The North American Datum of 1983 Outside of North America, Part 2 of 2

In Part 1 of this series (April 2004) I discussed plate tectonics and the International Terrestrial Reference Frame (ITRF). We concentrated on geodetic control on the North American plate. In this second and final part of the series, I will discuss the Pacific plate and how NGS found it necessary to define two new spatial reference frames: NAD 83 (PACP00) and NAD 83 (MARP00). Their calculations were based on GPS data gathered in 1993 by the NGS. It's expected, in my opinion, that within the next few years NGS will have cooperative CORS in several nations located on the Pacific plate, allowing NAD 83 to be extended further west.

I stated in Part 1 that the positional coordinates of a point changes with time because of crustal motion, especially that associated with plate tectonics; the rate of change exceeds 9 mm per year everywhere throughout the United States. After a transformation by NGS, points located within the interior of the North American tectonic plate experienced little or no motion relative to NAD 83 (CORS96).

Figure 1. A visual representation showing where the North American plate meets the Pacific plate.

What about the Pacific?

Such is not the case for points located on other plates or within a few hundred kilometers of the North American plate boundary. In his paper titled "Introducing Two Spatial Reference Frames for Regions of the Pacific Ocean,"[1] Richard Snay, manager of the National and Cooperative CORS Program for NGS, reported that Honolulu, Hawaii, moves northwestward about 80 mm per year relative to NAD 83 (CORS96). Figure 1 shows where the North American plate meets the Pacific plate.

Figure 1 also shows the boundaries of the Pacific plate, the Mariana plate and the Philippine plate. The island nations under U.S. jurisdiction, referenced in Snay's article, are Hawaii, American Samoa and Guam. Because of the large movement in these Pacific nations, NGS introduced two new spatial reference frames in 2000, one for the Pacific tectonic plate called NAD 83 (PACP00) and one for the Mariana tectonic plate called NAD 83 (MARP00). These new spatial reference frames were established so users do not have to cope with changing positional coordinates for those positioning applications that are confined to the interior of one of these tectonic plates. Each of these reference frames is defined in terms of a transformation from ITRF00.

Figure 2. In addition to the two VLBI stations, there are six active CORS stations in Hawaii.
In order to arrive at these new spatial reference frames, NGS performed an adjustment on eight VLBI (Very Long Baseline Interferometry) stations on the North American Plate (shown in Figure 1) to estimate the seven transformation parameters to get ITRF92 positional coordinates on the eight stations. This transformation was used to convert ITRF92 positional coordinates to two VLBI stations in Hawaii, one on Kauai and the other on Maui. This eventually led to a transformation from ITRF00 to the new spatial reference frame NAD 83 (PACP00). In addition to the two VLBI stations, there are six active CORS stations in Hawaii, shown in Figure 2.

Guam is on the Mariana tectonic plate. Unfortunately, the Mariana plate contains only one reference station: the CORS station on Guam (named GUAM). However, the GUAM station's horizontal ITRF00 velocity is well-determined. Two stations are required to define the three parameters needed to define horizontal motion. NGS then used the CORS station on Saipan, named CNMI, and assumed it had the same horizontal ITRF00 velocity as GUAM. This information led to a transformation from ITRF00 to the new spatial reference frame NAD 83 (MARP00). In Figure 1, the two CORS stations CNMI and GUAM are shown at the left edge of the Philippine plate.

With these two new reference frames, users of these frames will not need to worry about changing positional coordinates in the regions.

Figure 3. Palau is not far from the Philippines.

The Palau Conference

In the fall of 2001, I was contacted by Jon Abrams, a cadastral surveying consultant from Colorado, who asked if I would participate in a surveyors' conference in Guam. After some time, the final decision was to change the conference location to Koror, Palau, on March 10-14, 2003. The name of the conference was "Western Pacific Regional GPS, GIS, Cadastral Survey Conference and Technical Programs." I was informed that Dave Doyle, senior geodesist at NGS and Ed Carlson, NGS geodetic advisor to the state of Hawaii, would join me on the program.

