaned toward the term "adjustment" for projects updating the coordinates of multiple points simultaneously, because the process is via a statistical tool called a "least squares adjustment." The use of "re" implies a second adjustment, setting a poor precedent and begging the question of whether the next project should be a "re-readjustment." Moreover, NA2011 contains approximately 30 percent more vectors than the 2007 National Readjustment (nearly all of these additional vectors are from projects performed since the November 2005 cutoff for the 2007 project). Because NA2011 includes substantially more vectors than the 2007 project, it represents a new set of observations and in that sense be considered essentially a new adjustment. For these reasons, NGS has chosen to simplify matters, calling the NA2011 Project an "adjustment."
4. Please see the previous question. For the NAD 83(NSRS2007) National Readjustment, NGS had not considered the confusion that might be caused by using the term "readjustment," rather than simply "adjustment."
5. A passive station is, in NGS terminology, a "conventional" ground station, e.g., a brass disk set in a substantial structure, a steel rod driven vertically into the ground to refusal, or other such stable physical marks that can be occupied with survey equipment. An active station is a GNSS antenna (and associated receiver) in a fixed location, such as a CORS.
6. NGS' goal had been to complete the adjustment phase of the NA2011 Project by the end of calendar year 2011. But final results will be officially released later (target of June 30, 2012), to allow time for updates of all affected NGS products and services (such as a new hybrid geoid model, as discussed in question #20). Because there are numerous technical and logistical challenges in executing a project of this magnitude, it is difficult to estimate the time necessary to accomplish all the required tasks. Rather than set an exact date, project completion will instead be governed by when criteria for quality and completeness-both for the adjustment itself and for its implementation-have been met. Updates to the Project time line will be posted on the NGS website.
7. No. The NA2011 Project will update coordinates within the geometric reference frames (previously referred to as "horizontal datums") which NGS already defines and maintains. There are three of these geometric frames, all named "NAD 83," but with each affixed to either the North American, Pacific, or Mariana tectonic plates, accordingly. (Please see the following questions for more information on frames fixed to different tectonic plates). The proper terminology is to refer to the new sets of coordinates on CORS and passive control as new "realizations" of these three existing reference frames.
8. This is a highly complicated, but frequently overlooked topic, relevant to the names of the new realizations that will follow the MYCS and NA2011 Projects. Specifically, NGS currently defines and maintains three different reference frames, each affixed to a different tectonic plate. The most recent realizations of these, prior to the completion of MYCS and NA2011 are:
| North American Plate: | NAD 83(CORS96) | active control | |
| NAD 83(NSRS2007) | passive control | ||
| Pacific Plate: | NAD 83(PACP00) | active control | |
| NAD 83(1993) | passive control (Hawaii) | ||
| NAD 83(2002) | passive control (American Samoa) | ||
| Mariana Plate: | NAD 83(MARP00) | active control | |
| NAD 83(2002) | passive control | ||
9. Both the MYCS and the NA2011 Project will provide coordinates for the 2011 realizations of the North American Datum of 1983 (NAD 83), affixed to each of three tectonic plates. (Please see the previous question). The names of these new realizations are:
| North American Plate: | NAD 83(2011) | active and passive control | ||
| Pacific Plate: | NAD 83(PA11) | active and passive control | ||
| Mariana Plate: | NAD 83(MA11) | active and passive control | ||
The parenthetical portion (2011, PA11, or MA11) of the realization titles is also referred to as the "datum tag." The same datum tag is used for both active and passive control, because the MYCS is the basis for NA2011 passive control, and the realizations will be consistent enough with one another to remove the necessity for different datum tags. To fully specify the realization, the name (and datum tag) should be followed by an epoch date, for example "NAD 83(2011) epoch 2010.00." The epoch date is described in more detail in a later question.
