Evolution of NAD 83 in the United States: Journey from 2D Toward 4D
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Evolution of NAD 83 in the United States: Journey from 2D toward 4D Richard A. Snay1 Abstract: In 1986, Canada, Greenland, and the United States adopted the North American Datum of 1983 (NAD 83) to replace the North American Datum of 1927 as their official spatial reference system for geometric positioning. The rigor of the original NAD 83 realization benefited from the extensive use of electronic distance measuring instrumentation and from the use of both TRANSIT Doppler observations and very long baseline interferometry observations. However, the original NAD 83 realization predated the widespread use of the global po- sitioning system and the use of continuously operating reference stations. Consequently, NAD 83 has evolved significantly in the United States since 1986 to embrace these technological advances, as well as to accommodate improvements in the understanding of crustal motion. This paper traces this evolution from what started as essentially a two-dimensional (2D) reference frame and has been progressing toward a four-dimensional (4D) frame. In anticipation of future geodetic advances, the U.S. National Geodetic Survey is planning to replace NAD 83 about a decade from now with a newer, more geocentric spatial reference system for geometric positioning. DOI: 10.1061/(ASCE)SU.1943- 5428.0000083. © 2012 American Society of Civil Engineers. CE Database subject headings: History; Datum; Transformations; Geodetic surveys; North America. Author keywords: History; Datum transformations; Dynamic datums; Crustal deformation; NAD 83. Introduction height, and how these three coordinates vary over time. NAD 27 provided a foundation for measuring only geodetic latitude and lon- In 1986 a group of institutions representing Canada, Greenland, and gitude; therefore, it is considered a horizontal datum. NAD 27 was the United States of America adopted the North American Datum of established using triangulation data together with 112 taped baseline 1983 (NAD 83) as their official spatial reference system for geo- lengths to provide scale and 175 astronomic azimuths to provide metric positioning (Schwarz 1989). The National Geodetic Survey orientation. The origin of NAD 27 was established by prescribing (NGS), an office of the U.S. National Oceanic and Atmospheric specific values for both the geodetic latitude and longitude of the Administration, represented the United States, where the first NAD reference station in Kansas known as Meades Ranch—a point located 83 realization is referred to as NAD 83 (1986). NAD 83 replaced the near the geographic center of the conterminous United States North American Datum of 1927 (NAD 27), and the NGS has already (CONUS). expressed its intention to replace NAD 83 about a decade from In 1927, the United States’ horizontal reference network contained now with a newer geometric reference system (NGS 2010). Dracup approximately 25,000 reference stations. By 1983, this network had (1989) provides a history of horizontal geodetic control in the United expanded to include approximately 272,000 reference stations. States prior to the adoption of NAD 83. Craymer (2006) discusses NAD 27 served the nation well until electronic distance measuring the evolution of NAD 83 in Canada since its adoption in 1986. (EDM) instrumentation came into widespread usage in the 1960s. Here, the term reference system pertains to an abstract collection EDM instrumentation enabled surveyors and others to measure of principles, fundamental parameters, and specifications for quan- baseline lengths with relative ease and with an accuracy of a few titatively describing the positions of points in space and how these parts per million (ppm). With the existing triangulation network, positions vary over time, whereas the term reference frame pertains geodesists often had to propagate a taped baseline length more than to the materialization or realization of such a system. Traditionally, 100 km via the laws of trigonometry and geometry to compute the a reference frame consists of a network of reference stations on the distance of a baseline connecting a pair of NAD 27 reference sta- ground with adopted positional coordinates (and sometimes veloc- tions. The distance of this same baseline could be measured much ities), which may be used to help determine coordinates for other more accurately with EDM instrumentation. As such, EDM results locations. NAD 83 is a reference system, whereas NAD 83 (1986) is exposed significant regional and local distortions among NAD 27 just one of many reference frames associated with NAD 83. coordinates for reference stations. In some cases, the relative error Modern geometric reference systems are intended to provide a between the NAD 27 