Where is Palau? It's a 21⁄2 hour flight from Guam in the southwest direction at 7º30' north latitude, 133º30' east longitude. Far to the southwest of Micronesia, the Republic of Palau consists of an archipelago of 343 islands, spread north to south over 100 miles. For three decades it was part of the United Nations Trust territory of the Pacific under U.S. administration. In 1978, they asked for independence rather than join the Federated States of Micronesia. A compact of free association with the United States was approved in 1986 and later ratified in 1993. Palau gained independence in 1994. Now, as an independent nation, its official currency is the U.S. dollar and the U.S. Postal Service handles the mail. The postal zip code for Palau is 96940 and a 37-cent stamp sends a letter from the United States. Figure 3 shows the location of Palau, not far from the Philippines; Figure 4 is a map of Palau.

Figure 4. A map of Palau.
Dave Doyle, Ed Carlson and I flew on the same flights from Honolulu to Guam to Palau. The conference was held at the Palau Community College in Koror, and attendees were from the Pacific nations of: American Samoa, Commonwealth of the Northern Mariana Islands (Saipan), Guam, Kosrae, Marshall Islands, Republic of Palau and Yap, and Pohnpei in the Federated States of Micronesia. (I'm not a history buff, but many readers will recognize some of these nations as being occupied by Japan in World War II. Like Palau, all nations today are part of the U.S. Postal Service.)

Dave Doyle, Ed Carlson and I attended every session of the conference, but had some time available to visit the Office of the Palau Automated Land and Resources Information System, Ministry of Resources and Development. The minister, Fritz Koshiba, who is a surveyor by profession, treated us like royalty. He held a beach party for the conference attendees with the governor of Palau in attendance. Our special contact at the Ministry was Kelly L. Raleigh Moses, an American woman originally from Wisconsin who is a program manager. In her office is a GPS receiver, owned by the University of Tokyo, operating as a CORS. Jon Abrams, a consultant to the Republic of Palau, has an office in the Ministry.

The walls of the Ministry had several large-scale maps of Palau, compiled by the U.S. Geological Survey when Palau was placed under U.S. jurisdiction. All maps showed Palau on the Guam datum. Dave Doyle took issue with this since the datum of Palau is actually Bab South Astro. When the USGS decided to map Palau, they had to establish a known position. They established an astro station on Babeldaob, the large island north of Koror (see Figure 4). This astro position was held fixed, and a triangulation network was established to generate additional control for the mapping project. Dave later reported the error to the USGS.

Establishing control anywhere in the world is no problem today. Ed Carlson had brought several GPS receivers with him and was given permission to establish GPS control points on the islands. At the end of the conference, Ed remained to continue establishing geodetic control and to photo ID his work so the imagery could be geo-referenced. The geodetic control Ed established was adjusted to certain CORS in the Pacific, making Palau part of NAD 83. The CORS in the Ministry may soon be a Cooperative CORS, giving NGS additional information on the plate tectonics in that area.

I have also been involved in establishing geodetic control in the Pacific. In 1969, I wrote a master's thesis titled "The Use of Geodetic SECOR for an Inter-island Tie in the Southwest Pacific." SECOR was an early geodetic satellite system, similar in many ways to today's GPS. No SECOR stations were on the island nations that attended this conference.

Differing Spatial Reference Frames

It is important to remember that ground control stations are not fixed in position. On the North American plate, NGS has provided a transformation that shows little or no motion relative to NAD 83 (CORS96). This is not the case on the Pacific plate or the Mariana plate. As such, NGS introduced a new spatial reference frame for the Hawaii area named NAD 83 (PACP00) and also a new spatial reference frame for the Guam area named NAD 83 (MARP00); data continues to be gathered in other island nations like Palau.

The Western Pacific Regional Conferences are bringing together friendly Pacific nations that may, eventually, have geodetic control on NAD 83, extending it further west. In November, another Western Pacific Regional GPS, GIS, Cadastral Survey Conference will take place in Guam.

North American surveyors in California, Oregon, Washington and western Canada, however, have a more complex situation where a new spatial reference frame cannot be established because of excessive motion. Surveyors in this area will have to use the horizontal time-dependent software, HTDP, since the geodetic position will change with time.

Crustal motion near the North American plate?

The crustal motion occurring near the North American plate is so complex that no single reference frame can be defined to remove this motion. NAD 83 users surveying in this area need to expect changing coordinates at points located within a few hundred kilometers of this boundary. To help these users, NGS has developed the HTDP (horizontal time-dependent positioning) software, accessible from the NGS website "Geodetic Tool Kit."