10. Yes. NA2011 will yield coordinates for all areas where acceptable results are obtained for GNSS passive control tied to CORS included in the MYCS. However, as mentioned in the previous question, the datum tag used for the station coordinates will depend on the tectonic plate used as a fixed reference frame. Three fixed-plate reference frames are currently defined by NGS, and they will be used for both active and passive control with the following naming conventions:
a. "NAD 83(2011) epoch 2010.00" is the realization of the reference frame fixed to the North American tectonic plate. This realization is used for the vast majority of CORS and NA2011 passive marks, and it includes all stations on, and nearly adjacent to, the North American and Caribbean tectonic plates. These stations are located in the conterminous United States (CONUS), including California, and this realization also includes Alaska, Puerto Rico, and the US Virgin Islands.
b. "NAD 83(PA11) epoch 2010.00" the realization of the reference frame fixed to the Pacific tectonic plate, and it consists of stations located in Hawaii and American Samoa.
c. "NAD 83(MA11) epoch 2010.00" " is the realization of the reference frame fixed to the Mariana tectonic plate, and it consists of stations located in Guam and the Commonwealth of the Northern Mariana Islands (CNMI).
The NA2011 stations referenced to the Pacific and Mariana plate-fixed frames will be adjusted separately from the simultaneous adjustment of the NAD 83(2011) stations referenced to the North American plate. Note that stations in the Pacific and Mariana frames were not computed in the NAD 83(NSRS2007) National Readjustment.
11. The "epoch date" is the date associated with the coordinates of a control station. An epoch date is a necessary part of a complete datum or reference frame name, because coordinates can change with time (i.e., they often have non-zero velocities relative to some chosen, stable coordinate reference). For NA2011, the positions will be referenced to midnight January 1 of 2010 (epoch 2010.00). NGS has provided epoch dates for CORS and related products (such as OPUS) for many years, for all passive stations included in the 2007 national readjustment and for passive stations in California since 1992. This practice will continue for NA2011, and it will become more prevalent and consistent throughout NGS. Typically, the epoch date is displayed with two decimal places, although this may vary depending on the application. If the velocity of a station is known, then its position can be computed at different epochs; thus one datum tag may have multiple epochs. However, the accuracy of the computed position will depend on the accuracy of the published velocity, which varies by station and location. Epoch dates on some stations may also change without changing the datum tag, if a local episodic event (such as an earthquake) occurs, requiring a local adjustment to determine new positions. In summary, when providing coordinates, epoch dates are needed, along with the datum tag, to accommodate the precision of GNSS measurements and to recognize the dynamic nature of the Earth and its effect on coordinates.
12. In 2007, the use of separate datum tags was chosen to reflect the variety of subtle, yet significant, differences between the NSRS2007 and CORS96 realizations, despite the fact that the two realizations were very closely aligned (median values of 1 cm in horizontal and 2 cm in vertical). Unfortunately, the separate datum tags caused unnecessary confusion in the user community. For the MYCS and NA2011, NGS has chosen to use the same datum tag for both the active and passive realizations of NAD 83 in 2011. Firstly, efforts to improve consistency between passive and active stations-over and above those made in 2007-are being attempted with NA2011. Secondly, NGS hopes to avoid the confusion caused by the 2007 naming choices. It is NGS' intention to have one realization, accessed primarily through the active control, and secondarily through passive control, but with only one realization of NAD 83 at a time (per tectonic plate).
13. Yes. The CORS and CORS96 datum tags refer to the same realization of NAD 83, and CORS96 is the official datum tag for that realization. NAD 83(CORS) has been displayed on NGS datasheets for CORS to accommodate the fixed format of the datasheet. Note that for other NGS products (such as those associated with OPUS), this realization is displayed as NAD 83(CORS96). Furthermore, NGS has displayed NAD 83(PACP00) and NAD 83(MARP00) coordinates on CORS stations as "NAD 83(CORS)," as well, further confusing the situation. This will be remedied at the conclusion of the MYCS project.
14. GNSS observations (vectors) for all passive control stations in the NGS Integrated Data Base (NGSIDB) as of March 29, 2012 that are connected (directly or indirectly) to CORS with MYCS coordinates have been included in the Project (the observation dates for these vectors span from April 12, 1983 through December 21, 2011). However, being "included in the project" only assures the observations will be evaluated for their possible inclusion in the final adjustment. As with any survey control network adjustment, observations that do not "fit" well with other observations (i.e., "outliers") will be rejected. In some cases, this may result in passive control stations being excluded from NA2011, in which case they will retain their previous datum tag. Projects received by NGS through March 29, 2012 will be published with NAD 83(NSRS2007) coordinates until NA2011 is completed, at which time they will be published with NAD 83(2011) coordinates. Stations in the Pacific or Mariana tectonic plate areas will continue to use the most recently available NAD 83 realizations until NA2011 is complete. After NA2011 is completed, those projects received after March 29, 2012 will be individually adjusted to the new realization in the order received and published on NAD 83(2011), NAD 83(PA11), or NAD 83(MA11), epoch 2010.00, as the location warrants.