coordinates of two separate reference stations foundation for measuring geodetic latitude, longitude, and ellipsoid Downloaded from ascelibrary.org by University of Minnesota - Twin Cities on 10/04/16. Copyright ASCE. For personal use only; all rights reserved. exceeded 1 m. The 1960s also witnessed the dawning of yet another innovative 1 technology for measuring positional coordinates; namely, the use of Retired; formerly, National Geodetic Survey, 9505 Aspenwood Ct., artificial earth-orbiting satellites. In particular, the U.S. Navy’s Montgomery Village, MD 20886. E-mail: [email protected] TRANSIT Doppler constellation of satellites enabled its users to Note. This manuscript was submitted on November 22, 2011; approved measure the three-dimensional (3D) coordinates of a point relative to on January 30, 2012; published online on February 1, 2012. Discussion period ’ open until April 1, 2013; separate discussions must be submitted for in- the earth s center of mass (hereafter referred to as the geocenter) with dividual papers. This paper is part of the Journal of Surveying Engineering, a precision of approximately 1 m. An earth-based receiver did so by Vol. 138, No. 4, November 1, 2012. ©ASCE, ISSN 0733-9453/2012/4- tracking the Doppler shifts of satellite-transmitted radio signals. The 161e171/$25.00. receiver needed to collect data at a reference station for at least one JOURNAL OF SURVEYING ENGINEERING © ASCE / NOVEMBER 2012 / 161 J. Surv. Eng., 2012, 138(4): 161-171 week to achieve such meter-level precision. TRANSIT technology illustrates the magnitudes of the two-dimensional (2D) coordinate may seem quite outdated compared with today’s global navigation shifts associated with the transition from NAD 27 to NAD 83 (1986). satellite system (GNSS) technology; however, TRANSIT Doppler The NGS developed the NADCON software (Dewhurst 1990), results were popular back in the 1960s and 1970s. TRANSIT en- which enables users to convert between NAD 27 and NAD 83 (1986) abled geodesists to measure all three spatial dimensions simulta- positional coordinates. Although NAD 83 started as 3D in scope, the neously and in an absolute sense; that is, relative to the geocenter NGS adopted only horizontal coordinates (geodetic latitude and rather than to some arbitrary point such as a reference station located longitude) for over 99% of the approximately 272,000 United States in Kansas. This facilitated various countries, if they so desired, to use reference stations that were involved in its first realization. This first the geocenter as a common origin for their respective reference realization did include a few global positioning system (GPS) systems. The U.S. Department of Defense (DoD) employed observations; however, this first realization occurred before GPS TRANSIT Doppler observations extensively for establishing the blossomed into the technology that has helped to make the vertical original realization of the World Geodetic System of 1984 (WGS 84) dimension economically accessible. (True 2004). Indeed, the NGS and DoD collaborated to make the original NAD 83 coordinates and the original WGS 84 coordinates consistent with each other in the sense that both sets of coordinates GPS Changes Everything are referred to the same origin and share the same orientation and scale. Only a submillimeter difference in the semimajor axis of the Just before the NGS adopted NAD 83 (1986), the agency began chosen reference ellipsoid differentiates the original NAD 83 using GPS, instead of triangulation and/or trilateration, for hori- coordinates from the original WGS 84 coordinates in a systematic zontal positioning. In the 1980s, surveyors, hydrologists, and other way. users of vertical positions somewhat overlooked GPS results be- It was both EDM and TRANSIT Doppler that provided the im- cause they required orthometric heights rather than ellipsoid heights. petus for replacing NAD 27 with NAD 83. The NGS, in collabo- As shown in Fig. 2, an orthometric height, denoted as H, at a location ration with other institutions, performed numerous very high quantifies the distance along the curved plumb line (through that accuracy trilateration surveys with EDM instrumentation and posi- location) above or below the geoid, an equipotential surface ap- tioned 612 reference stations with TRANSIT Doppler observations proximating mean sea level. Orthometric heights depend on the prior to establishing the geocentric NAD 83 reference system. Fig. 1 earth’s gravity field, and these heights are traditionally measured Downloaded from ascelibrary.org by University of Minnesota - Twin Cities on 10/04/16. Copyright ASCE. For personal use only; all rights reserved. Fig. 1. Magnitude of shifts from NAD 27 horizontal coordinates to NAD 83 (1986) horizontal coordinates 162 / JOURNAL OF SURVEYING ENGINEERING © ASCE / NOVEMBER 2012 J.