such as the European Reference Frame (EUREF) and the North American Reference Frame (NAREF), are represented within the IAG structure through their representation in IAG Commission 1. These regional entities play a major role in redefining regional and national datums and their relationship to the ITRF.
From 1987 to 1997, the National Geodetic Survey (NGS), in cooperation with other federal, state, and local surveying agencies, conducted a resurvey of the United States using GPS observations often referred to as High Accuracy Reference Networks (HARNs). Continued improvements in GNSS/GPS technology and requirements from the users of spatial data will eventually require a transition to an improved reference frame based on the ITRF. Positions relative to the ITRF differ from the existing NAD83 by approximately one meter in horizontal position and one meter in ellipsoidal height. NGS already publishes ITRF coordinates for all Continuously Operating Reference Stations (CORS), and will implement, over the next 3–5 years, an adjustment to include the HARNs and other GPS data submitted to NGS.
The North American Datum 1983 is a common horizontal reference frame for the North American continent that is legally recognized by the United States and Canada. It is a fundamental element of the National Spatial Reference System (NSRS). Based on the first satellite geodetic results from early Doppler tracking and data from a few VLBI stations in the 1970s, the NAD83 took the North American reference frame into the space age, making obsolete the older NAD27 system that was based on ground-based classical surveys. Twenty-five years later, it would be timely and appropriate to upgrade the NAD system again, taking advantage of the latest global and national geodetic observations and geophysical models. The NAD83 differs from the ITRF at the level of one meter, a very large number considering the approximately one-millimeter level of precision of today’s geodetic techniques.
The difficulty for the United States and Canada is that the North American plate rotates, with the result that the coordinates of some stations would change by as much as one meter in 20 years. Fortunately, the rotation of the North American plate is highly stable and is well-understood by geodesists, so the effect of the plate rotation can be taken into account to provide stable coordinates. A NAREF working group of the IAG led by the U.S. NGS and Natural Resources Canada (NRCan) has researched ways to improve the NAD system. The working group, called “Stable North American Reference Frame,” is a collaboration between the NGS, NRCan, and university researchers who are experts in the latest geodetic techniques and modeling of geophysical effects. The participation of university researchers had been facilitated by funding from the U.S. National Science Foundation in recognition of the critical need for reference frame improvement for the scientific objectives of the EarthScope program. In addition, the NGS provides a service that enables users throughout the United States to compute the effect of modeled geophysical surface motion as a function of position, a service designed to bring station coordinates to rest after the correction is applied.
The NGS has to work within the legal definition of the NAD83, which was designed before the advent of the GPS in the 1980s. Modern surveyors typically use GNSS/GPS and do not necessarily need to use classical survey markers now that NGS provides data and coordinates from its CORS network. Nonetheless, in most areas, property laws govern the use of ground markers, which should be maintained on that basis. Furthermore, in areas where ground markers undergo large displacements as a result of geological processes, recovery and re-survey of these markers is a precious source of scientific information. The committee recognizes the vision and considerable efforts of NGS in modernizing the NSRS and encourages further developments to modernize the NAD system.