15. First, consult with NGS through the appropriate NGS State Geodetic Advisor or NGS headquarters personnel to ensure your project meets NGS requirements for projects performed during this time frame. (NGS contact information is available here). When performing the constrained adjustments, unless advised differently by NGS personnel, hold NAD 83(NSRS2007) coordinates for all published passive control and NAD 83(CORS96) coordinates for all CORS in the survey (for survey projects located on, or nearly adjacent to, the North American tectonic plate).
16. Yes, those stations will have network and local accuracies computed and published, along with the NAD 83(2011, or PA11, or MA11) epoch 2010.00 coordinates. As with the NA2011 Project itself, the horizontal and ellipsoid height accuracies will be provided in centimeters at the 95 percent confidence level, in accordance with the Federal Geographic Data Committee (FGDC) Geospatial Positioning Accuracy Standards, Part 1.
17. All CORS with NGS-archived GNSS data were included in the MYCS. The only CORS not included are those with no archived data, and they consist of only a small number of stations belonging to the now defunct Cooperative CORS category. Of these, the only CORS excluded were decommissioned prior to becoming part of the NGS CORS network and did not provide their historic GNSS data to NGS. Because the Cooperative CORS program is no longer in effect, all currently operating CORS are simply "CORS," and they are all included in the MYCS. A list of previously-operating CORS not used in the MYCS is available here.
18. It is likely that most, but not all, CORS with MYCS coordinates will be constrained for NA2011. The reason for this is that there are two categories of MYCS coordinates. The first category is for CORS (and IGS) sites with velocities computed from the GNSS observations at the CORS site itself. This requires processing of at least 2.5 years of data for the site, and NGS expects that all CORS in this category will be constrained for NA2011. For the second category of sites not meeting this criterion, there is not enough data to compute accurate velocities, so the velocities are modeled using the NGS Horizontal Time-Dependent Positioning (HTDP) software. These modeled HTDP velocities are also used to transform the coordinates of the sites from the mean time of the dataset to a common reference time (epoch). Because these MYCS coordinates are (in part) modeled, it is likely that at least some of them will not be constrained for NA2011. The determination of which CORS will be constrained in the final adjustment will be based on an analysis of the results. Regardless of whether or not CORS are constrained, none will be assigned new published coordinates based on NA2011; the published coordinates will continue to be from the MYCS.
19. Results from the MYCS provide insight into the positional changes between NAD 83(NSRS2007) and NAD 83(2011) epoch 2010.00. Note that, in making this comparison, NGS is treating NAD 83(CORS96) epoch 2002.00 as a proxy for NAD 83(NSRS2007), which is appropriate, as NAD 83(CORS96) was the basis for NAD 83(NSRS2007). Based on the preliminary MYCS results, the mean expected change is about 2 cm horizontally and 1 cm vertically (i.e., ellipsoid height). However, these changes-particularly the horizontal changes-vary significantly. In addition, the largest part of the horizontal change is due to propagation of positional velocities over the epoch difference, especially in tectonically active areas, such as California. When compared at a common epoch date (2002.00), the mean horizontal change decreases to only 0.2 cm. It is important to note that the coordinate changes are not evenly distributed across a region, and that shifts of many centimeters are expected to occur in some areas. Users are strongly encouraged to view the shifts in their locale by going to FAQ #6 on the NGS MYCS FAQ web page for a better indication of the coordinate changes at individual CORS.
20. Yes, a new hybrid geoid model (GEOID12) will be produced, and preliminary activities in this regard are currently underway. Development of this model first requires completion of NA2011, so that NAD 83(2011) ellipsoid heights are available for North American Vertical Datum of 1988 (NAVD 88) leveled control stations that have also been positioned in NA2011. NGS has decided that our user community would be best served if GEOID12 is released simultaneously with NA2011. It is mainly for this reason that the official release date of NA2011 will be postponed until GEOID12 is completed (target of June 30, 2012), as mentioned in question #6. However, if the development of GEOID12 is excessively delayed for some unforeseen reason, NA2011 may be released prior to completion of GEOID12. As with the NA2011 Project itself, completion of GEOID12 will be determined by meeting quality and completeness criteria rather than a deadline.
