The future of positioning is GNSS. The underlying reference frames for all GNSS systems are geocentric. The International Terrestrial Reference Frame (ITRF), used for globally consistent scientific applications such as the determination of sea level change, has gotten progressively more geocentric over the last ten years, so that now the origin of the ITRF coincides with Earth’s center to about 1 centimeter of accuracy. Furthermore, countries are increasingly choosing GNSS as their primary tool to access a vertical datum, minimizing their reliance upon unmonitored passive control.
In the United States, the official geometric, historically called “horizontal”, datum, NAD 83, has a known non-geocentricity of over two meters and the official vertical datum, NAVD 88, is accessed through a set of passive control that is fragile, inaccurate and rapidly deteriorating. The National Geodetic Survey (NGS) is working to define and adopt a geocentric reference datum for the United States to replace NAD 83 and is working to compute an accurate geoid model which will serve as the defining surface of a new vertical datum that is accessed through GNSS technology and which replaces NAVD 88. These two changes are dependent upon one another in a variety of ways and are currently planned to occur simultaneously. The decision to proceed with these changes was both obvious and difficult because NGS is cognizant of two important, but conflicting needs in the user community: accuracy and constancy. To fulfill its mandate to provide the geodetic reference frame for all United States geospatial activities, NGS must strive to be as scientifically accurate as possible. After much internal discussion, NGS has determined that it must address serious issues of inaccuracy in the current realizations of NAD 83 and NAVD 88. At the same time NGS recognizes that significant user resources have been invested in the current realizations of these datums.
In order to continue improving accuracy while minimizing the impact of new reference frame paradigms, NGS is working to implement this transition over the next 10 years. This will allow time for the user community to voice concerns, for NGS to address them, and to ensure that the transition will go as smoothly as possible.
For this reason, on May 11-12, 2010 NGS will convene the first in a series of Federal Geospatial Summits to address these proposed improvements to the National Spatial Reference System. The intent of these summits is to solicit user input and to provide documented solutions to address all concerns.
This white paper defines the issues as currently understood by NGS and is to serve as the catalyst for soliciting user comments, questions and concerns.
Click HERE to see a 348Kb PDF of the white paper
Information about the Summit, including presentations, transcripts, the webcast, a participant list and eventually the proceedings can be found HERE