The National Adjustment of 2011 Project
Alignment of passive control with the latest realization of the North American Datum of 1983: NAD 83(2011/PA11/MA11) epoch 2010.00
The mission of NOAA's National Geodetic Survey (NGS) is to define, maintain, and provide access to the National Spatial Reference System (NSRS). The NSRS is the official reference system for latitude, longitude, height, scale, gravity, and orientation throughout the United States and its territories. It is the foundation for the nation's transportation, mapping, and charting infrastructure, and it supports a multitude of scientific and engineering applications.
As part of continuing efforts to improve the NSRS, on June 30, 2012, NGS completed the National Adjustment of 2011 Project. This project was a nationwide adjustment of NGS "passive" control (physical marks that can be occupied with survey equipment, such as brass disk bench marks) positioned using Global Navigation Satellite System (GNSS) technology. The adjustment was constrained to current North American Datum of 1983 (NAD 83) latitude, longitude, and ellipsoid heights of NGS Continuously Operating Reference Stations (CORS). The CORS network is an "active" control system consisting of permanently mounted GNSS antennas, and it is the geometric foundation of the NSRS. Constraining the adjustment to the CORS optimally aligned the GNSS passive control with the active control, providing a unified reference frame to serve the nation's geometric positioning needs.
Current NAD 83 CORS coordinates were determined by re-processing all CORS data collected from January 1994 to April 2011 in the NGS initial Multi-Year CORS Solution (MYCS1) project. The resulting CORS coordinates were published by NGS in September, 2011, and constitute a new realization referred to as NAD 83(2011/PA11/MA11) epoch 2010.00. The realization name has two parts: the datum tag in parentheses after NAD 83, and the epoch date in decimal years. The datum tag refers to the year the realization was completed (2011) and the tectonic plate to which the coordinates are referenced (2011 refers to the North America plate, PA11 to the Pacific plate, and MA11 to the Mariana plate). The epoch date indicates that the published coordinates represent the location of the control stations on January 1, 2010 -- an important consideration in tectonically active areas (such as the western U.S.). In this way, the CORS coordinates (and thus the passive marks constrained to the CORS) are consistent across both space and time. Additional information on the MYCS1 realization of NAD 83 is available on the NGS CORS Coordinates web page.
To create the passive control network, 4267 individual GNSS survey projects (stored in the NGS database) were combined into an overall network of 81,055 stations, including 1195 CORS used as constraints. The stations were connected by 424,711 GNSS vectors observed between April 1983 and December 2011. Because of a lack of vector connections between the conterminous U.S. (CONUS), Alaska, and the Pacific, these three regions were each adjusted separately: CONUS with 79,546 stations (including 1113 CORS), Alaska with 968 stations (including 58 CORS), and the Pacific with 541 stations (including 24 CORS). The entire Pacific was adjusted as two individual networks, one referenced to the Pacific tectonic plate and the other to the Mariana plate. CONUS was further split into a Primary and Secondary network based mainly on the age of the observations, as described below. The CONUS Primary network consisted of 62,364 stations (including 1097 CORS) and the Secondary network 22,503 stations (including only 45 CORS), where 5321 stations (including 29 CORS) were common between the two networks. Because of the large size of the two CONUS networks, they were adjusted using a Helmert blocking strategy. The Helmert approach breaks a large network into separate (but connected) smaller "blocks" to reduce computation time. The blocks are each adjusted individually and combined to give results identical to what would be obtained if the entire adjustment were performed simultaneously.
For the final constrained adjustments, the median network accuracy for all stations was 0.9 cm horizontal and 1.5 cm vertical (i.e., ellipsoid height) at the 95% confidence level. The median change in coordinates from the previous published values was about 2 cm horizontally and vertically. However, some station coordinates changed by more than 1 meter horizontally and 60 cm vertically. Although some of the large coordinate changes resulted from new data and adjustment strategies, most horizontal changes greater than about 6 cm occurred in geologically active areas and were likely due to tectonic motion.
Results of the 2011 national adjustment for 79,677 passive control marks are available on NGS Datasheets, including their network and local accuracies. Of these passive marks, 79,161 are referenced to the North America tectonic plate as the 2011 realization (including CONUS, Alaska, and the Caribbean); 345 are referenced to the Pacific plate as the PA11 realization (the central Pacific, including Hawaii, American Samoa, and the Marshall Islands); and 171 are referenced to the Mariana plate as the MA11 realization (the western Pacific, including Guam, Palau, and the Commonwealth of the Northern Mariana Islands). Although the passive marks are referenced to three different tectonic plates, all refer to a common 2010.0 epoch date. With the completion of the national adjustment, all passive marks on NGS Datasheets with NAD 83(2011/PA11/MA11) epoch 2010.00 coordinates will be consistent with results obtained using CORS and the NGS Online Positioning User Service (OPUS). Note that 183 stations were excluded from the final national adjustments due to lack of enabled vector connections; where possible, these stations will be reconnected to the network in subsequent individual adjustments.
A new NGS hybrid geoid model, GEOID12A, was developed by combining NAD 83(2011/PA11/MA11) epoch 2010.00 ellipsoid heights on leveled bench marks with a new gravimetric geoid model, USGG2012. The GEOID12A model is for converting between NAD 83(2011/PA11/MA11) epoch 2010.00 ellipsoid heights and orthometric heights in the respective vertical datums for different regions, such as the North American Vertical Datum of 1988 (NAVD 88) for CONUS and Alaska. Previous hybrid geoid models (such as GEOID09) should not be used to convert NAD 83(2011/PA11/MA11) epoch 2010.00 ellipsoid heights to orthometric heights.
A number of technical challenges were confronted in performing the 2011 national adjustment. One, as mentioned previously, was that the networks were referenced to three different tectonic plates (North America, Pacific, and Mariana plates, as mentioned above). In some cases, stations referenced to one plate were located on a different plate (e.g., stations in coastal California and the Caribbean were referenced to the North America plate). This was handled by modeling tectonic motion (including earthquakes) using the NGS Horizontal Time Dependent Positioning (HTDP) software to transform the GNSS vectors to NAD 83 at the common 2010.00 epoch date. For the 2011 national adjustment, HTDP version 3.1.2 was used for CONUS, Alaska, and the Caribbean, and version 3.2.2 was used for the Pacific.
Other technical issues addressed in the project include 1) appropriate down weighting of the up component of GNSS vectors to account for subsidence in the northern Gulf Coast region of CONUS; 2) use of variable weighted (stochastic) constraints for CORS based on formal accuracy estimates derived from the NGS MYCS1; 3) scaling of GNSS vector error estimates for all projects to ensure consistent weighting of observations; 4) use of down weighting (rather than removal) for vector rejections; 5) splitting the conterminous U.S. into a Primary and Secondary network, as mentioned above, such that vectors observed prior to about 1994 were assigned to the Secondary network. This allowed the Primary network to be adjusted separately without the problems associated with older observations (e.g., single frequency receivers, no antenna phase center models, poor orbit accuracy, incomplete satellite constellation, lack of CORS, etc.). Each of these technical challenges (and others) was satisfactorily resolved, and completion of the National Adjustment of 2011 Project represents a significant step toward a more integrated, consistent, and accurate NSRS.
We welcome your feedback and comments
