The main thing to understand about the NSRS readjustment is this: It is a readjustment, not a new datum. Both the North American Datum of 1983 (NAD 83) and International Terrestrial Reference Frame (ITRF) coordinates will be produced and published. Since the datum will remain NAD 83, let's review what NAD 83 is and why the readjustment is necessary.
NAD 83NAD 83 is the third horizontal geodetic datum of continental extent in North America. It replaces both the original North American Datum and the North American Datum of 1927 (NAD 27). The fundamental task of NAD 83 was to perform a simultaneous least squares adjustment involving 1,785,772 observations and 266,436 stations in the United States, Canada, Mexico and Central America. Greenland, Hawaii and the Caribbean Islands were connected to the datum through Doppler satellite and Very Long Baseline Interferometry (VLBI) observations. The computations were performed with respect to the ellipsoid of the Geodetic Reference System of 1980 (GRS 80). The ellipsoid is positioned in such a way as to be geocentric and the orientation is that of the Bureau International de l'Heuer (BIH) Terrestrial System of 1984 (BTS 84). NAD 83 is similar to the World Geodetic System of 1984 (WGS 84) of the U.S. Department of Defense.
The NAD 83 project extended from July 1, 1974, to July 31, 1986; the cost to NGS was $37 million. All coordinates from that adjustment were designated NAD 83 (1986). There were no GPS observations in the NAD 83 (1986) adjustment.
Adding GPS Data to the NSRSThe launch of Block I GPS satellites began in 1978 and ended in 1985. The first satellite of Block II was launched in February 1989. At that time, a few surveyors jumped onto the GPS bandwagon early and were surveying using Block I GPS satellites only. The observing window was four hours per day. With three or four GPS receivers, a surveyor could establish a significant amount of horizontal control in four hours. All of their networks were referenced to triangulation or traverse stations of the NAD 83 (1986) adjustment. Even though GPS data was gathered before the completion of NAD 83 (1986), none was included in the adjustment.
High Accuracy Reference NetworksWith GPS, observations can be several orders of magnitude higher in accuracy than conventional observations. In May 1988, the Federal Geodetic Control Committee (FGCC) published the document "Geometric Geodetic Accuracy Standards and Specifications for using GPS Relative Positioning Techniques." The document was revised on August 1, 1989, and to this day is the bible for establishing geodetic control that is to be submitted to the NSRS. (It is available on the NGS website.) Table 1 of that document, "Geometric relative positioning accuracy standards for three-dimensional surveys using space system techniques," is reproduced here. As the table shows, all terrestrial-based observations are categorized as Order C. GPS observations are categorized as Orders B, A and AA; Order B is an order of magnitude higher in accuracy than the best of order C, Order A is an order of magnitude higher than Order B, etc. I personally have never found an Order AA station when working on surveying projects.
The NAD 83 adjustment cannot be readjusted at will. Rather, NGS ap-proached each state, requesting their cooperation in a program to establish a GPS control network in their respective state; the network of stations would then be adjusted separately. These networks of stations are called High Accuracy Reference Networks (HARNs). Tennessee was the first state to establish a HARN. Figure 1 shows the status of HARNs in 1993.
With these adjustments, however, a problem developed. In some cases, each state, or two or more states, were adjusted separately. Colorado and New Mexico were adjusted together in adjustment GPS 419 in 1992. Arizona was also adjusted in 1992 but in a different adjustment: GPS 376. Geodetic control in Arizona NAD 83 (1992) is in a different adjustment than Colorado/New Mexico NAD 83 (1992). The HARNs are state networks, useful only in the state or states of the same adjustment. Notice the many different adjustments shown in Figure 2. [Note: There have been some additions to the data shown in Figure 2. Some areas in California are referenced NAD 83 (1998)]. Because of the separate adjustments, there are some inconsistencies at the borders between some states.
Continuously Operating Reference Stations AdjustmentIn the mid 1990s, the Continuously Operating Reference Stations (CORS) network was established. A CORS is a dual-frequency carrier-phase GPS receiver at a permanent location that gathers code and phase data every day. At the end of each day, all data gathered for that day is transmitted to NGS in Silver Spring, Md. Figure 3 is a map showing the location of all CORS stations, a network that is constantly expanding. As mentioned in my September 2004 column (The North American Datum of 1983 Outside of North America, Part 2 of 2), many CORS stations are outside North America.
The CORS network is yet another adjustment. The CORS network is adjusted daily and generates positioning accuracies that approach a few centimeters relative to the NSRS, both horizontally and vertically. (Note: Vertically refers to ellipsoid heights, not orthometric heights). However, with the advent of the CORS network, additional inconsistencies have been discovered between the CORS and HARNs (FBN/CBN) coordinates.
A New ReadjustmentFrom September 1997 through July 2003, NGS re-observed the FBN, and in some cases, assisted in re-observing and/or densifying the CBN. The purpose of this project was to improve the accuracy of the horizontal and ellipsoid height components of FBN stations. Accuracy requirements for these surveys were set to ensure 2 cm local accuracy in the horizontal component as well as 2 cm in ellipsoid heights. Ties were made between CORS and FBN/CBN stations. The availability of this data suggested to NGS that the time is right for a new national readjustment.
On September 24, 2003, NGS' Executive Steering Committee approved a plan for the readjustment of horizontal positions and ellipsoid heights for GPS stations in the contiguous United States. Key points of this readjustment are:
- Only GPS will be adjusted. Classical geodetic observations will not be included.
- CORS stations will serve as control; i.e., CORS positional coordinates will be held fixed.
- The FBN/CBN surveys are a key element since these are the high accuracy (2 cm) surveys that tie traditional geodetic marks from the HARN network to the CORS throughout the contiguous United States.
- A Helmert Blocking strategy will be used for the adjustment. (Note: This is a least squares algorithm that allows NGS to adjust the network in geographic blocks.)
- Both NAD 83 and ITRF coordinates will be produced and published. NAD 83 coordinates will be designated NAD 83 (NSRS).
- User densification projects will be included if observed with GPS, tied to the HARNs and submitted prior to the June 1, 2005 deadline.
- Network and local accuracies will be implemented with the readjustment of the NSRS.
- In the event of a delay in software development, testing and implementation of the new network and local accuracies, a contingency option was adopted. This option endorses immediate statewide GPS readjustments of both horizontal positions and ellipsoid heights.
The readjustment is scheduled to commence in June 2005 and is scheduled for completion Feb. 10, 2007.