21. Possibly. NGS will investigate the need for, and feasibility of, performing a nationwide vertical adjustment of all GNSS-derived orthometric heights in the NSRS. If performed, the GNSS-derived orthometric heights determined for all stations in the vertical adjustment would be consistent with NAD 83(2011) epoch 2010.00 and GEOID12, and would be referenced to the North American Vertical Datum of 1988 (NAVD 88) for CONUS and Alaska. If pursued, this project would likely encompass similar orthometric adjustments in other US territories, tied to their regional datums (if they exist).
22. Most, but not all, of the NAD 83(2011) stations have very small, more-or-less randomly oriented horizontal velocities (on the order of 1 to 2 mm/year), because most are located on the stable part of the North American tectonic plate. However, stations located in tectonically active areas, and/or on other plates, may have significant velocities. The largest NAD 83(2011) velocities (exceeding 50 mm/year) occur in southwestern California on the Pacific plate (i.e., on, and west of, the San Andreas fault system). Other stations with considerable velocities occur along the Pacific Northwest coast of the United States (approximately 15 mm/yr) and south-central Alaska (up to 40 mm/yr). Stations located in Puerto Rico and the US Virgin Islands also have fairly large NAD 83(2011) velocities (about 20 mm/yr), since they are located on the Caribbean tectonic plate.
23. A vast majority of the station coordinates determined in NA2011 will be referenced to a frame fixed to the North American tectonic plate, and these will be denoted as NAD 83(2011). As stated in the previous question, most of these stations will have velocities of nearly zero relative to this frame. Similarly, users of NGS control far away from North America (such as Hawaii) typically prefer the coordinates change with time as little as possible. It is for this reason NGS developed reference frames fixed to the Pacific and Mariana plates, so the control coordinates would have local velocities of nearly zero. Otherwise, if the coordinates for Pacific and Mariana regions are referenced to NAD 83(2011), the velocities would be quite large (for example, approximately 80 mm/year in Hawaii, 40 mm/year in CNMI, and 25 mm/year in Guam).
24. Yes. The NGS Horizontal Time-Dependent Positioning (HTDP) software can be used to convert coordinates between the North American, Pacific, and Mariana tectonic plates. HTDP can also be used to compute coordinates referenced to different time frames (i.e., at different epoch dates). Thus, a user will be able to convert, for example, North American plate NAD 83(2011) epoch 2010.00 coordinates to Pacific plate NAD 83(PA11) coordinates at the same or different epoch date. The algorithms already exist in HTDP to perform these transformations, and the reference frame names used by HTDP will be updated to match the new MYCS and NA2011 naming convention used in these FAQs.
25. Any passive station that does not have acceptable GNSS data will be excluded from NA2011, as will stations that are not (directly or indirectly) connected to a CORS in the MYCS. For passive stations meeting these criteria, NGS intends to provide NAD 83(2011), NAD 83(PA11), and NAD 83(MA11) coordinates at epoch 2010.00 everywhere that is appropriate and can be determined accurately with respect to the MYCS. Stations not included in NA2011 will continue to have their positions referenced to the realization indicated by the datum tag currently on NGS datasheets (i.e., prior to the publication of NA2011 results).
26. The answer to this question is entirely dependent upon whether an acceptable GNSS survey is ever performed at that passive control mark. For non-GNSS (i.e., "classically" determined) control, the most accurate way to determine coordinates consistent with a modern realization of NAD 83 is to resurvey using geodetic-quality GNSS data.
27. A list of coordinate changes, from NAD 83(NSRS2007) epoch 2002.00 to NAD 83(2011) epoch 2010.00 will be provided to the public. Coordinate changes will also be provided between the current NAD 83 realizations for the Pacific and Mariana tectonic plate regions and NA2011 results. These coordinate changes will be provided simultaneously with the release of NA2011 results.
28. For the purposes of transforming the official coordinates of each geodetic control point, NGS does not recommend the use of simple transformation models, unless the coordinate shifts are sufficiently systematic over large areas. Interpolative transformation tools, such as NADCON and VERTCON, function only to apply gross consistent shifts to entire areas. The original version of NADCON provides a transformation between the North American Datum of 1927 (NAD 27) and the original (1986) realization of NAD 83. This transformation yields systematic shifts of dozens of meters across the country that can be applied to small scale products such as USGS topographic maps and regional GIS datasets. With continuing refinements of NAD 83, the coordinate changes have decreased to the centimeter level and have become less regionally systematic. One consequence of this is that the formal error of the coordinate shifts may be of similar magnitude as the shifts themselves, and NGS is concerned that such transformation models could be misused, should they be provided. Such transformations could actually be a disservice to our customers, by degrading the spatial accuracy of the data and misleading people into a false sense of complacency about such transformed datasets. The MYCS provides an indication of the expected irregularity of the coordinate and height changes for NA2011; maps of these changes may be viewed under FAQ #6 on the NGS MYCS FAQ web page.
Despite the challenging technical issues with developing interpolative datum transformation models, NGS is committed to providing tools and guidance to help our broad community of users satisfactorily and correctly reference their geospatial data to the NSRS. Toward that end, NGS will actively pursue the following actions.
a. As stated in the previous question, NGS will continue to provide coordinate difference datasets whenever new datum realizations are defined. This will allow surveyors and others who work with raw observations tied to geodetic control to continue to follow best practices.
b. NGS will perform a scientific investigation centering on the possibility of producing transformation models, akin to such products as NADCON and VERTCON. It will include analyses of methods for separating signal from noise, determination of the potential errors involved in using such transformations, and comparisons to the best practice of returning to original control. The study will consider the transformation between NAD 83(NSRS2007) and the previous NAD 83 realizations, as well as between NAD 83(NSRS2007) and NAD 83(2011, PA11, and MA11) epoch 2010.00.
c. NGS will produce useful guidance about datums, datum realizations, the use and misuse of transformation grids, and pitfalls of misidentified geodetic control (i.e., the importance of correctly identifying the actual datum and datum realization of a dataset prior to invoking a transformation).
Questions added on November 8, 2011:
29. NGS will continue to accept projects referenced to NAD 83(NSRS2007) for two months following the date the NA2011 project is officially released (i.e., once the new NAD 83 coordinates are published on the datasheets). During this two-month transition period, NGS will also accept projects referenced to the NAD 83(2011, PA11, and MA11) realizations.
30. Yes, the version of the NGS program ADJUST used to perform the adjustments will change. ADJUST will be modified to have a graphical user interface, compute local and network accuracies, and provide additional more "user friendly" output. Different procedures and input will also be required for using ADJUST, as well as different checking and analysis programs for ADJUST output. However, once NA2011 is complete, NGS will accept projects referenced to the NAD 83(2011, PA11, or MA11) realizations using the version of ADJUST available at that time, even if the procedures (and other modifications) to ADJUST associated with NA2011 are not yet finalized.
31. Projects submitted after March 29, 2012 will be published as soon as NGS has verified the project is acceptable. Station datasheets will have positions referenced to NAD 83(NSRS2007), as provided by the submitter. Once the 2011 adjustment is completed, NGS will adjust those projects to provide NAD 83(2011) epoch 2010.00 coordinates and update the datasheets.
32. No, NGS will not constrain NA2011 to coordinates determined by the CSRC for continuously operating GNSS base stations in California. The main reason for this is that CSRC coordinates are referenced to an epoch date of 2011.00 based on the previous NAD 83(NSRS2007) realization. In addition, the CSRC coordinates were processed independently by CSRC and SOPAC (Scripps Orbit and Permanent Array Center), and so they are not part of the simultaneous processing performed by NGS for the MYCS. In addition, the CSRC/SOPAC solution is based on a different global reference frame (ITRF 2005), GNSS orbits, and (importantly) GNSS antenna models. As such, the CSRS coordinates are inconsistent with the MYCS constraints used for NA2011 and thus cannot be reliably used for NA2011. Please visit the CSRC website for additional information.
33. Yes, NGS will provide NA2011 results in the so-called Readjustment Distribution Format (RDF), which was developed for the initial release of NSRS2007 adjustment results. This format has been added to the official NGS "Bluebook" data format, and so it is no longer referred to by the name "RDF" (although many NGS documents and web pages still use that name). The addition to the Bluebook format consist of three new record types for network accuracies, local accuracies, and variance factors (the *91*, *92*, and *93* records, respectively). Details are given in NGS Bluebook Chapter 2, Horizontal Control Data, pages 2-87 through 2-89 (note that the "RDF" name is not used in the Bluebook).
